Rsc Cfns Manual

Chemistry for Non-Specialists Course Book

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Organised by: Royal Society of Chemistry The RSC is the UK Professional Body for chemical scientists and an international Learned Society for advancing the chemical sciences. Supported by a network of over 46,000 members worldwide and an internationally acclaimed publishing business, our activities span education and training, conferences and science policy, and the promotion of the chemical sciences to the public. www.rsc.org

Supported by an unrestricted educational grant from: GlaxoSmithKline GlaxoSmithKline – one of the world’s leading research-based pharmaceutical and healthcare companies – is committed to improving the quality of human life by enabling people to do more, feel better and live longer. www.gsk.com

In collaboration with: National Network of Science Learning Centres Science Learning Centres provide the highest quality Continuing Professional Development for everyone involved in science education, at all levels. With a network of ten Centres across the country access to innovative and inspiring courses is within easy reach. www.sciencelearningcentres.org.uk

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Chemistry for Non-Specialists Welcome to the Chemistry for Non-Specialists training course, organised by the Royal Society of Chemistry (RSC) in collaboration with the national network of Science Learning Centres (SLCs) and supported by an unrestricted educational grant from GlaxoSmithKline (GSK). Developed by the RSC, in conjunction with the Department for Education and Skills (DfES) and GSK, the course has trained over 1500 teachers between 2006 and 2009. This course is designed to provide teachers with the confidence, flair and enthusiasm to teach chemistry at KS3 or KS4. It is specifically aimed at those who are not chemistry specialists. The four-day courses comprise a two-day residential and two one-day follow-up events at approximately one-term intervals. The dates of the follow-up days should be available from the Science Learning Centre. The trainers on this course are experienced chemistry teachers, trained by the RSC.

Course learning outcomes By the end of the four-day course you will have:  increased your understanding of chemistry topics at KS3 and/or KS4,  become more confident and competent in the teaching of chemistry,  rehearsed relevant and interesting practical experiments and demonstrations to help

inspire and engage your students,  developed an understanding of common student misconceptions and how these can

be addressed,  developed the effective use of scientific models relevant to the teaching of chemistry.

About the course book and CD-ROM This course book and CD-ROM are © Royal Society of Chemistry 2010. This course book and accompanying CD-ROM are intended for your use during and after the training course. All contents of this book are available on the CD-ROM as PDF files from which you can make copies. The CD-ROM may be loaded onto your school’s network should you wish to share this resource with colleagues; however, they should not be loaded onto a publicly available website. If colleagues would like to attend the course, please visit www.rsc.org/chemnonspec for a list of forthcoming course dates. The CD-ROM has been thoroughly checked for errors and viruses. The RSC cannot accept liability for any damage to your computer system or data which occurs while using this CD-ROM or the software contained on it. If you do not agree with these conditions, you should not use the CD-ROM. Please note the RSC will not offer support or guidance on how best to network the files. If you wish to purchase a copy of this course book and CD-ROM visit www.rsc.org/books. A catalogue record for this book is available from the British Library. ISBN: 978-1-84973-112-6

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Contents

Page

Health and safety guidance

4

The CLEAPSS® science resources web site and CD-ROM

6

CLEAPSS® helpline

10

Experiments

11

No Title 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

2

Diffusion of gases – ammonia and hydrogen chloride Handling liquid bromine and preparing bromine water Diffusion of gases – a safer alternative to bromine Diffusion in liquids Allotropes of sulfur Making glass Chocolate and structure Experiments with hydrogels – hair gel and disposable nappies Experiments with hydrogels – plant water storage crystals Cross-linking polymers – alginate worms Extracting limonene from oranges by steam distillation Generating, collecting, and testing gases An alternative to using compressed gas cylinders Making a reaction tube Ammonia fountain Titrating sodium hydroxide with hydrochloric acid Using indigestion tablets to neutralise an acid A thermometric titration Universal indicator ‘rainbow’ An effervescent universal indicator 'rainbow' Neutralisation circles Indicators and dry ice: demonstration Thermal decomposition of calcium carbonate Reacting elements with oxygen Reacting elements with chlorine Identifying the products of combustion The ‘Whoosh’ bottle demonstration Fat-pan fire! Money to burn The methane rocket The thermal decomposition of nitrates – ‘writing with fire’ Iron and sulfur reaction The reaction between zinc powder and sulfur Reaction between aluminium and iodine Reaction of zinc with iodine The combustion of iron wool The change in mass when magnesium burns

13 15 16 18 20 24 27 30 33 37 41 45 47 51 53 57 61 64 66 68 69 71 74 77 81 83 86 88 91 93 95 97 100 102 104 107 109

38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80

Competition for oxygen Displacement reactions between metals and their salts Extracting metals with charcoal Extraction of iron on a match head! The reaction between zinc and copper oxide The thermite reaction Making silicon and silanes from sand Reduction of copper(II) oxide with methane Finding the formula of copper oxide The reaction of magnesium with steam The 'blue bottle' experiment Turning copper coins into 'silver' and 'gold' Alkali metals Heating Group 1 metals in air and in chlorine Reactions of aqueous solutions of the halogens Halogen reactions with iron Colourful electrolysis Electrolysis of zinc chloride Identifying the products of electrolysis Electrolysis of potassium iodide solution Endothermic solid-solid reactions Exothermic or endothermic? Chemiluminescence – cold light Spontaneous exothermic reaction The effect of concentration on reaction rate Iodine clock reaction Rates and rhubarb Catalysts for the decomposition of hydrogen peroxide Controlled explosion of a hydrogen-air mixture Controlled explosion of a methane-air mixture Exploding balloons The howling/screaming jelly baby Hydrogen/oxygen explosion Exploding bubbles of hydrogen and oxygen An oscillating reaction The fractional distillation of crude oil Cracking hydrocarbons Making nylon - the 'nylon rope trick' PVA polymer slime Reactions of positive ions with sodium hydroxide (microscale version) Testing for negative ions Flame tests (wooden splint method) Flame colours – a demonstration

113 116 119 121 123 125 128 131 133 136 140 142 145 148 152 156 161 164 168 173 176 178 181 184 186 189 192 195 197 201 204 207 211 213 216 218 221 225 228 232 236 241 243

Self test questionnaire

246

RSC resources

255

Index

257

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Health and safety guidance See the health and safety notes in each experiment. The notes on pages 4 and 5 are for general guidance. Health and safety in school and college science affects all concerned: teachers and technicians, their employers, students, their parents or guardians, as well as authors and publishers. These guidelines refer to procedures in the United Kingdom. If you are working in another country you may need to make alternative provision.

Health and safety checking As part of the reviewing process, the experiments in this course book have been checked for health and safety. In particular, we have attempted to ensure that: ● all recognized hazards have been identified, ● suitable precautions are suggested, ● where possible, the procedures are in accordance with commonly adopted model (general) risk assessments, ● where model (general) risk assessments are not available, we have done our best to judge the procedures to be satisfactory and of an equivalent standard, ● experiments have been checked by CLEAPSS®.

Assumptions It is assumed that: ● the practical work is carried out or supervised by a qualified science teacher with adequate knowledge of chemistry and the equipment used, ● practical work is conducted in a properly equipped and maintained laboratory, ● rules for student behaviour are strictly enforced, ● mains-operated equipment is regularly inspected, properly maintained and appropriate records are kept, ● care is taken with normal laboratory operations such as heating substances and handling heavy objects, ● good laboratory practice is observed when chemicals are handled, ● eye protection is worn whenever risk assessments require it, ● any fume cupboard used operates at least to the standard of Building Bulletin 88, ● students are taught safe techniques for such activities as heating chemicals, smelling them, or pouring from bottles, ● hand-washing facilities are readily available in the laboratory.

Teachers' and their employers' responsibilities Under the COSSH Regulations, the Management of Health and Safety at Work Regulations, and other regulations, employers are responsible for making a risk assessment before hazardous procedures are undertaken or hazardous chemicals used or made. Teachers are required to co-operate with their employers by complying with such risk assessments. However, teachers should be aware that mistakes can be made and, in any case, different employers may have different local rules.

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Therefore, before carrying out any practical activity, teachers should always check that what they are proposing is compatible with their employer’s risk assessments and does not need modification for their particular circumstances. Any local rules issued by the employer must always be followed, whatever is recommended here. However, far less is banned by employers than is commonly supposed.

Reference material Model (general) risk assessments have been taken from, or are compatible with: CLEAPSS® Hazcards (see CLEAPSS® Science Resources website and annually updated CD-ROM) CLEAPSS® Laboratory handbook (see CLEAPSS® Science Resources website) CLEAPSS® Recipe cards (see CLEAPSS® Science Resources website) ASE Safeguards in the school laboratory 11th edition 2006 ASE Topics in Safety 3rd edition, 2001 ASE Safety reprints, 2006 or later

Procedures Clearly, you must follow whatever procedures for risk assessment your employers have laid down. As far as we know, almost all the practical work and demonstrations in this course book are covered by the model (general) risk assessments detailed in the above publications, and so, in most schools and colleges, you will not need to take further action, other than to consider whether any customisation is necessary for the particular circumstances of your school or class.

Special risk assessments Only you can know when your school or college needs a special risk assessment. But thereafter, the responsibility for taking all the steps demanded by the regulations lies with your employer.

External websites The Royal Society of Chemistry is not responsible for the content of non-RSC websites which may be linked from pages in this course book. This health and safety guidance has been adapted from the Practical Chemistry website: http://www.practicalchemistry.org/health-and-safety/

Key to health and safety symbols The following symbols are used throughout this course book and are used to draw your attention to the health and safety precautions needed. The design draws on a common style of sign but we have produced some new ones to meet the needs of this book. Note that the text should be read to determine exactly the type of gloves, eye protection etc that is needed for each different experiment.

• Eye Protection

• Fume Cupboard

Gloves

• Do Not Inhale

Wash Hands

Ear Protection

Face Mask

Safety Screen

5

The following 5 pages contain a CLEAPSS® guidance leaflet on making the most effective use of CLEAPSS® resources and information about the CLEAPSS® helpline. We have reproduced them here for readers' convenience.

The CLEAPSS Science Resources web site and CD-ROM 1 The purpose of this guidance This guidance is intended for Science Consultants, Advisers and Inspectors or Health & Safety Advisers who may have meetings, eg, for a local technicians’ group or with science subject leaders or newly-qualified teachers as well as subject leaders, ie, heads of science, senior technicians and others who might lead schoolbased training. It presents some ideas for making the CLEAPSS Secondary Resource (on the CLEAPSS web site) and the CLEAPSS Science Publications CD-ROM better known and more effectively used. You will probably find it helpful to give the training a theme. Health and safety is used here as an example but you could develop other themes targeted at identified needs within the science department or local authority. When running courses, typically we find that fewer than half the teachers and technicians have made significant use of Secondary Resource, either on-line or on the CD, and many, teachers, especially, have never looked at it. On courses, when participants have had the opportunity to dip into it for 5 minutes on about 5 or 6 occasions during the day, there is great enthusiasm – and some frustration that they hadn’t encountered it earlier. The ideas presented here are some we have found effective but we would welcome further ideas so that we can improve this guidance. 2 Distribution and dissemination All the information that has, for several years, been on the CLEAPSS Science Publications CD-ROM is also available on our web site under the title Secondary Resource. The URL is www.cleapss.org.uk/secure/secondary/ or, alternatively, go to www.cleapss.org.uk then click the ‘Secondary schools & colleges’ button and then click the ‘Secondary Resource’ button. The password required for the online version is given in the spring term Bulletin and is also given on the CD-ROM - click on the ‘Password’ button. The online version has the advantage that we can update it when existing publications are amended or new publications are produced. Even though our resources for science are on our web site, for some time yet we will continue to update and distribute the CD every year. We send out bulk supplies to local authorities in late November or early December and ask for them to be distributed to every secondary school and college, and relevant special schools, PRUs, etc. Associate members (ie, most independent schools and post-16 colleges) receive theirs in the first week of st January, in the same envelope as the Spring Term Bulletin. The previous year’s CD stops working on 31 January. (Finding a way round this would be a breach of CLEAPSS copyright and those concerned would be using out-of-date health & safety information.) Every teacher and technician in the science department needs easy access to the Internet and the CD-ROM. We also strongly encourage schools to copy the CD-ROM onto stand-alone computers and teachers’ laptops, so they can have access to it at home1. It can also be put onto internal networks. Occasionally, there are complications in running it on some systems. If so, look at the FAQs on our web site and, if that does not help, e-mail [email protected]. You might want to discuss with participants how distribution works in your school or local authority. Does the CD arrive at the school in plenty of time? Does it go unlabelled into the school bag? Is it put in an addressed envelope? Where does it go to when it reaches the school? Is it copied quickly onto stand-alone computers and laptops? Is it copied onto the school network without delay? In one local authority, the science adviser has written to all head teachers and IT managers informing them that they will be personally responsible if an accident occurs as a result of science staff not having easy access to the CD. 3 Getting started It’s best if all participants can have a computer in front of them – one between two works quite well so they can help each other. We recommend that the most up-to-date version of Adobe Reader is installed on the computer. This can be downloaded from http://get.adobe.com/uk/reader/. Remove older versions of Adobe Reader before installing the new version. Ideally the presenter should have a data projector so that s/he can show on a screen what the participants should be looking at. Start the web version of Science Resource or CD-

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To do this, create a folder on the desktop and call it CLEAPSS. Copy all the files and folders from the CD-ROM into this - it will take about 5 minutes. Open the desktop CLEAPSS folder. Right click the index.htm file and select Create Shortcut. This will create Shortcut to index.htm. Drag this shortcut from the CLEAPSS folder to the desktop. Rename it ‘CLEAPSS Science 2009’ (or whatever year is relevant). Double clicking ‘CLEAPSS Science 2009’ will start the CD on your desktop.

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© CLEAPSS , The Gardiner Building, Brunel Science Park, Uxbridge UB8 3PQ Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk

ROM so that the search page is showing. When first run you may be asked if you want to allow active content to run from CD or in files on my computer. You must answer ‘yes’ in order for the search function to work. Point out that all CLEAPSS favourites are there - Hazcards, the Laboratory Handbook, the termly Bulletin (back to 1990), Recipe Cards, about 50 guides, about 70 guidance leaflets and address lists as well as the Student Safety Sheets. But, at this stage, don’t go through every button - some arise more naturally later on. 4 Using the Search facility Participants may be a bit daunted by the amount of information on the Science Resource; how can they find what they want? Secondary Resource starts with the search page, otherwise click the ‘Search’ button at the top left. Type into the window an item that you are searching for. For example, staff sometimes wonder if they can wedge open a heavy fire door to avoid risk of injury when carrying trays through it. Type in ‘wedge’, click on SEARCH and about 7 references appear. The top one is PS49 Fire Risk Assessments for School Laboratories. Click the link and the document opens into a new window. View this one and you can find the answer to the question. The search term, in this case ‘wedge’ should be highlighted within PS49. If not then type wedge into the ‘find’ text box in Adobe reader then hit the return key. To perform another search, return to the search page. Do not click the ‘New search’ button in Adobe Reader. Participants might then want a little time to explore briefly what the other references show up. Searching has to be done quite carefully. In order to find information about Van de Graaff generators, you have to be able to spell ‘Van de Graaff’ correctly! or you could just type in ‘van’. You can use the wildcard characters * and ? in your search terms to search for multiple words and return a larger set of results. An asterisk character in a search term represents any number of characters, while a question mark character represents any single character. This allows you to perform advanced searches such as therm*; which would return all pages containing words beginning with therm - such as thermit, thermite, thermometer, etc. *chlor* would search for any words containing the word chlor - for example, chlorine, chloroform, dichloromethane, etc. Similarly, ethan?l would return all pages containing seven letter words beginning with ‘ethan’ and ending with ‘l’ - ie, ethanal and ethanol. Searching for ‘locust?’ or ‘locust*’ should return results that contain the word ‘locust’ or ‘locusts’. Tips and a tutorial are also provided - click the ‘Tutorial’ button toward the bottom of the page. To return to the search page, close any open documents (or tabs) until you can see the search page again or reselect it from the taskbar (or tab list) or click the ‘search’ button again. 5 The e-documents Go back to the search page. Click the button labelled ‘Electronic documents’. These are files that only exist electronically; there are no paper versions and hence teachers and technicians may be unaware of them. Click on E232 Common safety signs and hazard symbols and click, say, on the Word file (E232.doc). Scroll through this document and virtually all the safety signs a school could want are shown there. In effect, this document is a catalogue. For printing purposes you should click the ‘All graphics’ link in the document. This will open a folder that contains separate black & white and colour graphic files. (Obviously you can print in black and white from a colour file, but you end up with shades of grey and white.) The folder also contains separate files for poster-size printing and small symbols suitable for including in worksheets. Some schools print off a poster-size version of the WEAR EYE PROTECTION symbol, laminate it and use Blu-Tack to put it on the wall on those occasions when the class is meant to be wearing eye protection. Ask if there are any further ideas for using the safety symbols. Close E232 and click instead on E229 Illustrations of Basic Laboratory Equipment. Again, click on the Word version (E229.doc). Scrolling through this document will reveal many useful diagrams in colour and black & white, as 3-dimensional pictures and as traditional line diagrams. Some schools have printed these off to provide attractive labels for cupboard doors; teachers can adjust their size and insert them into worksheets. In some schools for children with special needs, they have been used as flash cards. Ask the audience if they have any further ideas for using these equipment illustrations. Finally, close E229 and click on E252 CLEAPSS font. This time, if you click on CLEAPSS font.pdf you will find a whole range of symbols that can be typed providing you install the font into your font folder (instructions are provided.) So, rather than typing the characters QWERTYUIOP{ } or qwertyuiop[ ], instead you can type:

WERTIOP{} or wertuio[] 6 Guides A common problem in many schools is teachers or technicians whose qualifications and/or experience do not match what the department really requires. A school needs to assess their capabilities in order to identify where training has to be provided. On the search page, click on the ‘Guides’ button. If your audience is technicians, go to L234 Induction and Training of Science Technicians; if it is teachers go instead to L238 Health and Safety Induction and Training of Science Teachers. Both guides have a similar layout with the bulk of each guide comprising quite long checklists of the activities technicians (sections 4 and 5 of L234) or teachers (section 3 of L238) might carry out. In some schools, these lists have been printed out and given to new staff who have then been asked to carry out a self-assessment. Of course, the accuracy of that self-assessment needs to be at least partially checked by a more-experienced member of staff. However, the gaps in knowledge that are PS66 TPB 10/09 Page 2 of 4

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identified then define the training needed - and the checklists give references to where guidance can be found, mostly on the CD-ROM or web site. The problem is, these are fairly general lists, covering work in all three sciences up to A-level. No one teacher or technician would be expected to know all of the items listed. Also, some of the items will be irrelevant to some schools, perhaps because they don’t have a sixth form or don’t do some of the listed activities. Hence the lists need to be customised to the requirements of particular schools and/or individuals. The file you are looking at is a PDF (Portable Document Format) file and these are notoriously difficult to modify. We use PDF files deliberately - after all, we don’t want somebody to be able to delete the word “not” too easily. Go back to the search page and click on the ‘Customisable Documents’ button. There you will find a whole series of documents as Word files, including L234 and L238, so they can be easily modified to suit a school’s needs. Ask the audience which of the other customisable documents could be useful in the department. Also ask the audience to look at the titles of all the guides. Which ones are they familiar with? Which might be useful in the department? 7 Guidance leaflets and address lists Quite often schools phone CLEAPSS to ask for the contact details of Registered Waste Contractors or where they can find repair companies for electrical equipment. In fact, they already have the information on the Science Resource. Click on the ‘Guidance Leaflets’ button. Click on either PS5 Waste disposal contractors or PS41 Repair and service agents for electrical laboratory equipment. These are both address lists. The problem is that such lists rapidly become out of date. CLEAPSS updates such lists several times over a year so schools should check the on-line version of the Secondary Resource rather than that on the CD. Ask participants if they have ever used the CLEAPSS web site. Did they know that all the CD-ROM resources are now online? 8 The CLEAPSS Laboratory Handbook Technicians usually know quite a lot about the Laboratory Handbook but most teachers have rarely opened it. Yet there is a huge range of material of direct relevance to them. Go back to the search page, click on the ‘Laboratory Handbook’ button. This takes you to a list of the contents. (Each section is a separate file, so that sections can be printed one at a time). Ask the biologists to have a look at microorganisms (click on “15 Mainly Biology, K-Z”, then 15.2), the chemists to look at accepted practice when handling chemicals (click on “13 Mainly Chenistry”, then 13.1) and the physicists to look at high voltage (click on “12 Mainly Physics”, then 12.9). Technicians might want to look at manual handling (click on “3 Personal Safety”, then 3.7). Ask your audience if they knew all this information was there. Also ask them if they still use the paper version of the Laboratory Handbook . Is there a paper copy in the Prep Room? Fetch it and see when it was last updated by checking the Contents page of the Handbook. It will probably say “Updated Autumn 2001”, which was the last paper update we issued for free. However, the Handbook has been updated several times since then and the process will continue. Generally, we expect to issue a Handbook update on the CD every year. Go back to the search page and click on the ‘Updates’ button. There are various files here that allow schools to print off just the pages changed this year, so they can keep the Handbook (and other CLEAPSS publications) up to date. Of course, if you haven’t updated since 2001 it will probably be easier to start from scratch and print off the whole of the Handbook by going to the ‘Laboratory Handbook’ button on the search page. The ‘Laboratory Handbook’ button takes you to separate files for each of the sections of the Handbook, so you don’t have to print it all in one go. After clicking on the ‘Laboratory Handbook’ button it would also be worthwhile clicking on the ‘Contents & Preface’ link. Scroll down to page (iii) which shows the correct date of issue of every page, so that you can check just how out of date your paper copy of the Handbook actually is. Another option is to contact CLEAPSS and ask us to send the complete paper Handbook - but that will incur a cost (currently £25 + handling charge). It is important to emphasise that if the school has not updated the Handbook, it will be using out-ofdate information. A department could scrap the paper Handbook altogether and just go electronic. But if it decides to keep a paper version, it must keep it up to date – it could be dangerous not to do so. 9 Hazcards and Recipe Cards We intend to continue issuing Hazcards and Recipe Cards as A5-size cards. However, having them on-line or on the CD does offer some advantages. Teachers, for example, can take a copy of the file home and incorporate information onto worksheets. It is common practice for technicians to issue relevant Hazcards with a set of apparatus. These could now be printed so that the original card is kept safely in the prep room set. (Each Hazcard and each Recipe Card is a separate file, to make searching easy. However, if you want to print the whole lot it could take some time. In that case, click on the ‘Print versions’ button - these files are gathered together in single documents for easy printing – but note that these files are not covered by the search facility.) Some people are put off because they cannot find a card for the chemical they are interested in. This is often because individual cards can cover several chemicals. Therefore you need to use the (paper) index or the search facility. Ask the participants to find health and safety information about naphthalene or pyrogallol or perhaps technicians might be asked to find the recipe of solutions for the Belousov-Zhabotinski Reaction.

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10 What else is there on the CD-ROM? Many teachers are unaware of our ‘pupil-speak’ version of Hazcards, ie, Student Safety Sheets, which are intended to support the teaching of health and safety, especially at KS4 and post-16. They are likely to be particularly useful for Applied Science and similar courses. One of the problems when using the search facility is that you come up with too many references. Generally, those from Student Safety Sheets are least useful, because they are intended for pupils and may not give the detail teachers and technicians want. References to the Laboratory Handbook, Hazcards and Recipe Cards are usually the most useful although, for specialist topics, guides guidance leaflets may be most useful. It is also important to look at the date of a document that a search has found. For example, some of sections of the Handbook have not been updated for some time. Guidance leaflets such as PS 76, Electron-beam tubes and PS 80, How to use a model steam engine, are much more recent. Ask the participants to look at one or two of these leaflets and see how the advice compares with that in the Handbook. When the Handbook is updated, these leaflets may be withdrawn. Ask the audience to try typing a few things into the search engine. For example, try looking for information about class size, or soldering or locusts or dinitrophenylhydrazine. Ask them also to look at the list of titles of all the ‘Guidance Leaflets’. Which ones are they familiar with? Which might be useful in the department? 12 At the end Discuss with participants how the department should use the Secondary Resource on the CLEAPSS web site or CD in future. How can every member of the department have easy access to it? How will newcomers be informed about it? How will the next update be copied onto the network and individual computers? Which parts of the Secondary Resources (web site or CD) will individuals definitely use, soon? How will the confident encourage the less confident and increase the awareness and use of CLEAPSS information? How could the safety symbols and the diagrams of laboratory equipment be used? Will the CLEAPSS font be installed on all departmental computers? Are there other useful files in the ‘Electronic Documents’ list? If participants have difficulties with the Secondary Resource on the web site or CD in future, they could always try looking at the tutorial. In fact, that might be the last thing to try on the training session - and the first thing to try back at school.


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A

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Helpline

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Is it safe to .... ?

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How do I dispose of .... ?

Where can I buy .... ?

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What’s the recipe for .... ?

Why won’t / doesn’t .... work ?

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How can I .... ?

Which is the best .... ?

Is .... good value for money ?

•

•

How many technician hours.... ? Is it true we’ve got to ....?

The Helpline is for: ❏ teachers in nursery, primary, middle, secondary and special schools

❏ lecturers teaching science in colleges up to GCE A-level, or equivalent, and in initial teacher training ❏ technicians, headteachers and governors

❏ science advisers, inspectors, consultants and advisory teachers

❏ local-authority staff, eg, safety advisers and purchasing officers ❏ architects involved in laboratory design work

Use of the Helpline is free for any establishment in a local authority which is a member (currently, all of them). Independent schools and similar establishments, incorporated colleges and initial teacher-training institutions can become associate members (around 2000 are). The Helpline is especially for queries about practical work in science, ie, technicians, resources, laboratories, equipment, materials, chemicals, living organisms, storage and safety in teaching science, together with some aspects of technology. Many of the queries received on the Helpline can be answered by accessing information on the CLEAPSS Secondary Resource and D&T Resource online and on CD. These contain all CLEAPSS publications currently in print. The CD-ROMs have been issued to all member and associate secondary and FE establishments.

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Helpline S

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01895 251496

CLEAPSS®, The Gardiner Building, Brunel Science Park, Uxbridge UB8 3PQ Tel: 01895 251496 Fax: 01895 814372 E-mail: [email protected] Web site: www.cleapss.org.uk A consortium of local authorities supporting practical science and technology

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Experiments The experiments in this book have deliberately not been matched to any age of student, specification or Key Stage. This is because many of the experiments can be adapted to suit the class you are teaching. Some of the experiments have guidance on whether to use as a teacher demonstration or a class practical, again this can vary depending on the class you are teaching at a given time. If you are in doubt how you could incorporate any of the experiments into your teaching please discuss this with the course trainers who will be happy to help.

Apparatus

Technical notes 1 Cyclohexene and cyclohexane are flammable and irritant. 2 Bromine water is toxic and irritant. The concentration should not exceed 0.3% v/v. 3 0.001M potassium manganate (VII) solution is oxidising and harmful. This is a substitute for bromine water for student use.

Procedure HEALTH & SAFETY: Wear eye protection

Stage 1

•

a Grate the outer orange coloured rind of two oranges and add to 100 cm3 of distilled water in the 250 cm3 round bottomed flask. Add anti-bumping granules to the round bottomed flask. b Set up the distillation apparatus as shown in the apparatus section. c Heat the flask so that distillation proceeds at a steady rate, approximately one drop per second of distillate. (Note: Take care not to let the liquid in the round bottomed flask boil too strongly). d Collect approximately 50 cm3 of distillate in the measuring cylinder. The oil layer will be on the surface.

gauze

e Using a dropping pipette carefully remove the oil layer into a test tube for the next stage.

Stage 2

tripod

Odour

Bunsen burner

f

Cautiously smell the extracted oil by wafting the fumes towards the nose. Do not breathe in directly from the test tube.

Action of bromine water g Measure out approximately 1 cm3 of bromine water into each of three test tubes.

heat resistant mat

h Add a few drops of the limonene oil to one test tube, a few drops of cyclohexane to another, and a few drops of cyclohexene to the third. Place in the bungs and agitate. If the bromine water is decolourised the molecule contains double bonds. i

0.001M potassium manganate (VII) can be substituted for the bromine water for class use. However, students need to know the action of bromine water.

Extrac by steating limonene m distil f This expe lation rom oranges riment de monstrat The es

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Additional notes The amount of oil extracted varies considerably with the variety, season and storage of the oranges. However, it is always possible to extract sufficient. Do not distill more than 50% of the initial volume of water or solid “jam” will form in the flask which is difficult to remove. Always use a gauze on the tripod or the orange will burn.

Teaching notes Limonene (1-methyl-4-prop-1-en-2-yl-cyclohexene) is an unsaturated hydrocarbon, classed as a terpene. At room temperature it is a colourless oily liquid with the smell of oranges. Its molecular formula is C10H16 and its boiling point is 176 °C. H3C CH3 H2C

Limonene is a chiral molecule with two optical isomers (enantiomers). The major biological form d-limonene, the (R)-enantiomer, is used in food manufacture and medicines. It is also used as a fragrance in cleaning products, a botanical insecticide, and due to its flammability, a potential biofuel. The (S)-enantiomer, l-limonene, is also used as a fragrance but has a piney, turpentine odour. It is possible to allow students to observe the optical activity of chiral molecules by comparing saturated glucose solution with distilled water in a polarimeter.

Reference This experiment was written by Andrew Thompson on behalf of the RSC

44

11

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•

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12

1

Diffusion of gases – ammonia and hydrogen chloride Concentrated ammonia solution is placed on a bud in one end of a tube and concentrated hydrochloric acid on a bud at the other. After about a minute the gases diffuse far enough to meet and a ring of solid ammonium chloride is formed.

Lesson organisation This demonstration is best performed in a fume cupboard. A black background, such as a sheet of black sugar paper, behind the demonstration helps the white ring to be seen more clearly. Actually performing the demonstration takes only a few minutes.

Apparatus and chemicals For one demonstration: Eye protection (goggles) Access to a fume cupboard Protective gloves, preferably nitrile

•

•

A length of glass tube about half a metre long with an inside diameter of about 2 cm (see note 1) Retort stands with bosses and clamps, 2 Cotton wool buds, 2 Bungs with small hole to hold cotton buds Glass rod (optional) Strip of Universal indicator paper (optional) Concentrated hydrochloric acid (Corrosive), a few cm3 (see note 2) 880 ammonia solution (Corrosive, Dangerous for the environment), a few cm3

Technical notes Concentrated hydrochloric acid (Corrosive) Refer to CLEAPSS® Hazcard 47A. Produces hydrogen chloride gas (Toxic, Corrosive) Refer to CLEAPSS® Hazcard 49.

●

880 ammonia solution (Corrosive, Dangerous for the environment) Refer to CLEAPSS® Hazcard 6. Produces ammonia gas (Toxic) Refer to CLEAPSS® Hazcard 5.

●

1 It is very important that the tube is clean and completely dry for this experiment. If necessary, the tube can be dried by pushing a cotton wool pad soaked in propanone through the tube and leaving it for a few minutes. 2 The concentrated hydrochloric acid and the 880 ammonia solution are easier to handle in small bottles than in Winchesters (large bottles) for this demonstration. Care should be taken when opening the bottle of ammonia solution, particularly on hot days when pressure can build up in the bottle. If the bottle of ammonia is kept for a long time, its concentration may decrease which will lessen the effectiveness of the demonstration.

13

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Procedure HEALTH & SAFETY: The demonstrator should wear goggles and protective gloves.

cotton bud soaked in concentrated HCI (aq)

glass rod with damp UI paper strip attached

cotton bud soaked in concentrated ammonia (aq)

a Working in the fume cupboard, clamp the glass tube at either end, ensuring that it is horizontal. The use of the glass rod, with damp Universal Indicator attached, is optional. b Open the bottle of ammonia solution cautiously, pointing the bottle away from both you and the audience. Open the bottle of hydrochloric acid and hold the stopper near the mouth of the ammonia bottle. Note the white clouds of ammonium chloride that form. c Put the end of one of the cotton buds (held in the bung) into the ammonia solution. Push the bung into one end of the tube. Replace the lid on the bottle of ammonia. d Repeat this procedure quickly with the second bung/cotton bud and the hydrochloric acid. Put this into the other end of the tube. Replace the lid on the bottle of hydrochloric acid. e Watch the tube and observe a ring of white powder forming near the middle of the tube. This is ammonium chloride.

Teaching notes The reaction which is taking place is: ammonia + hydrogen chloride → ammonium chloride NH3 (g) + HCl (g) → NH4Cl (s) The exact time taken for the ring to form will depend on the dimensions of the tube, the amount of the solutions which are put on the cotton wool buds and the temperature of the room. The ring usually forms nearer to the hydrochloric acid end of the tube because hydrogen chloride diffuses more slowly than ammonia. This is because hydrogen chloride has almost twice the molecular mass of ammonia, and the rate of diffusion is inversely proportional to the square root of the molecular mass of the gas. It is worth noting that the rate of diffusion is not the same as the speed at which the gas molecules travel (which is hundreds of meters per second). The gas molecules follow a zigzag path through the tube as they collide with the molecules of the gases in the air that are present in the tube. The purpose of the glass tube is to eliminate air currents and to see if the gas molecules will move on their own.

Reference This experiment has been adapted from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/states-of-matter/diffusion-ofgases-ammonia-and-hydrogen-chloride,184,EX.html

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Health & Safety checked, April 2008 Updated Jan 2010

2

Handling liquid bromine and preparing bromine water Opening ampoules Wear heavy duty, chemically resistant (e.g. nitrile) gloves, Do NOT use disposable plastic gloves (refer to CLEAPSS® Guidance leaflet PS50). Wear suitable eye protection. Use a fume cupboard.

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Do not handle the ampoules for longer than necessary (heat from the hand will build up pressure). Make a scratch on the neck of the ampoule with a good glass knife and snap off the neck. For many purposes, the ampoule may be crushed where it is required.

Bromine water Wear heavy duty, chemically resistant (e.g. nitrile) gloves, Do NOT use disposable plastic gloves (refer to CLEAPSS® Guidance leaflet PS50). Wear suitable eye protection. Work in a fume cupboard.

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Add 0.5 ml of bromine to 100 ml of water. Or crush an ampoule under 200 ml of water and decant the liquid into a bottle. Refer to CLEAPSS® Hazcard 15B for alternative method. Bromine is VERY TOXIC, CORROSIVE and a Danger to the environment – refer to CLEAPSS® Hazcards 15A and 15B and CLEAPSS® Recipe Card 28. Wherever bromine liquid is used or stored, have 500 ml of 1 mol dm–3 (25 %) solution of sodium thiosulfate to hand. See CLEAPSS® Hazcard 15A.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/standard-techniques/handling-liquid-bromine-andpreparing-bromine-water,59,AR.html

Health & Safety checked, November 2007 Updated 3 Dec 2007

15

3

Diffusion of gases – a safer alternative to bromine Myth: Bromine is banned. It is too dangerous to handle.

Diffusion into air Bromine (VERY TOXIC and CORROSIVE) (see Hazcard 15) has caused a number of accidents to both teachers and technicians, with serious burns to the skin or breathing difficulties. Bromine is not banned but if it is to be used, a knowledgeable colleague should be in the vicinity to provide assistance in case of an accident. Any person handling bromine for the first time, or who does not handle it regularly, should receive training from an experienced colleague. Nitrogen dioxide (VERY TOXIC and CORROSIVE) is a heavy, brown gas. Despite the similar hazard warnings, there is a lower risk of serious injury with nitrogen dioxide than bromine and the gas provides a safer alternative. A known volume of concentrated nitric acid (CORROSIVE) is added to an excess of copper turnings to produce enough nitrogen dioxide so that a gas jar of known volume is nearly filled. Another gas jar of air is placed over a gas jar of nitrogen dioxide. Over the next 20 minutes, the brown gas diffuses into the upper jar.

Procedure

•• clamp

copper turnings

Fig i

Fig ii

Fig iii

a Using water and a 250 ml measuring cylinder, establish the volume of the gas jar. Do not use this wet gas jar for the following demonstration. b Using a retort stand, boss and clamp, adjust the fitting of a dry inverted gas jar over another dry gas jar of the same size and set it to one side. c Place at least 1 g, but no more than 2 g, of copper turnings in the gas jar (fig i). Knowing that 8 ml of concentrated nitric acid produces 1000 cm3 of nitrogen dioxide at room temperature and pressure, estimate the volume of acid needed to just fill the gas jar with gas. Wearing eye protection and suitable gloves, place 1 ml less than the estimated volume of nitric acid (CORROSIVE and OXIDISING AGENT) in a 10 ml measuring cylinder. Empty the contents of the measuring cylinder into the gas jar with copper and watch the brown gas rise (fig ii).

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d Once the reaction stops, invert the second jar over the jar containing the gas. Clamp this jar into position with care (fig iii). Diffusion takes place in 20 minutes.

Controls and hints If the above procedure is followed, a fume cupboard is not required because nitrogen dioxide, being heavier than air, remains in the gas jar. Gloves are not required when an automatic pipettor is used.

Disposal If possible, move the gas jars to a fume cupboard. Add water to each gas jar and pour the contents down a foul-water drain, adding more water. Unreacted copper turnings can be dried and reused. If there is no fume cupboard in the room, carefully insert gas-jar lids to cover both jars. Seal with sellotape and remove to a fume cupboard.

Extension The demonstration can be performed along with a similar set up using bromine to show that gases diffuse at different rates. To fill a 1 litre gas jar, use no more than 2 ml of liquid bromine. Adjust the volume of bromine liquid to the capacity of the gas jar that is available. It takes time for bromine to vaporise. Use a fume cupboard, wear goggles or a face shield and nitrile or latex chemical-resistant gloves. A bucket of 1 M sodium thiosulfate solution should be available in case bromine splashes onto the skin or is spilled.

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Reference This experiment has been reproduced from CLEAPSS®, L195 Safer chemicals, safer reactions p.28 with permission from CLEAPSS®.

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4

Diffusion in liquids This class practical shows that diffusion takes place in liquids. Students place colourless crystals of lead nitrate and potassium iodide at opposite sides of a petri dish of deionised water. As they dissolve and diffuse towards each other they form clouds of yellow lead iodide.

Lesson organisation This practical activity takes around 30 minutes.

Apparatus and chemicals Per pair or group of students: Petri dish Forceps White tile or piece of white paper Lead nitrate (Toxic, Dangerous for the environment), 1 crystal Potassium iodide (Low hazard), 1 crystal Deionised water

Technical notes Lead nitrate (Toxic, Dangerous for the environment) Refer to CLEAPSS® Hazcard 57A Potassium iodide (Low hazard) Refer to CLEAPSS® Hazcard 47B

Procedure

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HEALTH & SAFETY: Wear eye protection a Place a petri dish on a white tile or piece of white paper. Fill it nearly to the top with deionised water. b Using forceps, place a crystal of lead nitrate at one side of the petri dish and a crystal of potassium iodide at the other. c Observe as the crystals begin to dissolve and a new compound is formed between them. potassium iodide crystal lead nitrate crystal petri dish

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Teaching notes The lead nitrate and potassium iodide each dissolve and begin to diffuse through the water. When the lead ions and iodide ions meet they react to form solid yellow lead iodide which precipitates out of solution. lead nitrate + potassium iodide → lead iodide + potassium nitrate Pb2+ (aq) + 2I- (aq) → PbI2 (s) The precipitate does not form exactly between the two crystals. This is because the lead ion is heavier and diffuses more slowly through the liquid than the iodide ion.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/diffusion-in-liquids,185,EX.html

Health & Safety checked, April 2008 Updated 29 Oct 2008

19

5

Allotropes of sulfur Sulfur is heated slowly and steadily from room temperature, so that all the changes in colour and consistency as it melts and eventually reaches boiling point, can be observed. A fresh sample of sulfur is heated to just above the melting point, then allowed to cool and crystallise slowly as monoclinic sulfur. A further sample is heated to boiling point, and the liquid rapidly chilled in cold water to form plastic sulfur. A separate sample of sulfur is dissolved in a warm solvent, and the solution allowed to cool and evaporate, leaving crystals of rhombic sulfur. All the observed changes in properties can be related to the different molecular structures of the three solid forms of sulfur, and to the changes in structure as the temperature of the liquid changes.

Lesson organisation This practical is described here as a demonstration. However, some teachers may wish to consider whether certain parts could be used as class practicals with appropriately skilful and reliable classes. A demonstration, without any accompanying discussion about the possible reasons for the changes in properties in terms of structure, would take up to 45 minutes. However, to derive maximum benefit from the experiment, more time needs to be allowed for such discussion.

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Apparatus and chemicals The teacher will require: Eye protection Heat resistant gloves Access to a fume cupboard Flexicam or similar camera, digital microscope, digital projector and screen or other method of projecting images of small crystals to the class (as available). Test-tubes, 2 Test-tube holders, 2 Test-tube rack Beaker (250 cm3), 3 Beaker, 1 dm3 Thermometer, 0 – 250 °C Bunsen burner Heat resistant mats, 2 Filter paper, about 18 - 20 cm diameter Spatula Paper clips Heat resistant gloves Damp cloth (to extinguish small sulfur fires) Sulfur, powdered roll (100 g) Boiling tube Watchglass Hand lens Sulfur, powdered roll (Low hazard), 100 g Dimethylbenzene (Flammable, Harmful, Irritant), 5 cm3

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Technical notes Sulfur (Low hazard) Refer to CLEAPSS® Hazcard 96A The sulfur used must be roll sulfur, crushed to a powder. To crush the rolls of sulfur, place in a strong plastic bag on a hard surface. Use a hammer or a vice to break up the roll sulfur into small pieces, then crush to a powder in a mortar and pestle. ‘Flowers of sulfur’ is not suitable because it contains a lot of insoluble amorphous sulfur. During the experiments sulfur may catch fire, releasing sulfur dioxide (Toxic), which may cause breathing difficulties to some students. If this happens, extinguish quickly by placing a damp cloth over the mouth of the test-tube. If the combustion cannot be extinguished quickly, the test-tube should be placed in fume cupboard, and the fan left running. The preparation of the saturated solution of powdered roll sulphur in dimethylbenzene at 40 °C must be undertaken using a waterbath of warm water in a fume cupboard.

Procedure HEALTH & SAFETY: Wear eye protection. Work in a fume cupboard.

1. Plastic sulfur

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Work in a fume cupboard a Half fill the 250 cm3 beaker with cold water. b Half fill a test-tube with powdered roll sulfur and heat gently. The sulfur will melt to a transparent, amber, mobile liquid. c Continue to heat the molten sulfur gently over a small Bunsen flame, keeping the contents moving to prevent local overheating. The liquid gets darker and, fairly suddenly, becomes a viscous, gel-like substance. This occurs at about 200 °C. d The tube can be inverted and the sulfur will remain in it. Show that the mobile liquid reforms on cooling. e Now heat the sulfur slowly and steadily beyond the gel-like stage. The sulfur liquefies again to a very dark red-brown liquid. Note that during this heating the sulfur may catch fire and sulfur dioxide will be produced. Have a heat resistant mat or damp cloth to hand to place over the mouth of the tube to extinguish the blue flames. f When the sulfur begins to boil (441 °C), pour the liquid sulfur in a slow stream into a beaker of cold water. A tangled mass of brown plastic sulfur will form. g Allow this to cool thoroughly. The inside of the plastic sulfur may remain molten after the outside has solidified. h Remove the plastic sulfur from the water and show that it is rubbery – it can be stretched and will return to its original shape. i The shiny surface of the plastic sulfur begins to dull and some of the elasticity is lost within 30 minutes, as it begins to turn back to the more stable rhombic sulfur.

2. Crystals of sulfur Work in a fume cupboard a Prepare a filter paper cone (double layer) held together by a paper clip and supported in a 250 cm3 beaker. b Half fill a test-tube with powdered roll sulfur and heat gently. The sulfur will melt to a transparent, amber, mobile liquid. c Pour the molten sulfur into the filter paper cone. Allow the sulfur to cool slowly and solidify, forming a crust.

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d Break the crust with a spatula and, handling the filter paper cone with heat resistant gloves, tilt it so that any remaining liquid flows out of the cone of solidifying sulfur on to a piece of scrap paper or card (for disposal). Needle-shaped crystals of monoclinic sulfur will be seen inside the hollow cone. e When cool, the cone can be passed around. It may be necessary to break the cone open to see the crystals more easily. f Over the next day or two, look carefully at the needle crystals from time to time. They will slowly go cloudy, yet retain their needle shape, as the monoclinic form slowly turns back to the more stable rhombic sulfur – each needle becomes a mass of tiny rhombic crystals. paper clip molten sulfur

filter paper

beaker

3. Rhombic crystals of sulfur a Working in a fume cupboard measure approximately 5 cm3 of dimethylbenzene into a boiling tube. b Place the boiling tube into a 250 cm3 beaker approximately half full of warm water. c Once the dimethybenzene has reached 40 °C, using a spatula, add the powdered roll sulfur until a saturated solution is formed (no more will dissolve). d Decant the solution into a glass watch glass. e Small rhombic crystals will form as it cools. f Once the liquid has evaporated the crystals can be examined with a hand lens.

Teaching notes Very slow heating is essential if all of the changes on heating sulfur are to be seen clearly. Sulfur is a poor thermal conductor, hence the changes can overlap one another if the heating is too fast. Crystalline sulfur consists of puckered S8 rings in the shape of crowns. These can be packed together in two different ways – to form rhombic crystals and to form needle-shaped monoclinic crystals, as shown below: rhombic sulfur

monoclinic sulfur

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Below about 96 °C, rhombic sulfur is the more stable allotrope. On melting at about 118 °C, sulfur first forms a mobile, amber liquid containing S8 rings. If this is allowed to cool, monoclinic sulfur forms as crystallisation occurs above 96 °C. Monoclinic sulfur will turn slowly into the more stable rhombic form on standing below 96 °C. Further heating of the S8-containing liquid breaks the rings into S8 chains. These may join to form longer chains which tangle, causing an increase in viscosity. At higher temperatures, these chains break into shorter ones, perhaps as short as S2, and the viscosity decreases again. Rapid cooling of this liquid traps the resulting solid sulfur in the tangled chain state – this is plastic sulfur. On stretching, the chains uncoil and on releasing the tension they return to the partly coiled state. If solid sulfur is formed below 96 °C by crystallisation from a solution, the stable rhombic form is produced. S8 rings (packed in solid state)

HEAT

HEAT

S8 rings (mobile, amber liquid)

S8 chains HEAT

short chains of sulfur atoms (e.g. S4) in runny black liquid

HEAT

longer chains of sulfur atoms entangled in viscous liquid, and in plastic sulfur

Chart with Temperature and Composition 445 °C

Boiling Point

200 °C

Chains Shorten

180 °C

Brown and Viscous

160 °C

S8 breaks up, forms chains

113 °C

Melting Point


Health & Safety checked, December 2009

23

6

Making glass A sample of glass is made by heating a mixture of lead oxide, zinc oxide and boric acid strongly until it melts. The glass formed can be coloured by adding traces of various transition metal oxides.

Lesson organisation This class experiment demands careful manipulation of very hot apparatus by students. Teachers will need to be satisfied that a class is capable of doing so in a safe and orderly manner before using this experiment. The experiment itself may take up to 60 minutes, given the need for careful handling and weighing of toxic and harmful metal oxides, careful heating of the crucible to a high temperature with stirring of the contents, and finally adding a trace of a transition metal oxide to the melt with continuous stirring.

Apparatus and chemicals

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Each working group will require: Eye protection Access to top-pan balance (±0.1 g) (see note 2) Boiling tube (see note 3) Rubber bung, to fit boiling tube Spatula Crucible, low (squat) form, approx 15 - 20 cm3 capacity, with lid (see note 4) Crucible tongs Pipe clay triangle (see note 4) Bunsen burner Heat resistant mat Tripod Heat resistant mat Plastic weighing boats Paper clips, large enough to form a long stirrer wire when straightened. Boric acid (boracic acid) (Low Hazard) about 5 g Lead(II) oxide (Toxic, Dangerous for the environment), about 8 g Zinc oxide (Dangerous for the environment), about 1 g Copper(II) oxide (Harmful, Dangerous for the environment), trace Cobalt(II) oxide (Harmful), trace Manganese(IV) oxide (Harmful), trace Chromium(III) oxide (Low Hazard), trace

Technical notes Boric acid (Low Hazard) Refer to CLEAPSS® Hazcard 14 Lead(II) oxide (Toxic, Dangerous for the environment) Refer to CLEAPSS® Hazcard 56 Zinc oxide (Dangerous for the environment) Refer to CLEAPSS® Hazcard 108B Copper(II) oxide (Harmful, Dangerous for the environment) Refer to CLEAPSS® Hazcard 26 Cobalt(II) oxide (*Harmful, Dangerous for the environment) Refer to CLEAPSS® Hazcard 25 Manganese(IV) oxide (Harmful) Refer to CLEAPSS® Hazcard 60 Chromium(III) oxide (Low Hazard) Refer to CLEAPSS® Hazcard 24

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1 This recipe for making a glass uses several toxic and harmful chemicals, so quantities for student access should be minimised as indicated above, and made available to each working group if possible in small snap-lid plastic sample pots or similar containers. Note also that some of the transition metal oxides used can be very expensive to buy, so it is worth keeping quantities small for this reason as well. Note that chromium(III) oxide must not be confused with chromium(VI) oxide, which is a very hazardous substance. Also, do not use nickel(II) oxide in this experiment. 2 The smooth progress of this experiment depends on each group having easy access to a top-pan balance weighing to +/- 0.1 g. Each group has to make three weighings, which may well take up to 5 minutes, so enough balances will be needed for three groups to each balance. In addition there is the issue of spillage and dust from some of the toxic oxides. It is therefore probably better if the mixture is made up in bulk in a fume cupboard in a large, self-sealing plastic bag. The mixture could then be distributed in small plastic bags. If these cannot be provided, then some or all of the three chemicals required may have to be supplied in pre-weighed amounts. Use disposable plastic weighing boats, if available. 3 A large, 150 x 25 mm, test-tube. 4 Crucibles: ceramic crucibles are best but are liable to breakage in class use. Stainless steel crucibles, if available, are more durable, but should be absolutely clean to avoid contamination by any metal oxides from previous use. The crucibles should sit securely in, not just resting on, the pipe-clay triangles, which should also be in good condition (not broken or bent out of shape) to avoid the risk of crucibles falling through or tipping over during the experiment. It is suggested that the crucibles should be dedicated to this experiment. 5 After use, the crucibles used to make this glass should be immersed in 1 mol dm-3 dilute nitric(V) acid (Corrosive) for cleaning. Allow the glass to dissolve, and dilute to 1 litre before pouring down a foul-water drain. The crucibles must be thoroughly dried before re-use.

Procedure HEALTH & SAFETY: Wear eye protection. Avoid inhaling lead oxide dust. Wash hands after handling lead compounds.

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a Weigh 6.5 g of lead(II) oxide, 3.5 g of boric acid and 0.5 g of zinc oxide into the boiling tube and stopper firmly, taking great care not to spill any of these chemicals in this process. b Insert the bung in the tube and shake the contents to ensure thorough mixing, and transfer into the crucible. c Straighten out a paper clip to form a wire stirrer, and stir the mixture again. d Place the lid on the crucible, and carefully into seat the crucible onto a pipe clay triangle on a tripod on a heat resistant mat. crucible pipe clay triangle

tripod Bunsen burner

heat resistant mat

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e Heat carefully at first, then strongly with a hot Bunsen flame, until the mixture becomes molten and runny. f Taking great care, remove the Bunsen flame from underneath the crucible, then use tongs to remove the lid and lift the crucible off the tripod. Pour one or two drops of the molten glass onto the heat resistant mat. Replace the crucible onto the tripod, and keep heating. g Allow the glass beads to cool on the mat for 5 minutes and then examine them. h Use the straightened paper clip to pick up a tiny speck of one of the metal oxides provided and stir this into the remaining molten mixture. Do not add too much powder or you will produce a very dark piece of glass. i Remove the Bunsen flame, and use tongs to pour out one or two drops of the coloured glass from the crucible to form beads on the mat. Note the colour of the glass you have now produced. Place the crucible on the mat to cool. j Allow all the apparatus to cool before clearing away.

Teaching notes Very little background knowledge is required, and practical skills are more important in this experiment, including the use of balances and the handling of hot apparatus. The glass produced is very brittle and difficult to keep. This type of glass is not used commercially, but for further information about the composition of a wide range of glasses made industrially, see links below. Different groups can be allocated different transition metal oxides to produce coloured glass.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/elements-compounds-andmixtures/making-glass,267,EX.html

Useful resources A very informative introduction to the composition and manufacture of different glasses for different uses, together with a brief history of glass-making, can be found at: http://www.britglass.org.uk/AboutGlass/TypesofGlass.html#3 This link is to a school science club website whose students have carried out this experiment. It contains excellent photographs of the apparatus and of the glass made. http://www.st-johns.org.uk/sciclubweb/glass/glass.html (Websites accessed December 2009)

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Health & Safety checked, August 2008 Updated 29 Oct 2008

Chocolate and structure

7

The structure of a substance affects its properties, including melting point, and this is also true for chocolate. When chocolate is melted and re-hardened it sets into a different structure, which gives it a different taste, texture and melting point.

Lesson organisation There are two parts to this activity; taste tests and melting point tests. For the taste tests, students must not be in a laboratory as they are going to eat. They should be given two pieces of milk chocolate, one from an ordinary chocolate bar and one from a bar of chocolate that has previously been melted and quickly re-set. This should not be a blind trial; they should know which is which. Emphasise to students that the chemical composition is unchanged as the chocolate is heated and cooled within the wrapper. Once they have eaten the chocolate, they can go into the laboratory. They should be warned not to eat any more of the chocolate which is used during the lesson. For difficult classes it is probably best to have the chocolate broken up into individual pieces prior to the lesson. For students who you can trust to do so sensibly, it is better to have them break up the chocolate themselves so that they can get a better idea about its ‘snap’. The melting point tests can then be done, graphs drawn, and students given information about the melting points of the various polymorphs present in the chocolate (see Teaching notes) and asked to work out which form is present in each of their samples.

Apparatus and chemicals Each experiment requires Taste tests: At least two half squares of milk chocolate per student (some needs to be melted and rehardened first (see note 1) Melting point tests: Per pair or group Milk chocolate, 1 square Chocolate that has been melted and re-hardened (same type as above), 1 square (see note 1) Boiling tubes, 2 Beaker (250 cm3) (or similar sized container) Thermometer (0 - 100 °C) Timer Access to: Kettle (for boiling water)

Technical notes 1 For pre-melted chocolate, take a whole chocolate bar – the ones that are fully wrapped in one sealed wrapper are best. Dairy milk generally works well. Put it somewhere warm, such as on a central heating radiator, to melt it. Once it has melted, put it into a refrigerator – not one where chemicals are stored – to harden quickly. Remove once it is set and have at room temperature prior to the lesson. The remaining chocolate should be of the same make and type but simply stored at room temperature. 2 For the melting point tests it is easiest if the chocolate is already broken up into pieces small enough to fit in the boiling tubes and placed in labelled containers.

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Procedure Taste tests a Take two pieces of chocolate, one that has been melted and re-hardened and one that has not. Note the differences between the two. Try snapping the pieces and see what happens. Then eat the two pieces separately and note any differences in taste and texture.

Melting point tests a Put some hot water (at no more than 50 °C) into the beaker. b Place a few small pieces of chocolate (enough to cover the bulb of a thermometer when melted) into a boiling tube and put in a thermometer. c Take the temperature of the chocolate, then put the boiling tube into the hot water and start the timer. d Stir continuously with the thermometer and record the temperature of the chocolate every 15–30 seconds for about five mins. Note any other changes. A results table is useful here. e Draw a graph of each set of results and use them to decide the melting point of the samples and if the samples have the same structure.

Teaching notes This activity is a good introduction to how structure can make a big difference to the properties of a substance. It’s also fun to do, interesting, and the taste tests are always popular! The taste of the chocolate is partly determined by the recipe used in making the product but there is more to it than that. This is because the taste of chocolate depends on the microscale structure of the chocolate. Chocolate is made up of tiny particles and crystals ranging from 0.01 to 0.1 mm in diameter. These govern how the chocolate is perceived by the consumer. To register taste, flavour compounds have to reach the mouth and the nose, but the texture is important too. The overall flavour is a result of both chemical make-up and also how the material melts and breaks up in the mouth. Chocolate is a mixture of many chemical compounds of which about 400 have been identified. Taste, texture, gloss, 'snap' and other properties can be varied according to how the mixture is processed. Manufacturing chocolate is a very complex multi-step process. Making a chocolate bar begins with mixing the ingredients and grinding them to give a mixture of correctly sized particles. Size is critical to the 'mouth feel' of the product, and is typically about 0.02 mm. The next stage is known as 'conching' and involves removing volatile compounds and adjusting moisture content and viscosity. This gives the end product its desired flavour. The mixture is melted, sheared (stirred) and cooled in a complex process known as tempering. The temperature and shearing have to be very carefully controlled or the chocolate ends up brittle, crumbly and tasting different. This experiment models the process. A key ingredient is cocoa butter. It is a fat and it can come in at least six different crystalline forms. This means that the atoms are the same but they are arranged differently. The different arrangements can lead to different properties including melting point, how easily it snaps, strength, glossiness and texture. It’s a bit like lego bricks. You can use the same bricks to make different structures; some are stronger, and some look better.

28

The ability of the structure to take on many different crystalline forms is called polymorphism. ('poly' means many; 'morph' means shape). The details of the polymorphism of chocolate are very complex and this is still an area of active research. One of the six polymorphs – form V – has a far superior taste and texture than the others. It is also the glossiest and snaps well. The table below shows some of the characteristics of different cocoa butter polymorphs. Cocoa butter polymorphs Polymorph Conditions to make the polymorph

Melting point (°C)

Form I

Rapidly cooling molten chocolate

17.3

Form II

Cooling the molten chocolate at 2 °C

23.3

Form III

Solidifying the molten chocolate at 5-10 °C or storing Form II at 5-10 °C

25.5

Form IV

Solidifying the molten chocolate at 16-21 °C or storing Form III at 16-21 °C

27.3

Form V

Solidifying the molten chocolate while stirring. Needs a special process called 'tempering'

33.8

Form VI

Storing Form V for four months at room temperature

36.3

Using their melting point graphs students should be able to work out which forms or polymorphs are present in their two samples, given the melting points of the polymorphs. The ordinary sample usually has a melting point of around 33–35 °C, showing that form V is probably present. This chocolate has a good ‘mouth feel’ and students may notice a cooling effect on eating it as melting is an endothermic process. The melted and re-hardened sample melts at a much lower temperature and is probably form II or III. The chocolate often tastes stronger in this sample, but it does not snap so well and has a less smooth texture. The cooling effect in the mouth is less pronounced. This is a genuine experiment as the results are difficult to predict exactly. As can be seen from the table above, the cocoa butter polymorphs will readily change from one form to another at the sort of temperatures experienced by chocolate. The plots which students generate from this may therefore be rather ragged and are rarely as clear to read as those which they may get from stearic acid, for example. In spite of this, the practical is well worth doing as genuine research is always interesting. There is almost always a clear difference between the plots and students enjoy studying a 'real life' example of chemistry.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/structure-and-bonding/chocolateand-structure,122,EX.html

Useful resource Inspirational chemistry on Learnnet has more information about the structure of chocolate. www.chemsoc.org/networks/learnnet/inspirchem.htm (Website accessed December 2009)

Health & Safety checked, April 2008 Updated 30 Apr 2008

29

8

Experiments with hydrogels – hair gel and disposable nappies In this pair of activities students investigate hydrogels – polymeric smart materials. They are found in many commonly available products including disposable nappies and cheap hair gel. The practical work is fun to do and the results are sudden and dramatic.

Lesson organisation To do all the practical work takes about 30 mins. The hair gel experiment is a good quick introduction to hydrogels, while the nappy experiment is more detailed. If time is available, it is worth considering combining this experiment with another experiment with hydrogels, using plant water crystals. See experiment 9: Experiments with hydrogels – plant water storage crystals. It is a good idea to ask students to make detailed observations of each part of the experiment.

•

Apparatus and chemicals Eye protection Hair gel Each working group requires: Hair gel (see note 1) Salt Petri dish or lid Teaspoon or similar – an ordinary spatula is a bit small Disposable nappies Each working group requires: A disposable nappy (see note 2) Scissors A large ice cream tub or similar container (see note 3) Dessert spoon or similar measure Stirring rod Large beaker or plastic tub to hold at least 600 cm3 Plastic gloves for those with sensitive skin Access to: Distilled water, about 500 cm3 per group (see note 4) Salt

Technical notes 1 For the hair gel the cheaper and nastier the better. Allow about one large teaspoonful per group. 2 Pampers Baby Dry® nappies work well, but any ultra absorbent disposables should be fine. As an alternative to using nappies and extracting the hydrogel, it is possible to order sodium polyacrylate (Low hazard) from Sigma Aldrich.

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3 The ice cream tub is for collecting the inside of the nappy and is safer than collecting it over newspaper or similar. If tubs are in short supply, large zip-lock bags can be used. Students put the nappy in the bag, zip it up and manipulate it until all the hydrogel is extracted and then proceed as per the directions. 4 If distilled water is not available, tap water can be used but the results are not as spectacular.

Procedure

•

HEALTH & SAFETY: Wear eye protection

Hair gel a Put a blob of hair gel onto the petri dish lid. A large teaspoonful is fine. b Gently sprinkle salt from a spatula over the hair gel.

Disposable nappy a Cut the middle section out of the nappy – the thicker piece that is designed to absorb the urine. Discard the other piece. b Make sure the ice cream container is completely dry - wipe it with a paper towel if necessary. Any moisture in the tub stops the experiment from working properly. c Wear eye protection for the next step. Put the centre piece of the nappy into the ice cream container and gently take it apart. Small white grains should start coming away and this is what you are trying to collect. Keep gently pulling the nappy apart until you have collected as many of the grains as you can. Do not do this roughly or you will lose your product and put a lot of dust and fluff into the air. Avoid breathing in any of the dust. d Remove and dispose of all the fluff and other parts of the nappy, keeping the grains in the bottom of the tub. They are heavier and fall to the bottom, which makes it easier to separate them out. e Estimate the volume of the grains. f Pour them into the large beaker and add about 100 cm3 of distilled water. Stir. Keep adding distilled water until no more can be absorbed and stir between each addition. Estimate the final volume of the hydrogel. g Add a dessert spoonful of salt and stir.

Teaching notes This activity can be used to enhance the teaching of ionic and covalent bonding, or hydrogels can be considered as an interesting polymer as well as an example of a smart material. Hydrogels are smart materials because they change shape when there is a change in their environment – in this case it is the change in the concentration of ions. Students need to have some knowledge and understanding of ionic and covalent bonding, reversible reactions, and acids and bases to understand what is happening. Hydrogels are polymers that can retain many times their own weight in water. They are often polymers of carboxylic acids that ionise in water, leaving the polymer with several negative charges down its length. This has two effects. First, the negative charges repel each other and the polymer is forced to expand. Secondly, polar water molecules are attracted to the negative charges. This increases the viscosity of the resulting mixture still further as the polymer chain now takes up more space and resists the flow of the solvent molecules around it.

n O

+ H2O O

n –

O

+ H3O+ O

H

31

The polymer is in equilibrium with the water around it, but that equilibrium can be disturbed in a number of ways. If the the ionic concentration of the solution is increased – eg by adding salt – the positive ions attach themselves to the negative sites on the polymer, effectively neutralising the charges. This causes the polymer to collapse in on itself again. Adding alkali removes the acid ions and moves the equilibrium to the right; adding acid has the opposite effect. There are a large number of hydrogels and they are sensitive to different pHs, temperatures and ionic concentrations. By using a mix of monomers to create the polymer these characteristics can be fine-tuned. The hydrogels that are commonly available and are used in this practical activity are sensitive to salt concentration, but do not show much change across the pH range that can be readily investigated in the classroom. However, they do lend themselves very well to a range of investigative practical work. For example, their volume in different amounts of water or in different salt concentrations can be measured. For this type of investigation it is best to use either plant water crystals or to order sodium polyacrylate from Sigma Aldrich – this has a smaller crystal size and gives faster results. Students should make detailed notes on their experiments, noting changes in volume, colour and any other observations. Some expected observations could include: Hair gel The hair gel shrinks in size very quickly when the salt is added. After a couple of minutes all that is left is some liquid in the petri dish. Disposable nappy About 10 cm3 of hydrogel can be extracted from the nappy core. (Exactly how much depends on the make and the size of the nappy.) The hydrogel swells up extremely quickly (much quicker than with plant water storage crystals). It absorbs about 500 cm3 of distilled water giving a very viscous mixture. When salt is added, the viscosity immediately reduces and the mixture is easier to stir. The hydrogel releases the water and settles on the bottom of the beaker.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/structure-and-bonding/ experiments-with-hydrogels-hair-gel-and-disposable-nappies,143,EX.html

Useful resource Inspirational chemistry on Learnnet has more information about hydrogels. www.chemsoc.org/networks/learnnet/inspirchem.htm (Website accessed December 2009)

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Health & Safety checked, February 2008 Updated 29 Oct 2008

Experiments with hydrogels – plant water storage crystals

9

In this activity students investigate plant water storage crystals, a product that contains hydrogels – polymeric smart materials. The practical work is fun to do, and the results are clear and easy to see.

Lesson organisation To complete all parts of this experiment takes over an hour. If lessons are shorter than that then part 1 can be done in a prior lesson. The crystals keep for a few days if they are covered in water. During the time that the crystals have to be left, other experiments with hydrogels, using hair gel and disposable nappies, could be carried out. See experiment 8: Experiments with hydrogels – hair gel and disposable nappies. It is a good idea to ask students to make detailed observations of each part of the experiment. The water crystals can be coloured with a few drops of food colouring (for wonderful, lurid colours), with strong tea solution (which stains some containers but provides a useful model of a drug delivery system – see teaching notes) or not at all (which seems a bit of a shame as they look great when coloured.)

Apparatus and chemicals Eye protection Part 1 Each working group requires:

•

Large beaker or plastic tub (at least 1 dm3) – ice cream or similar tubs are fine Access to: Water crystals (see note 1) Either strong tea, 500 cm3, or a few drops of food colouring (optional) (see note 2) Part 2 Each working group requires: Beakers (250 cm3), 3 Dessert spoon or similar – plastic disposable spoons are fine and can be re-used White paper - to place under beakers to see what is happening more easily Stirring rods, 3 Petri dishes – lids not required, 2 Access to: Sieve (the plastic ones used for sifting flour are fine) or large funnel and either paper towels or filter paper – groups can share these Sodium chloride (table salt) solution, very concentrated or saturated, 200 cm3 per group Distilled water, 400 cm3 per group Sugar Sieve or tea strainer – if a funnel is used earlier, tea strainers are needed now or sieves can be used for both

33

Technical notes 1 The water crystals are available from garden centres and are sold under various names including Phostrogen Swellgel. Each group needs about a teaspoonful. 2 For the strong tea use two tea bags per litre, pour on boiling water and leave to brew overnight. This tea stains some containers. 3 If distilled water is not available, tap water can be used but the results are not as spectacular.

Procedure

•

HEALTH & SAFETY: Wear eye protection

Part 1 a Estimate the volume of the water crystals. b Put about 500 cm3 of tea, tap water or water coloured with a few drops of food colouring into the beaker or tub. Add one teaspoonful of water crystals, stir gently and leave on one side for at least half an hour, or overnight.

Part 2 a Sieve the water crystal mixture. It is best to do this over a large tub rather than the sink in case you drop it. Wash the gel crystals carefully once or twice in water to remove any excess tea or food colouring if you used it. Estimate the new volume of your crystals. b Stand the three 250 cm3 beakers on a piece of white paper. c Put two dessert spoons of the gel crystals into each beaker, estimate their volume and then add about 200 cm3 of salt solution to one and 200 cm3 of distilled water to each of the others. Add a spoonful of sugar to one of the beakers with water in it. Label the beakers. d Stir the mixtures gently – using a separate stirring rod for each one so that the solutions do not become cross-contaminated. Leave for 10–15 mins, stirring occasionally. e If you used tea, pour some of the solution from each beaker into a petri dish placed on the white paper. Use a tea strainer to prevent any crystals getting onto the petri dish. Note carefully the colour of each liquid. f Sieve the remaining mixtures, discarding the excess liquid and returning the crystals to the beakers. Estimate their new volumes.

34

Teaching notes This activity can be used to enhance the teaching of ionic and covalent bonding, or hydrogels can be considered as an interesting polymer as well as an example of a smart material. Hydrogels are smart materials because they change shape when there is a change in their environment – in this case it is the change in the concentration of ions. Students need to have some knowledge and understanding of ionic and covalent bonding, reversible reactions, and acids and bases to understand what is happening. Hydrogels are polymers that can retain many times their own weight in water. They are often polymers of carboxylic acids that ionise in water, leaving the polymer with several negative charges down its length. This has two effects. First, the negative charges repel each other and the polymer is forced to expand. Secondly, polar water molecules are attracted to the negative charges. This increases the viscosity of the resulting mixture still further as the polymer chain now takes up more space and resists the flow of the solvent molecules around it. The polymer is in equilibrium with the water around it, but that equilibrium can be disturbed in a number of ways. If the the ionic concentration of the solution is increased – eg by adding salt – the positive ions attach themselves to the negative sites on the polymer,

n O

+ H2O O

n –

O

+ H3O+ O

H

effectively neutralising the charges. This causes the polymer to collapse in on itself again. Adding alkali removes the acid ions and moves the equilibrium to the right; adding acid has the opposite effect. There are a large number of hydrogels and they are sensitive to different pHs, temperatures and ionic concentrations. By using a mix of monomers to create the polymer these characteristics can be fine-tuned. The hydrogels that are commonly available and are used in this practical activity are sensitive to salt concentration, but do not show much change across the pH range that can be readily investigated in the classroom. However, they do lend themselves very well to a range of investigative practical work. For example, their volume in different amounts of water or in different salt concentrations can be measured. For this type of investigation it is best to use either plant water crystals or to order sodium polyacrylate (Low hazard) from Sigma Aldrich – this has a smaller crystal size and gives faster results. Students should make detailed notes on their experiments, noting changes in volume, colour and any other observations. Some expected observations could include: The crystals swell up from about 5 cm3 to about 500–600 cm3. They take on the colour of the tea (or food colouring), showing that the tea has also been absorbed.

35

When distilled water is added to the hydrated crystals, they swell up further. The tea remains absorbed in the crystals and the water does not change colour. When salt water is added to the hydrated crystals, they begin to shrink and the water changes colour as the tea is released. It is possible to measure the aproximate size of individual pieces of the hydrogel too, and to show that the pieces have swollen or shrunk. The hydrated crystals in the sugar solution have the same volume as the ones in the distilled water. If they are left for up to 15 mins the tea is not released. (After this time, the water in the hydrated crystals is in equilibrium with the water in the beaker and some tea may begin to be observed.) These observations show that the hydrogel responds to changes in the ionic concentration of the solution – the salt, which is ionic, causes the hydrogel to collapse but the covalent sugar does not. Research is currently being done to see if it is possible to use hydrogels and similar materials as a drug delivery system – a way to get drugs and medicines to where they are required in the body. The experiment with tea and the hydrogel is a model of this type of drug delivery system. The drug is first loaded onto the carrier and then it is released at the right location. The tea represents the drug and the hydrogel is the carrier.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/structure-and-bonding/ experiments-with-hydrogels-plant-water-storage-crystals,170,EX.html

Useful resource Inspirational chemistry on Learnnet has more information about hydrogels. www.chemsoc.org/networks/learnnet/inspirchem.htm (Website accessed December 2009)

36


10

Cross-linking polymers – alginate worms Sodium alginate is a polymer which can be extracted from brown seaweed and kelps. It is one of the structural polymers that help to build the cell walls of these plants. It has some unusual properties and a wide variety of uses. The polymer can be represented like this: CO2– Na+

CO2– Na+

CO2– Na+

CO2– Na+

CO2– Na+

CO2– Na+

CO2– Na+

CO2– Na+

When sodium alginate is put into a solution of calcium ions, the calcium ions replace the sodium ions in the polymer. Each calcium ion can attach to two of the polymer strands. This is called cross-linking and can be represented like this: Ca2+

Ca2+ CO2–

CO2– CO2–

CO2– Ca2+

CO2–

CO2– Ca2+

CO2–

Ca2+

CO2– Ca2+

37

Collect approximately 5 cm3 sodium alginate suspension or Gaviscon® solution.

Questions 1 Describe the sodium alginate suspension or Gaviscon®. _______________________________________________________________________ 2 What is the formula of a. a sodium ion? _______________________________________________________________________ b. a calcium ion? _______________________________________________________________________ 3 Why can the calcium ion attach to two strands of the polymer, but the sodium ion to only one? _______________________________________________________________________ _______________________________________________________________________ 4 Predict how you think the properties of the polymer will change when it is poured into a solution of calcium ions. _______________________________________________________________________ _______________________________________________________________________


•

Eye protection Approx 5 cm3 sodium alginate suspension or Gaviscon® Dropping pipette 2 x 150 cm3 beakers Approx 100 cm3 sodium chloride solution Approx 100 cm3 calcium chloride solution Labels for the beakers

38

Procedure HEALTH & SAFETY: Wear eye protection a Put the calcium chloride solution into one of the beakers and the sodium chloride solution into the other. Label the beakers clearly.

•

b Using the pipette, squirt the sodium alginate or Gaviscon® into the calcium chloride solution. You are aiming to make ‘worms,’ although you can make beads if you prefer. c Remove a few of your worms straight away and put them into the beaker of sodium chloride solution. d Swirl both beakers gently and observe what happens to the worms in each one. You can remove and squeeze the worms as well as observing their appearance. You will need to wait a few minutes for all the reactions to be complete.

Questions 5 Describe how the polymer changed when it was poured into the calcium ion solution. Did this agree with what you predicted? _______________________________________________________________________ _______________________________________________________________________ 6 Describe what happens when the ‘worms’ are placed in sodium chloride solution. _______________________________________________________________________ _______________________________________________________________________ 7 Explain what happens in this experiment in terms of the ions and the polymer molecules. Use the term ‘cross-linking’ in your answer. _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________

Technical notes 1 Alginate is used in many applications and new ones are being found all the time. The uses range from applications in the food industry to wound dressings, medicines and dental impression materials. 2 Calcium alginate (the cross-linked polymer) is used in wound dressings. These dressings are particularly useful for slow healing wounds like leg ulcers, which can continue to bleed and weep for a long time. Part of the blood clotting mechanism involves calcium ions and on contact with blood the calcium alginate releases calcium ions in exchange for sodium ions – just as you observed in the experiment above. These extra calcium ions can help the blood to clot and encourage healing. It is easy to remove any excess calcium alginate when the dressing has to be changed.

39

Question 8 What could the wound be rinsed with to remove the excess calcium alginate?

_______________________________________________________________________

Technical note Alginate is a common food additive, E400. It is used as a thickener, stabiliser and gelling agent. It is often found in ice cream, where it is used to thicken the product so that even if it melts, it does not drip too much.

Question 9 Find five other foods that contain alginate. Try to think of a reason why it might be included in at least two of the products you have found. _______________________________________________________________________ _______________________________________________________________________

Reference This experiment has been reproduced from Inspirational Chemistry, Royal Society of Chemistry, London, Index 3.1.9

40


Extracting limonene from oranges by steam distillation

11

This experiment demonstrates the extraction of plant oils. The peel of oranges is boiled in water and the oil produced (limonene) distilled in steam at a temperature just below 100 °C, well below its normal boiling point. The immiscible oil can then be separated. Direct extraction by heating would result in decomposition whereas steam distillation does not destroy the chemicals involved. The experiment also links for tests for unsaturation, and at a higher level, chirality.

Lesson organisation This experiment can be conducted as a demonstration at secondary level as an introduction to some of the ideas about the extraction of plant oils. It can be used to stimulate discussion about the commercial extraction of plant oils – how science works. As described this demonstration will take a full lesson of approximately 50 minutes. This can also be conducted as a class practical at key stages 4 and 5. It can stimulate further discussions as to the process of steam distillation where oil with a boiling point of 176 °C is “distilled” at just under 100 °C. Limonene is an unsaturated hydrocarbon which can be tested for using bromine water or potassium manganate (VII). At a higher level, it is also a chiral compound and the experiment can lead to a discussion of optical enantiomers.

Apparatus and chemicals Eye protection Grater Bunsen burner Heat resistant mat Tripod and gauze Oranges, 2 110 °C thermometer Measuring cylinder (100 cm3) Measuring cylinder (50 cm3) Distillation apparatus 250 cm3 round bottomed flask Still head Thermometer pocket Condenser Receiver adapter Test tubes and bungs, 3 Dropping pipette Anti-bumping granules

•

Bromine water, no more than 0.2% v/v (Irritant) Potassium manganate(VII). 0.001M, (Oxidising, Harmful) Cyclohexene (Flammable, Irritant) Cyclohexane (Flammable, Irritant) Distilled water, 100 cm3.

41

Apparatus

gauze


heat resistant mat

42

Technical notes 1 Cyclohexene and cyclohexane are flammable and irritant. 2 Bromine water is toxic and irritant. The concentration should not exceed 0.3% v/v. 3 0.001M potassium manganate(VII) solution is oxidising and harmful. This is a substitute for bromine water for student use.


Stage 1

•

a Grate the outer orange coloured rind of two oranges and add to 100 cm3 of distilled water in the 250 cm3 round bottomed flask. Add anti-bumping granules to the round bottomed flask. b Set up the distillation apparatus as shown in the apparatus section. c Heat the flask so that distillation proceeds at a steady rate, approximately one drop per second of distillate. (Note: Take care not to let the liquid in the round bottomed flask boil too strongly). d Collect approximately 50 cm3 of distillate in the measuring cylinder. The oil layer will be on the surface. e Using a dropping pipette carefully remove the oil layer into a test tube for the next stage.

Stage 2 Odour f Cautiously smell the extracted oil by wafting the fumes towards the nose. Do not breathe in directly from the test tube. Action of bromine water g Measure out approximately 1 cm3 of bromine water into each of three test tubes. h Add a few drops of the limonene oil to one test tube, a few drops of cyclohexane to another, and a few drops of cyclohexene to the third. Place in the bungs and agitate. If the bromine water is decolourised the molecule contains double bonds. i 0.001M potassium manganate(VII) can be substituted for the bromine water for class use. However, students need to know the action of bromine water.

43

Additional notes The amount of oil extracted varies considerably with the variety, season and storage of the oranges. However, it is always possible to extract sufficient. Do not distill more than 50% of the initial volume of water or solid “jam” will form in the flask which is difficult to remove. Always use a gauze on the tripod or the orange will burn.

Teaching notes Limonene (1-methyl-4-prop-1-en-2-yl-cyclohexene) is an unsaturated hydrocarbon, classed as a terpene. At room temperature it is a colourless oily liquid with the smell of oranges. Its molecular formula is C10H16 and its boiling point is 176 °C. H3C CH3 H2C

Limonene is a chiral molecule with two optical isomers (enantiomers). The major biological form d-limonene, the (R)-enantiomer, is used in food manufacture and medicines. It is also used as a fragrance in cleaning products, a botanical insecticide, and due to its flammability, a potential biofuel. The (S)-enantiomer, l-limonene, is also used as a fragrance but has a piney, turpentine odour. It is possible to allow students to observe the optical activity of chiral molecules by comparing saturated glucose solution with distilled water in a polarimeter.


44


12

Generating, collecting, and testing gases Instructions about procedure for different gases Refer to CLEAPSS® Laboratory Handbook, section 13.2.2 Gases give rise to particular hazards so great care must be taken when preparing, collecting or testing. How the gas is to be used will differ from experiment to experiment – it is essential to read carefully the specific instructions given or referred to in the experiment details. This is especially important if the gas needs to be dried. Gases can be collected by upward or downward delivery or over water. Refer to specific information on each gas.

Gas preparation (general) The diagram below shows a typical set of apparatus which can be used to prepare a range of gases. liquid reagent

gas

tube must be below the level of the liquid

solid reagent

Gas collection methods

bee-hive shelf

downward delivery (upward displacement of air)

upward delivery (downward displacement of air)

over water

45

Gas Preparation (specific gases)

•(

)

HEALTH & SAFETY: Wear eye protection (and work in a fume cupboard when generating chlorine). Wear appropriate eye protection. The amounts given below are sufficient to generate 1 litre (1 dm3) of each of the named gases:

Carbon dioxide 42 cm3 of 2 mol dm–3 hydrochloric acid (Irritant) is slowly added to an excess of marble chips. Collect gas by downward delivery or over water (slightly soluble). Refer to CLEAPSS® Recipe Card 26 and CLEAPSS® Hazcards 20 and 47A.

Hydrogen 28 cm3 of 3 mol dm–3 hydrochloric acid (Corrosive) is slowly added to excess zinc granules and 1 g of hydrated copper sulphate (Harmful). Collect gas by upward delivery or over water. Refer to CLEAPSS® Recipe Card 26 and CLEAPSS® Hazcards 47A and 48. Hydrogen gas is extremely flammable – ensure there are no naked flames.

Oxygen 50 cm3 of 20 vol hydrogen peroxide (Irritant) is slowly added to manganese(IV) oxide powder (Harmful). Collect gas over water. Refer to CLEAPSS® Recipe Card 27 and CLEAPSS® Hazcards 50, 60 and 69. Oxygen is an oxidising agent.

Chlorine Work in a fume cupboard. 14 cm3 of concentrated hydrochloric acid (Corrosive) is added to at least 3 g of potassium manganate(VII) (Oxidising, Harmful, Danger to the environment). Double-check that the acid is hydrochloric and NOT sulfuric. Refer to CLEAPSS® Recipe card 26 and CLEAPSS® Hazcards 22A, 47A and 81. Alternatively 5 mol dm–3 hydrochloric acid (Irritant) is added to 30 cm3 of recently purchased (10–14% available chlorine) sodium chlorate(I) solution (Corrosive) with plenty of stirring. School samples often react too slowly because old sodium chlorate(I) is used. This will have less than the required 10% available chlorine (as it applies to both methods). Refer to CLEAPSS® Recipe Card 26 and CLEAPSS® Hazcards 22A, 47A and 89. Collect gas by downward delivery. Chlorine is classified as Toxic, Irritant and a Danger to the environment.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/standard-techniques/generating-collecting-and-testinggases,52,AR.html

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Health & Safety checked, November 2007 Updated 3 Dec 2007

An alternative to using compressed gas cylinders

13

Cylinders of hydrogen, oxygen and carbon dioxide are very expensive. Getting gas under pressure allows exciting demonstrations such as igniting balloons filled with hydrogen gas.

Introduction Comparing the density of gases and investigating Avogadro’s hypothesis is possible if balloons are filled with different gases. Igniting balloons filled with pure hydrogen gas is a very popular demonstration. Generating about 0.1 moles of common gases and filling balloons can be done with very basic equipment.

Lesson organisation Asking students how to fill a balloon with hydrogen gas to see what it is like to ignite it will probably generate some interesting ideas.

Apparatus and chemicals 2 L plastic fizzy drink bottle (empty) with top Glass trough or plastic washing-up bowl 2-hole bung to fit bottle fitted with glass delivery tubes 1-hole rubber bung fitted with glass delivery tubes Plastic or rubber tubing with adaptors Spring clips to close the rubber tubes Nichrome wire Side-arm flask, 250 cm3 Delivery tube Tap funnel with bung to fit side-arm flask. Hydrochloric Acid, 1 mol dm-3, 250 cm3 (Irritant, Refer to CLEAPSS® Hazcard 47A) Magnesium turnings, 4 g, (Highly flammable, Refer to CLEAPSS® Hazcard 59A) Clamp stand Boss head Clamp Cotton thread 1M wooden rule Splint Sticky tape Balloons (see note 3)

Technical notes 1 The description provided is for pressurising hydrogen gas, but the same method can be used for other gases. 2 Hydrogen is extremely flammable and explosive if mixed with oxygen. All flames should be extinguished during the preparation process. 3 If the balloon is inflated with air first to stretch it, it will fill more easily with hydrogen.

47

Procedure

••

HEALTH & SAFETY: Wear eye protection and ear protection Direct audience to cover their ears

Part 1

tap funnel

1 mol dm-3 HCI

hydrogen displaces water

fizzy drinks bottle

hydrogen bubbles Mg turnings

a Prepare the 2-hole bung with delivery tubes and clips as in the diagram. Wiring up the rubber tubes with nichrome wire will prevent them being pushed off under pressure. b Tape a splint to the 1M rule. c Half fill the trough with water. d Completely fill the bottle with water from the tap and screw on the top. e Invert the bottle over the trough and remove the top. (Water will come out and the bottle will deform at this stage. This does not matter; It will reform.) f Place the magnesium turnings in the flask, fit the delivery tube and put it under the water. g Fit the tap funnel and run hydrochloric acid to the flask until the level is close to the side-arm. h Allow the reaction to run for 30 seconds at least to flush air out of the flask and delivery tube. i Put the bottle over the delivery tube and fill it with hydrogen gas. The bottle will reform as the water is displaced with hydrogen gas. j When all the water has been displaced, keeping the neck of the bottle under water and fit the 2-hole bung. k Take the bottle out of the water and stand it on its base.

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Part 2

balloon

water from tap

gas forced out into balloon

fizzy drinks bottle

a Fit a balloon to the 1-hole bung. b Fit the other delivery tube to a laboratory water tap. c Slowly run water into the bottle and the balloon will inflate as the water displaces the hydrogen. d When the balloon has been filled with hydrogen gas, tie off the balloon. e Tie a cotton thread to the balloon and allow it to float in the air. f Remind audience to cover their ears. g Light the splint and ignite the balloon.

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Teaching notes A balloon filled with pure hydrogen will rise to the ceiling if not tied down. A large balloon may need 4 L of gas. (Small balloons will float with 2 L of hydrogen gas, larger ones may need more. More hydrogen can be forced into a balloon if more hydrogen is generated and collected in a larger container, eg a 4L milk bottle.) 4 dm3 of pure hydrogen in a balloon burns with a loud ‘woof’ and won’t damage ears, however some air may get into the balloon, so exercise caution. Caution This method could be used to fill a balloon with a stoichiometric (exactly reacting) mix of hydrogen and oxygen. The resulting explosion could have disastrous consequences to both people and property. See experiment 70: Hydrogen/oxygen explosion. Filling a balloon with oxygen gas and asking pupils what will happen when it is ignited and why often helps with misunderstandings. Many pupils predict an explosion ‘because oxygen is a reactive gas’. Filling balloons with equal volumes of carbon dioxide, oxygen, methane and hydrogen allows students to consider the density of gases.

Reference This experiment was written by Mike Thompson on behalf of the RSC

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Making a reaction tube

14

Many reactions between gases and solids are suitable for demonstrations and class practicals. Making reaction tubes is an excellent lesson in physical chemistry in its own right as well as being cheaper than buying in expensive material.

Introduction There are many reactions between gases and solids which illustrate chemical principles. Heating a solid in an atmosphere of the required gas can be carried out in a modified boiling tube or test tube. Heating a Pyrex test tube or boiling tube fitted with a bung, causes the pressure to rise inside and the glass to soften. Eventually the tube fails and a hole is made. Although this practical is not part of any Awarding Body specification, and although the purpose of making reaction tubes is for the reaction they will contain, the overall experience is so beneficial to students that most teachers say it is a practical worth doing in its own right.

Lesson organisation This experiment works well as a class demonstration, followed by a practical where students make their own. The demonstration takes about 5 minutes, the follow-on practical requires 15 minutes.

Apparatus and chemicals Pyrex test tube (or boiling tube) (must be a borosilicate tube, such as Pyrex) Bung to fit (see note 1) Bunsen burner Heat resistant mat Sharps bin required for broken glass. Dust pan and brush

Technical notes 1 The bung needs to be clean and undamaged and make a gas-tight seal on the tube 2 In step d of the procedure, if the glass balloons out into a large fragile globe this could pose a hazard. Students should be told to call over a teacher if this happens. 3 You must make sure there is no broken glass on the benches to harm the pupils coming in for the next lesson.

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•


The demonstration

a The bung is pushed very firmly into the tube. b Place the Bunsen burner on the mat , light it and adjust it to a roaring blue flame. c Hold the tube by hand around the top of the tube and hold the tube in the hottest part of the flame as shown in the diagram. Heat a single spot. Do not rotate the tube. d The glass will start to glow red and will begin to deform and then ‘pop’. It frequently blows the Bunsen burner flame out. Depending on the Bunsen burner, this usually takes between 30 seconds and two minutes. e Relight the Bunsen if necessary and heat the glass around the hole for a few seconds. The sharp glass will retract to form a neat edged hole. f Put the tube down on the mat to cool.

The practical Students repeat the process

Teaching notes If students are asked about what is happening to the gas inside the tube while it is heated, many say ‘It is expanding’ without thinking that it cannot expand inside a sealed tube. Others will say that the atoms are expanding. It makes an interesting discussion. Eventually they say the pressure increases. Students may be surprised by the pressure inside the tube, which is of the order of 2 - 3 atmospheres. Asking students about what is happening to the glass and why it behaves as it does is also informative. Hot and cold toffee makes quite a good analogy. Students may be surprised that the tube can be held by hand and that glass is a poor conductor of heat along the length of the tube. These discoveries are lost if the tube is held with a clamp stand.


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Ammonia fountain

15

This demonstration experiment can be used to illustrate the very high solubility in water of ammonia. Ammonia solution is seen to be alkaline and various indicator colour changes can be demonstrated. A flask, fitted with a glass jet, is filled with dry ammonia. Injecting water into the flask dissolves the ammonia and causes a fountain via the jet.

Lesson organisation The demonstration itself takes only a few minutes but needs careful advance preparation. With co-operative classes some of the preparation can be woven into the lesson so that the demonstration becomes the focal point.

Apparatus and chemicals Eye protection Access to a fume cupboard

•

For the fountain demonstration itself: Flask, 1 dm3 or 2 dm3 which is completely dry (see note 1) Rubber stopper, plain, to fit the flask Two-holed rubber stopper to fit the flask fitted with a glass jet (see note 2) Plastic syringe (10 cm3) Trough or large beaker which can hold more water than the flask Stand, clamp and boss along with a heavy weight or bench clamp to avoid apparatus toppling over White board or sheet of card to act as a background to improve visibility Acid-alkali indicator (e.g. Universal Indicator) For preparing the dry ammonia: Access to a fume cupboard Stand, clamp and boss Boiling-tube fitted with a one-holed rubber stopper holding a drying tube One-holed rubber stopper to fit the drying tube, fitted with a short length of glass delivery tube Length of rubber delivery tube Bunsen burner Heat resistant mat Ammonium chloride (Harmful), 5 g Calcium hydroxide (Irritant), 5 g Small lumps of calcium oxide (Irritant) as drying agent - sufficient for the drying tube (see note 4)

Technical notes Ammonium chloride (Harmful) Refer to CLEAPSS® Hazcard 9A Calcium hydroxide (Irritant) Refer to CLEAPSS® Hazcard 18 Calcium oxide (Irritant) Refer to CLEAPSS® Hazcard 18 Ammonia gas (Toxic, Dangerous for the environment) Refer to CLEAPSS® Hazcard 5.

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1 Use a round-bottom borosilicate flask or a thick-walled Buchner flask for this demonstration (other flasks may implode). Check carefully that the flask has no small cracks. Ensure that the flask is clean and scrupulously dry. The best way to achieve this is to put it in a glassware drying cabinet (or alternative) for an hour or so before it is required. Take it out and stopper it (with a dry stopper!) just before it is filled with ammonia. Even slight dampness will result in failure of the demonstration. 2 Take the two-holed stopper that fits the flask and insert the glass tube, which has been drawn out into a jet, through one of the holes. The glass tube must fit tightly into the rubber stopper – take great care. Insert a cork borer slightly larger than the diameter of the tube through the hole. Insert the tube, and reverse the cork borer. A 1 cm3 graduated pipette could be used as the glass jet. The tip of the jet should be positioned so that it is in the centre of the flask when the stopper is in place. About 20 cm of tube should protrude out of the flask. See second diagram. 3 Select a syringe which will fit tightly into the second hole of the two-holed stopper. 4 The drying agent, calcium oxide (corrosive) should be freshly prepared. 5 If this demonstration is to be repeated in future, it is worth making up a set of apparatus to keep for this specific purpose.

•

Procedure HEALTH & SAFETY: Wear eye-protection and work in a fume cupboard. Ammonia is Toxic and Dangerous for the environment.

Preparing dry ammonia and filling the flask This stage could form part of the demonstration. a Set up the boiling-tube and drying-tube as shown in the diagram.

fountain flask (1 or 2 dm3) drying tube

calcium oxide pellets

dry ammonia glass wool plugs

mixture of ammonium chloride and calcium hydroxide

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heat resistant mat

b Ensure that the flask is completely dry and clamp it in an inverted position. Arrange a delivery tube for upward delivery of the ammonia into the flask (ammonia is less dense than air). c Half-fill the drying tube with lumps of calcium oxide and half-fill the boiling tube with a solid mixture of calcium hydroxide and ammonium chloride. Care - the two solids begin to react immediately on mixing, and ammonia gas is evolved. d Gently warm the contents of the boiling tube for a few minutes with a small, blue, Bunsen flame. e Allow ammonia gas to pass through the drying tube into the upturned flask. The water vapour in the ammonia gas is removed by the calcium oxide. f Check that the flask is full of ammonia by holding barely damp Universal Indicator (or red Litmus) paper near the open neck of the flask and look for an alkaline indication. If in doubt, continue filling the flask for a little longer. g Stopper the flask with the plain, dry stopper. It is very important that no water comes into contact with the ammonia until all preparations are complete. h Fill the trough (or large beaker) with enough water to fill the fountain flask and add enough indicator to give an easily visible colour - this will be much more than is normally used for a titration. If preferred, add a little dilute acid so that the indicator starts in its acid colour.

The demonstration a Half-fill the syringe with water, dry the nozzle and carefully fit it into the second hole of the two-holed stopper (see diagram).

jet dry gas

water syringe (10 cm3 or 20 cm3) atmospheric pressure

atmospheric pressure water and indicator trough

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b Remove the plain stopper from the inverted gas-filled flask and quickly fit the stopper which holds the jet and syringe. Be careful not to prematurely inject water from the syringe. c Clamp (or get an assistant to hold) the flask over the trough or beaker of water so that the protruding glass tube is well below the water level. If clamping, bear in mind that the flask will be heavy when filled with water so take care that it will not overbalance. d Use the syringe to squirt a few cm3 of water into the flask and gently swirl to dissolve some of the ammonia gas. e As the gas dissolves, a partial vacuum forms inside the flask and the external air pressure will force water up the tube and through the jet - forming a fountain (see first diagram). The ammonia gas dissolves in the water emerging from the jet and the indicator changes colour. f The fountain continues for some minutes, depending on the size of the flask and the width of the jet. When the fountain finishes, a bubble of gas remains. This is air and its volume gives an indication of how well the flask was originally filled.

Additional notes on procedure When doing the demonstration, ensure that the flask is securely clamped and that the flask and clamp stand assembly cannot topple over when the flask fills with liquid. If you acidify the water in the trough, use only a few drops of acid. If you use too much, the fountain will show the alkali colour of the indicator at first and then change to the acid colour when all the ammonia has been neutralised. (Alternatively you might consider letting this happen and making a point of it.)

Teaching notes This is the equation for the preparation of ammonia: 2NH4Cl(s) + Ca(OH)2(s) → 2NH3(g) + CaCl2(s) + 2H2O(g)

Why is ammonia solution alkaline? Ammonia dissolves freely in water (1 cm3 of water dissolves at least 800 cm3 of ammonia under normal laboratory conditions). It also reacts reversibly with it. NH3(aq) + H2O(l) → NH4+(aq) + OH-(aq) It is the presence of OH- ions which make the solution alkaline.

Why use calcium oxide for drying ammonia? Ammonia will react with most other common drying agents such as concentrated sulfuric acid and calcium chloride.

Reference This experiment has been adapted from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/periodic-table/ammoniafountain,42,EX.html

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Health & Safety checked, November 2006 Updated 12 Feb 2009

Titrating sodium hydroxide with hydrochloric acid

16

In this experiment sodium hydroxide is neutralised with hydrochloric acid to produce the soluble salt sodium chloride in solution. This solution is then concentrated and crystallised to produce sodium chloride crystals.

Lesson organisation You have to decide if this experiment is suitable to use with different classes, and look at the need for preliminary training in using techniques involved in titration (see Teaching notes). What follows here assumes that teachers have judged the class to be capable of doing this experiment using a burette with reasonable expectation of success. Assuming that the students have been given training, the practical work should, if possible, start with the apparatus ready at each work-place in the laboratory. This is to avoid vulnerable and expensive glassware (the burette) being collected from an overcrowded central location.

Stage 1 Filling the burette, measuring out the alkali into the flask, and titrating it until it is neutralised takes about 20 minutes, with false starts being likely for many groups. In practice it does not matter if the end-point is overshot, even by several cubic centimetres, but the aim is to find the proportions for a roughly neutral solution.

Stage 2 Producing a neutral solution free of indicator, should take no more than 10 mins.

Stage 3 Evaporating the solution may take the rest of the lesson to the point at which the solution can be left to crystallise for the next lesson. Watching solutions evaporate can be tedious for students, and they may need another task to keep them occupied – e.g. rinsing and draining the burettes with purified water.

Apparatus and chemicals Eye protection Each working group requires:

•

Burette (30 cm3 or 50 cm3) (see note 1) Conical flask (100 cm3) Beaker (100 cm3) Pipette (20 or 25 cm3) with pipette filler Stirring rod Small (filter) funnel (about 4 cm diameter) Burette stand and clamp (see note 2) White tile (optional) (see note 3) Bunsen burner with heat resistant mat Tripod Pipeclay triangle (see note 4) Evaporating basin (at least 50 cm3 capacity) Crystallising dish Access to: Microscope or hand lens suitable for examining crystals in the crystallising dish

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Sodium hydroxide solution, 0.4 mol dm-3 (see note 5) (Irritant at this concentration), about 100 cm3 in a labelled and stoppered bottle Dilute hydrochloric acid, 0.4 mol dm-3 (Low hazard at this concentration), about 100 cm3 in a labelled and stoppered bottle Methyl orange indicator solution (Low hazard at this concentration) (or alternative) in small dropper bottle

Technical notes Sodium hydroxide (Irritant at concentration used) Refer to CLEAPSS® Hazcard 91, Recipe card 65 Dilute hydrochloric acid (Low hazard at concentration used) Refer to CLEAPSS® Hazcard 47A and Recipe card 31 Methyl orange indicator solution (The solid is Toxic but not the solution) Refer to CLEAPSS® Hazcard 32 and Recipe card 33 1 If your school still uses burettes with glass stopcocks, consult the CLEAPSS® Laboratory handbook, section 10.10.1, for their care and maintenance. This experiment will not be successful if the burettes used have stiff, blocked or leaky stopcocks. Modern burettes with PTFE stopcocks are much easier to use, require no greasing, and do not get blocked. Burettes with pinchcocks of any type are not recommended; while cheap, they also are prone to leakage, especially in the hands of student beginners. 2 Burette stands and clamps are designed to prevent crushing of the burette by overtightening, which may happen if standard jaw clamps are used. 3 The optional white tile is to go under the titration flask, but white paper can be used instead. 4 Ceramic gauzes can be used instead of pipeclay triangles, but the evaporation then takes longer. 5 The concentrations of the solutions do not need to be made up to a high degree of accuracy, but they should be reasonably close to the same concentration as each other, and less than 0.5 mol dm-3. 6 The evaporation and crystallisation stages may be incomplete in the lesson time. The crystallisation dishes need to be set aside for crystallisation to take place slowly. However, the dishes should not be allowed to dry out completely, as this spoils the quality of the crystals. With occasional checks, it should be possible to decide when to decant surplus solution from each dish to leave good crystals for the students to inspect in the following lesson.

Procedure

•

HEALTH & SAFETY: Wear eye protection 45 46

burette

47 48 49

evaporating dish titration solution pipe clay triangle

50

tripod conical flask hydrochloric acid sodium hydroxide and methyl orange indicator

58

Bunsen burner heat resistant mat salt solution crystallising dish

Stage 1 a Using a small funnel, pour a few cubic centimetres of 0.4 mol dm-3 hydrochloric acid into the burette, with the tap open and a beaker under the open tap. Once the tip of the burette is full of solution, close the tap and add more solution up to the zero mark. (Do not re-use the acid in the beaker – this should be rinsed down the sink.) b Use a pipette with pipette filler to transfer 25 (or 20) cm3 of 0.4 mol dm-3 sodium hydroxide solution to the conical flask, and add two drops of methyl orange indicator. Swirl gently to mix. Place the flask on a white tile or piece of clean white paper under the burette tap. c Add the hydrochloric acid to the sodium hydroxide solution in small volumes, swirling gently after each addition. Continue until the solution just turns from yellow-orange to red and record the reading on the burette at this point. This coloured solution should now be rinsed down the sink.

Stage 2 a Refill the burette to the zero mark. Carefully add the same volume of fresh hydrochloric acid as you used in (c) to another 25 (or 20) cm3 of sodium hydroxide solution, to produce a neutral solution, but this time without any indicator.

Stage 3 a Pour this solution into an evaporating basin. Reduce the volume of the solution to about half by heating on a pipeclay triangle or ceramic gauze over a low to medium Bunsen burner flame. The solution spits near the end and you get less crystals. Do not boil dry. You may need to evaporate the solution in, say, 20 cm3 portions to avoid over-filling the evaporating basin. Do not attempt to lift the hot basin off the tripod – allow to cool first, and then pour into a crystallising dish. b Leave the concentrated solution to evaporate further in the crystallising dish. This should produce a white crystalline solid in one or two days. c Examine the crystals under a microscope.

Teaching notes Titration using a burette, to measure volumes of solution accurately, requires careful and organised methods of working, manipulative skills allied to mental concentration, and attention to detail. All of these are of course desirable traits to be developed in students, but there has to be some degree of basic competence and reliability before using a burette with a class. The experiment is most likely to be suited to 14–16 year old students. This is discussed further below, but what follows here assumes that you have judged the class to be capable of doing this experiment using a burette with reasonable expectation of success. Students need training in using burettes correctly, including how to clamp them securely and fill them safely. You should consider demonstrating burette technique, and give students the opportunity to practise this. In this experiment a pipette is not necessary, as the aim is to neutralise whatever volume of alkali is used, and that can be measured roughly using a measuring cylinder. It is not the intention here to do quantitative measurements leading to calculations. The aim is to introduce students to the titration technique only to produce a neutral solution. Alternative indicators you can use include screened methyl orange (green in alkali, violet in acid) and phenolphthalein (pink in alkali, colourless in acid).

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Leaving the concentrated solutions to crystallise slowly should help to produce larger crystals. The solubility of sodium chloride does not change much with temperature, so simply cooling the solution is unlikely to form crystals. Under the microscope (if possible, a stereo-microscope is best) you can see the cubic nature of the crystals. If crystallisation has occurred in shallow solution, with the crystals only partly submerged, ‘hopper-shaped’ crystals may be seen. In these crystals, each cube face becomes a hollow stepped-pyramid shape.

Student questions Stage 1 1 What substances have been formed in this reaction? Write a word equation and a symbol equation. _______________________________________________________________________

Stage 2 2 Why must you use another 25 cm3 of sodium hydroxide solution, rather than making your crystals from the solution in Stage 1? _______________________________________________________________________

Stage 3 3 What shape are the crystals? _______________________________________________________________________

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/acids-alkalis-and-salts/titratingsodium-hydroxide-with-hydrochloric-acid,129,EX.html

Useful resource This website has a wealth of information on sodium chloride as a mineral. www.webmineral.com/data/Halite.shtml (Website accessed December 2009)

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Using indigestion tablets to neutralise an acid

17

Indigestion is caused by excess acid in the stomach. Indigestion tablets neutralise some of this acid. This experiment shows how you can measure the amount of hydrochloric acid neutralised by one tablet. This is one measure of the effectiveness of the tablet.

Lesson organisation Burettes are expensive and require a certain amount of skill to use. What follows here assumes that the class has been judged capable of doing this experiment using a burette. The practical work should if possible start with the apparatus ready at each work-place in the laboratory to avoid vulnerable and expensive glassware (the burette) being collected from an overcrowded central location. The experiment to test a single tablet should take no more than 25 mins. If it is extended to compare the effectiveness of two or more brands of tablet, allow another 15 – 20 mins per tablet. Different groups can be allocated one ‘standardisation’ brand of tablet to test, and one other brand, so that the class can compare a number of different brands in a one-hour lesson.

Apparatus and chemicals Eye protection Each working group requires:

•

Burette (30 cm3 or 50 cm3 capacity) (see note 1) Conical flask (100 cm3) Beaker (100 cm3) Pestle and mortar Stirring rod Spatula Filter funnel, small - about 35 mm diameter White tile (optional) Burette stand and clamp Dilute hydrochloric acid of appropriate concentration (Low hazard at this concentration), 100 cm3 (see note 1) Two indigestion tablets, one of one brand to be tested by all groups, and another tablet from a range of brands available to the class (see note 2) Original packets from which the tablets are taken, together with price information for each packet Methyl orange indicator solution (or alternative) (Low Hazard) (see note 3) Deionised or distilled water, about 100 cm3

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Technical notes Dilute hydrochloric acid (Low hazard at concentration used) Refer to CLEAPSS® Hazcard 47A and Recipe card 31 Methyl orange indicator. Refer to CLEAPSS® Hazcard 32 and Recipe card 33 1 The concentration of hydrochloric acid needed depends on the formulation of the tablets being tested. The aim is for each tablet tested to require around 20 – 30 cm3 of hydrochloric acid to neutralise. This can either be calculated from the tablet formulation if this is straightforward (some contain ingredients such as sodium alginate which make the calculation unreliable), or by running a test titration using an acid concentration of 0.1 mol dm-3. The latter result can then be used to calculate a suitable concentration. The concentration should not need to be greater than 0.4 mol dm-3. 2 It is sensible to select brands of tablets for which a comparison is straightforward, with active ingredients restricted to carbonates, bicarbonates and/or hydroxides, avoiding those containing other active ingredients. A total of, say, four or five brands should be sufficient for an interesting exercise in comparing brands. 3 The methyl orange indicator should be available in a dropper bottle on the teacher's bench.

Procedure

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HEALTH & SAFETY: Wear eye protection a Crush a tablet using a pestle and mortar and carefully transfer it to a conical flask, using a spatula to ensure complete transfer as far as possible. Rinse any remaining fragments into the flask with a few cm3 of deionised water.

45 46 47

burette filled with hydrochloric acid

48 49 50

b Add about 25 cm of deionised water to the flask, followed by three drops of methyl orange indicator. 3

c Using a small funnel, pour a few cm3 of the dilute hydrochloric acid provided into the burette, with the tap open and a beaker under the open tap. Once the tip of the burette is full of solution, close the tap and add more of the solution up to the zero mark. (Do not re-use the acid in the beaker – this should be rinsed down the sink.)

conical flask

indigestion tablet, water, and methyl orange indicator

d Add acid from the burette into the flask, 1 – 2 cm3 at a time, while slowly swirling the flask. Continue to add the acid until a red colour begins to be seen in the flask that quickly returns to yellow-orange. e When it begins to take longer for this to happen, add a smaller quantity of acid at a time – eg 0.5 cm3 – until you reach a point where the red colour remains after one minute. f Record the volume of acid used. g Rinse the flask with water, and repeat the experiment with a different indigestion tablet. Refill the burette, if necessary.

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Teaching notes Titrating a powdered tablet containing insoluble ingredients such as calcium carbonate, magnesium carbonate and magnesium hydroxide is slow, as you need to allow for the solid to react with the acid. If the tablets have been pre-tested for their expected titre values, students can be instructed to add acid from the burette rapidly to a point 5 cm3 below the lowest expected value for the brands being tested – this should save time. The experiment is designed to raise ‘fair-test’ principles for discussion, and students are expected to comment with rational arguments on the validity of the comparisons they make. In particular they ought, if possible without prompting, to read the instructions on each packet concerning the recommended dosage. Many comparisons are likely to be ‘grey’ rather than ‘black-and-white’. This could lead to suggestions for further investigations for improving comparisons, but it is unlikely that these will be feasible at school. This experiment is likely to be more useful in investigative-style work for 14–16 year olds, rather than illustrating the development of understanding of the concept of acidity. However, this experiment enhances such understanding for many students.

Student questions 1 Explain why each group is asked to test one tablet of the same brand, and one of another brand. _______________________________________________________________________ 2 Use your results to draw what conclusions you can about the value represented by each brand. _______________________________________________________________________ 3 Is this a fair way of comparing brands of indigestion tablet? Explain your answer. _______________________________________________________________________ 4 From the list of active ingredients on the packets, write word equations for the reactions that take place in your flask during the titrations. _______________________________________________________________________ 5 Write a symbol equation for at least one of the reactions that takes place in the flask.

_______________________________________________________________________

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/acids-alkalis-and-salts/usingindigestion-tablets-to-neutralise-an-acid,241,EX.html

Useful resource Information on indigestion and the variety of indigestion tablets: http://search.bupa.com/cgi-bin/query?mss=couksearch&i=live-bupacouk&q=indigestion+tablet s&Search.x=0&Search.y=0 However, there is a lack of easily found information on the quantitative composition of simple indigestion tablets on the web. (Website accessed December 2009) Health & Safety checked, May 2008 Updated 29 Oct 2008

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18

A thermometric titration Sodium hydroxide solution is titrated with hydrochloric acid. The temperature change is measured each time a portion of acid is added. The highest temperature indicates the endpoint of the titration, and this is used to calculate the concentration of the hydrochloric acid.

Lesson organisation This is best carried out individually or in pairs. The experiment takes about one hour.


•

Each group will need: Eye protection: goggles Thermometer (0 – 100 °C) (see note 1) Two insulated (polystyrene) cups Beaker (250 cm3) Burette (50 cm3) Burette stand Clamp and stand (optional) Cork, one-holed (optional - to fit thermometer) Pipette (20 or 25 cm3) Pipette safety filler Hydrochloric acid, 2.00 mol dm–3 (Irritant at this concentration), about 75 cm3 (see note 2) Sodium hydroxide solution, 1.50 mol dm–3 (Corrosive at this concentration), about 30 cm3 (see note 3)

Technical notes Hydrochloric acid (Irritant at concentration used) Refer to CLEAPSS® Hazcard 47A and Recipe Card 31 Sodium hydroxide solution (Corrosive at concentration used). Refer to CLEAPSS® Hazcard 91 and Recipe Card 65 1 Instead of using the thermometer to stir the titration mixture, it could be clamped in position in a cork, as shown in the diagram, and the mixture swirled after each addition of acid. Alternatively, a temperature sensor attached to a computer can be used in place of a thermometer. Data logging software could then be used to provide a detailed plot of the readings. 2 The concentration of the hydrochloric acid should not be indicated on bottle. 3 The concentration of the sodium hydroxide should be indicated on bottle. 4 The solutions need to be as concentrated as they are in order to achieve reasonable changes in temperature.

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Procedure

•

HEALTH & SAFETY: Wear goggles a Stand an insulated cup in a beaker for support.

45

b Using a pipette and safety filler, transfer 20 cm3 (or 25 cm3) of the sodium hydroxide solution into the cup, and measure the steady temperature.

46

burette

47 48 49 50

hydrochloric acid

c Using the burette, add a small portion (3 – 5 cm ) of dilute hydrochloric acid to the solution in the cup, noting down the actual volume reading. Stir by swirling the cup and measure the highest temperature reached. 3

d Immediately add a second small portion of the dilute hydrochloric acid, stir, and again measure the highest temperature and note down the volume reading.

cork clamp thermometer

polystyrene cup

e Continue in this way until there are enough readings to decide the maximum temperature reached during this experiment. You will need to add at least 30 cm3 of the acid. f Plot a graph of temperature against the volume of acid added, and use extrapolation of the two sections of the graph to deduce the maximum temperature reached without heat loss. g Use your results to calculate the concentration of the hydrochloric acid.

Teaching notes The main concern in this experiment is the heat loss. If possible a lid should be used. More reliable results can be achieved using two polystyrene cups (one inside the other). With abler or older students, it is possible to discuss the extrapolation of the cooling curve to estimate the maximum temperature reached without heat loss. The link below gives an example of how extrapolation is used to determine the maximum temperature reached. To reinforce the theory involved here, an indicator could also be used to show that the endpoint really did occur at the highest temperature.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/acids-alkalis-and-salts/athermometric-titration,279,EX.html

Useful resource This link gives an example of a typical plot of temperature vs volume of acid for this experiment, and the use of extrapolation to determine the maximum temperature change. http://www.creative-chemistry.org.uk/alevel/module2/documents/N-ch2-08.pdf (Website accessed December 2009)


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19

Universal indicator ‘rainbow’ A long glass tube is filled with a neutral solution of Universal indicator. Hydrochloric acid is added to one end and sodium hydroxide solution to the other. The tube is inverted a few times to mix the solutions and the ‘rainbow’ of Universal indicator colours appears.

Lesson organisation The demonstration itself takes only a few minutes. It provides a good attention-grabbing lesson starter or lesson endpoint.


•

Eye protection For one demonstration, the teacher will need: Glass tube (see note 1) Bungs (rubber), 2, to fit the glass tube Beaker (100 cm3) Dropper pipettes, 3 Clamp stand, boss and clamp Distilled or deionised water Hydrochloric acid, 0.1 mol dm-3 (Low hazard at this concentration) Sodium hydroxide solution, 0.1 mol dm-3 (Irritant) Universal indicator solution (Highly flammable) (see note 2)

Technical notes Hydrochloric acid (Low hazard) Refer to CLEAPSS® Hazcard 47A. Sodium hydroxide (Irritant) Refer to CLEAPSS® Hazcard 91. Universal indicator solution (Highly flammable) Refer to CLEAPSS® Hazcard 32 and 40A, and CLEAPSS® Recipe Card 36. 1 The glass tube needs to be about 60 cm long with an internal diameter of around 1 cm. 2 The concentrations of the solutions are not critical.

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Procedure HEALTH & SAFETY: Wear eye protection throughout a Add sufficient Universal indicator to about 60 cm3 of distilled or deionised water in a beaker, to give a solution with a visible green colour.

•

b Ensure that one end of the glass tube is firmly stoppered with a rubber bung. c Fill the tube to about 2 cm from the top with the Universal indicator solution. Then clamp the tube vertically. It is important to leave a space above the liquid in the tube so that there is an air bubble – this helps the mixing in step h. d Add 3-4 drops of the hydrochloric acid solution. The top few centimetres of the liquid should turn red. e Stopper the upper end of the tube, remove it from the clamp, carefully invert it and then clamp it vertically again. f Remove what is now the top stopper. Add 3-4 drops of the sodium hydroxide solution. The top few centimetres of the liquid should turn purple. g Stopper the tube. Both ends of the tube should now be firmly stoppered. h Remove the tube from the clamp and carefully invert it 2 or 3 times. The movement of the air bubble will mix the contents and produce a ‘rainbow’ in the tube, showing all the colours of Universal indicator from red through orange, yellow, green, blue and purple.

Teaching notes A white background is useful to show the colours.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/acids-alkalis-and-salts/universalindicator-rainbow,98,EX.html

Health & Safety checked, June 2007 Updated 29 Oct 2008

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An effervescent Universal indicator ‘rainbow’ Description Sodium carbonate solution is added to a burette containing a little hydrochloric acid and Universal Indicator. The two solutions react, with effervescence, and the liquid in the burette shows a ‘rainbow’ of all the colours of Universal Indicator from red through orange, yellow, green and blue to purple. This experiment will take around five minutes.


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Eye protection A 50 cm3 burette A retort stand with boss and clamp Cotton wool plug A few cubic centimetres of Universal Indicator solution About 10 cm3 hydrochloric acid solution (2 mol dm-3) (irritant) About 20 cm3 sodium carbonate solution (1 mol dm-3)

Procedure

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Health & safety: Wear eye protection. Your employer’s risk assessment should be consulted before carrying out this activity. This activity is covered by model (general) risk assessments widely adopted for use in UK schools such as those provided by CLEAPSS®, SSERC and ASE. Bear in mind, however, that these may need some modification to suit local conditions. a Clamp the burette vertically. Add about 0.5 cm3 of the Universal indicator solution followed by about 10 cm3 of the hydrochloric acid to give a clearly visible red colour. Now add about 20 cm3 of the sodium carbonate solution. Insert a loose plug of cotton wool in the top of the burette. The sodium carbonate and hydrochloric acid react, with effervescence, and the burette will be filled with liquid showing a ‘rainbow’ of all the colours of Universal indicator from red through orange, yellow, green and blue to purple. b A white background will show the colours to best advantage.

Reference This demonstration was developed in this form by Grant Birchby and Alan Matear of Blackburn College for the RSC

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‘Neutralisation circles’

21

Description Drops of dilute acid and alkali are placed a few centimetres apart on a sheet of filter paper and allowed to spread out until they meet. A few drops of Universal indicator are then placed over the moist area of the filter paper and a band of colours showing the range of colours of the Universal indicator is seen on the paper. A typical result is shown below. This experiment will take around 10 minutes.

Apparatus and chemicals Eye protection Each group of students will need:

•

Three dropping pipettes A pencil A white tile Access to sodium hydroxide solution, 0.1 mol dm-3 (Irritant at this concentration) Access to dilute hydrochloric acid, 0.1 mol dm-3 (Low hazard at this concentration) Universal indicator solution (Low hazard at this concentration) (or alternative) in small dropper bottle A sheet of filter paper, approximately 12.5 cm diameter (but the size is not critical). Whatman no. 1 works well, but chromatography paper appears to be less successful.

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Procedure Health & safety: Wear eye protection throughout Hydrochloric acid (Low hazard) Refer to CLEAPSS® Hazcard 47A. Sodium hydroxide (Irritant) Refer to CLEAPSS® Hazcard 91. Universal indicator solution (Highly flammable) Refer to CLEAPSS® Hazcard 32 and 40A, and CLEAPSS® Recipe Card 36. Students are given a piece of filter paper and asked to draw on it in pencil two circles about 1 cm in diameter and about 2 – 3 cm apart which they label ‘acid’ and ‘alkali’ respectively. The filter paper is then placed on a while tile and students use dropping pipettes to place a few drops of the appropriate solution in each circle. The concentrations of the acid and alkali are not critical but they should be approximately the same. The solution will begin to spread out on the filter paper. The students wait for a few minutes until the solutions have soaked through the filter paper towards each other and have met. Students then place drops of Universal Indicator solution on the area of the filter paper where the acid and alkali have met and reacted. A ‘rainbow’ is produced showing the range of colours produced by Universal indicator. Students can dry the filter papers and stick them into their notebooks.

Teaching notes This experiment is quicker, simpler and safer than the traditional method of illustrating neutralisation by titrating acid with alkali using a burette. It also uses more familiar equipment (a dropping pipette rather than a burette), uses little of the reagents and has the advantage of producing a permanent record of the colour changes. The reaction is HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Technical notes 1 Other acids and alkalis and other indicators (or mixtures of indicators) including ‘home made’ ones (from red cabbage, for example) could be tried. 2 Toilet roll and other white tissue may be used instead of filter paper but they appear to dry less successfully. 3 A hair drier or oven may be found useful to dry the filter papers quickly

Reference This experiment has been adapted from a version written by Ted Lister on behalf of the RSC

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Indicators and dry ice: demonstration

22

Dry ice is added to indicator solutions. Bubbles and a ‘fog’ are produced along with a gradual colour change. The experiment is a great way to demonstrate neutralisation reactions and pH changes, as well as to highlight that carbon dioxide forms weakly acidic solutions.

Lesson organisation This short but spectacular demonstration will be most memorable if it is done on a fairly large scale. Several different indicators could be used at the same time. This demonstration can be used when discussing acids, alkalis, indicators, or the properties of carbon dioxide. With the appropriate audience, it could also be used to introduce a discussion of the pH changes that take place during the titrations of weak acids with strong and weak alkalis - and hence buffers. It is also a good fun demonstration for more general audiences.

Apparatus and chemicals The teacher requires: Eye protection

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Measuring cylinders (1 dm3) - as many as the number of indicators to be used (see note 3) Expanded polystyrene cool-box to store the dry ice (see note 1) Tongs or large spoon/scoop for transferring dry ice Long stirring rod Gloves (leather or insulated) for handling dry ice Dry ice – allow 100 g for each indicator (see note 2) Access to a range of indicator solutions. Suitable ones include: Universal Indicator Phenolphthalein Thymolphthalein Thymol blue Phenol red Bromothymol blue Dilute ammonia solution and/or dilute sodium hydroxide solution (Irritant), 0.1 mol dm-3

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Technical notes Dry ice (solid carbon dioxide). Refer to CLEAPSS® Hazcard 20. Ammonia solution. Refer to CLEAPSS® Hazcard 6 and Recipe Card 4. Sodium Hydroxide solution (Irritant). Refer to CLEAPSS® Hazcard 91. Indicators (various hazards including Highly flammable). Refer to CLEAPSS® Hazcard 32. 1 For storing the dry ice, the expanded polystyrene box in which Winchester bottles are often supplied is ideal. Never put dry ice in a sealed container. 2 The dry ice should be bought, since dry ice made from a carbon dioxide cylinder will float and be much less effective at saturating the solutions. Dry ice can be obtained from universities or other higher education institutions, hospitals, industrial firms - and from some undertakers. 3 If 1 dm3 measuring cylinders are not available, 1 dm3 ‘tall form’ beakers are suitable substitutes. The measuring cylinders or beakers should be glass rather than plastic – the colour change is much easier to see. 4 0.1 mol dm-3 ammonia solution should be adequate for this demonstration.

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Procedure HEALTH & SAFETY: Wear eye protection and use gloves to handle the dry ice since it can cause severe frost burns. a For each indicator, fill a large measuring cylinder with water to the 1 dm3 mark, or a large beaker to within 5 cm of the top. Add enough indicator to give an easily visible colour. b Add a few drops of ammonia solution or sodium hydroxide solution to give an alkaline solution. Stir to mix the solution thoroughly. c Add a few lumps of dry ice. These will sink to the bottom and bubble as gaseous carbon dioxide is produced. A spectacular fog is produced at the top of the cylinder. After several minutes, the colour of the indicator will change.

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Teaching notes Carbon dioxide is a weakly acidic oxide which reacts with sodium hydroxide to produce sodium carbonate: 2NaOH + CO2 → Na2CO3 + H2O However, in this experiment the solution of sodium hydroxide is very dilute, and the reactions involved are more complex. Carbon dioxide dissolves in and reacts with water to produce hydrogen ions (H+). The acidic solution produced then reacts with and neutralises the alkali present. Carbon dioxide dissolves reversibly in water: CO2(g)

CO2(aq)

(This is the basis of the fizz you get when taking the top off a bottle of carbonated water – the CO2 comes out of solution when the pressure is released.) Some of the dissolved CO2 reacts reversibly with water to form an acidic solution: CO2(aq) + H2O(l)

HCO3-(aq) + H+(aq)

This acidic solution then reacts with the alkali present. If the alkali is sodium hydroxide, the equation for the neutralisation reaction is: HCO3-(aq) + H+(aq) + Na+(aq) + OH- (aq) → Na+(aq) + HCO3- (aq) + H2O(l) If the alkali is ammonia solution, the colour change takes place more slowly because ammonia, unlike sodium hydroxide, is a weak alkali. Ammonia itself reacts reversibly with water. NH3(g) + H2O(l)

NH4+(aq) + OH-(aq)

The equation for the neutralisation reaction involving ammonia is: HCO3-(aq) + H+(aq) + NH4+(aq) + OH-(aq) → NH4+(aq) + HCO3-(aq) + H2O (l) The final pH reached is about 4.5. It is best to use indicators which change colour at pH values above this, or use Universal Indicator. The expected colour changes (alkali – acid) for the suggested indicators are: Phenolphthalein: pink - colourless (pH range: 8.2-10.0) Thymolphthalein: blue - colourless (pH range: 8.3-10.6) Thymol blue: blue - yellow (pH range: 8.0-9.6) Phenol red: red - yellow (pH range: 6.8-8.4) Bromothymol blue blue - yellow (pH range: 6.0-7.6) You may want to demonstrate colour changes at lower pH values. If so add a few drops of concentrated hydrochloric acid at the end.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/acids-alkalis-and-salts/indicatorsand-dry-ice-demonstration,53,EX.html

Health & Safety checked, November 2006 Updated 29 Oct 2008

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23

Thermal decomposition of calcium carbonate Calcium carbonate is strongly heated until it undergoes thermal decomposition to form calcium oxide and carbon dioxide. The calcium oxide (unslaked lime) is dissolved in water to form calcium hydroxide (limewater). Bubbling carbon dioxide through this forms a milky suspension of calcium carbonate. Students are also asked to research the large-scale applications of these processes.

Lesson organisation This experiment can be carried out conveniently in groups of two or three and takes about 40 - 45 minutes.


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Eye protection Tripod Gauze Bunsen burner Heat resistant mat Tongs Boiling tubes, 2 (see note 1) Drinking straw (see note 4) Dropping pipette Filter funnel, small Filter paper Calcium carbonate (Low hazard) (see notes 1, 2 & 3) Universal Indicator solution

Technical notes Calcium carbonate (Low hazard) Refer to CLEAPSS® Hazcard 19B 1 Large (150 x 25 mm) test-tubes. 2 The calcium carbonate used should be in the form of pea-sized lumps of chalk. 3 Blackboard chalk should not be used, as it is likely to be mostly calcium sulfate. 4 Freshly purchased drinking straws should be used.

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Procedure

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HEALTH & SAFETY: Eye protection. gauze calcium carbonate

filter paper

filter funnel tripod heat resistant mat

Bunsen burner

boiling tubes water

clear filtrate

a You need to prepare a tabulated results sheet before you start your experiments. Method

Observation

Heat for 10 min Add 2 – 3 drops of water Add 10 cm3 more water Blow bubbles through solution Add Universal Indicator

b Set a lump of chalk (calcium carbonate) on a gauze. If your gauze has a coated central circle, use the edge where there is no coating. c Heat the chalk very strongly for 5 -10 minutes. Write down what you observe. d Let the chalk cool and use tongs to move it into a boiling tube. Add 2 – 3 drops of water with a dropping pipette. Write down your observations. e Add about 10 cm3 more water to the solid. What happens now? f Filter half the mixture into the other boiling tube and, using a straw, gently blow a stream of bubbles through the filtrate. What do you see? g Test the remaining half of the mixture with Universal Indicator solution. Write down what you observe.

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Teaching notes Keep an eye on less mature students who might be tempted to suck rather than blow through the filtrate. The results expected are as follows: Method

Observation

Heat for 10 min

The chalk should be seen to crumble slightly

Add 2 – 3 drops of water

More crumbling, steam given off, evidence that mixture has become hot

Add 10 cm3 more water

Some of the solid dissolves, white suspension

Blow bubbles through solution

Limewater turns cloudy.

Add Universal Indicator

Indicator goes from green to blue/purple

This set of experiments involves a variety of important reactions and types of reactions, with several references to industrial processes. The roasting of limestone and the hydration of the quicklime formed has relevance in the manufacture of plaster and cement, and in the laboratory limewater is a common reagent for the testing of carbon dioxide. Students could be asked to carry out web research on these applications.

Student questions and answers Here are some questions for your students, with answers 1 Why does the chalk crumble slightly on strong heating? Carbon dioxide/a gas is evolved; this forces its way out of the solid and breaks down its structure. 2 What type of reaction is taking place during the heating process? Write an equation for the reaction. Thermal decomposition; CaCO3(s) → CaO(s) + CO2(g) 3 Why is steam evolved when drops of water are added? Write an equation for the reaction occurring. The reaction is highly exothermic and the small amount of water added is partly converted to steam in the process: CaO(s) + H2O(l) → Ca(OH)2(s) 4 Why does the limewater turn cloudy? Write an equation for the reaction which is occurring. Insoluble calcium carbonate is being precipitated: Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l) 5 What does the colour change occurring when limewater is added tell you about the pH of the solution? Explain why the pH would be expected to have this value. The pH is about 11 - 14; soluble metal hydroxides are alkaline and therefore give high pH values

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/materials/thermal-decompositionof-calcium-carbonate,282,EX.html

Useful resource A detailed description of some of the reactions and commercial uses of calcium carbonate: http://scifun.chem.wisc.edu/CHEMWEEK/Lime/lime.html (Website accessed December 2009)

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Reacting elements with oxygen

24

Many elements react with oxygen on heating. These reactions and the properties of their products illustrate the periodic nature of the elements. The same procedure can be used to make samples of chlorides using chlorine gas (see experiment 25: Reacting elements with chlorine).

Introduction Many elements react with oxygen. The ease with which the reaction takes place, the vigour of the reaction and the properties of the compounds made provide excellent evidence to help understanding about the periodic nature of the elements in the Periodic Table and about many chemical principles. The difference in reactivity between reactions in air and in oxygen can be related to the relative concentrations.

Lesson organisation Students frequently say ‘Do it again.’ and ‘Use a bigger bit.’ when they see a spectacular demonstration. Both chemically and pedagogically it is much better for them to see a number of graded demonstrations rather than the same one again on a bigger scale. The reactions are spectacular and lend themselves well to a series of demonstrations which illustrate the periodic nature of the elements.

Apparatus and chemicals Fume cupboard Gas preparation apparatus: Side-arm flask 250 cm3 Tap funnel and bung to fit flask Connecting tubing Reaction tubes, one for each reaction (see experiment 14 for how to make a reaction tube) 1-hole bung and delivery tube to fit Clamp stand Boss head Clamp Hydrogen peroxide solution 15 cm3 20 vol per experiment (Harmful, Refer to CLEAPSS® Hazcard 50) (see note 1) Manganese(IV) oxide granular 5g (Harmful, Refer to CLEAPSS® Hazcard 50) The other elements Lithium (Highly flammable, corrosive, Refer to CLEAPSS® Hazcard 58) (see notes 4 and 5) Sodium (Highly flammable, corrosive, Refer to CLEAPSS® Hazcard 88) Potassium (Highly flammable, corrosive, Refer to CLEAPSS® Hazcard 76) Magnesium (Highly flammable, Refer to CLEAPSS® Hazcard 59A) (see note 6) Calcium (Highly flammable, Refer to CLEAPSS® Hazcard 59A) Aluminium foil (Low hazard, Refer to CLEAPSS® Hazcard 1) Carbon (Low hazard, Refer to CLEAPSS® Hazcard 21) Phosphorous Red (Highly flammable, Refer to CLEAPSS® Hazcard 73A) Sulfur (Low Hazard, Refer to CLEAPSS® Hazcard (96A)

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Technical notes 1 It is recommended that these experiments are done by the demonstrator before demonstrating to pupils, in order to gain experience, if they have not done it before. 2 A steady evolution of oxygen gas can be obtained by dripping 20 vol hydrogen peroxide solution onto manganese(IV) oxide. If granules are used the rate of reaction is more controlled. The other elements 1 Group 1 metals are stored under oil, this can be removed using paper tissue. 2 Cut pieces of Group 1 metals into cubes no bigger than 3mm. 3 For the other elements 0.1g is sufficient to see a spectacular reaction.

tap funnel

potassium metal 3mm cube

reaction tube

20 vol H2O2 Clamp at end

oxygen gas generated

Bunsen burner manganese(IV) oxide granules

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Procedure HEALTH & SAFETY: Wear eye protection at all times . a The apparatus is assembled in a fume cupboard, according to the diagram.

•(

)

b A reaction tube is clamped horizontally close to the bung end. c The tubes are used to connect the gas generator to the delivery tube in the bung. d A piece of the element is placed near the bottom of the test tube. e The bung is pushed gently into the reaction tube. f Start the oxygen gas generation by running hydrogen peroxide onto the catalyst. Use 20 cm3 hydrogen peroxide solution to flush the system of air for 60 seconds. g Heat the solid element using a Bunsen burner with a blue flame. h Add more hydrogen peroxide slowly i Remove the Bunsen burner when the element catches fire

Teaching notes The synthesis reactions of these binary compounds (two elements) will catch the attention of students because they are spectacular. Done in a series of reactions, students begin to appreciate ‘periodicity’ and other important aspects of chemistry. Group 1 and 2 – Students see the increase in reactivity going down the group (inferred from the heating time required). They will see the reaction start when the metal begins to melt. Melting allows fresh metal to flow out from under the oxide coating which inevitably forms on the surface of metals. Students understand why metals start to react vigorously with gases when they melt. They will not appreciate that most combustion reactions do not involve solids. The temperature of reaction and the melting points of elements can be related to their structure and bonding. Carbon and silicon burn in the solid state, unlike the other elements. The melting points, reaction with water and pH of the solutions of these compounds are all logical developments of this topic. For a comparison of these reactions with the reactions of chlorine and the properties of chlorides see experiment 25: Reacting elements with chlorine.

References This experiment was written by Mike Thompson on behalf of the RSC


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25

Reacting elements with chlorine Many elements react with chlorine on heating. The reactions and the properties of the products illustrate the periodic nature of the elements. The reactions require less energy input to initiate than those with oxygen (experiment 24: Reacting elements with oxygen).

Introduction Many elements react with chlorine. The ease with which the reaction takes place, the vigour of the reaction and the properties of the compounds made provide excellent evidence to help understanding about the periodic nature of the elements in the Periodic Table. The comparison of the properties of oxides and chlorides of various elements widens and deepens the understanding that students gain from seeing only one set of compounds.

Lesson organisation Students frequently say ‘Do it again’ and ‘Use a bigger bit’ when they see a spectacular demonstration. Both chemically and pedagogically it is much better for them to see a number of graded demonstrations rather than the same one again on a bigger scale. The reactions are spectacular and lend themselves well to a series of demonstrations which illustrate the periodic nature of the elements.

Apparatus and chemicals Fume cupboard Gas preparation apparatus: Side-arm flask 250 cm3 Tap funnel and bung to fit flask Connecting tubing Reaction tubes, one for each reaction (see Experiment 14 for how to make a reaction tube) 1-hole bung and delivery tube to fit Clamp stand Boss head Clamp For chlorine generation Concentrated hydrochloric acid, 14 cm3 5 mol dm-3 (Corrosive, Refer to CLEAPSS® Hazcard 47A) (see note 2) Potassium manganate(VII), 3g (Oxidising, Harmful, Danger to the environment Refer to CLEAPSS® Hazcard 81) Sodium thiosulfate solution 1 dm3, 40g dm-3 (Low hazard, Refer to CLEAPSS® Hazcard 96C) (see note 3) The other elements

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Lithium (Highly flammable, corrosive, Refer to CLEAPSS® Hazcard 58) (see notes 4 and 5) Sodium (Highly flammable, corrosive, Refer to CLEAPSS® Hazcard 88) Potassium (Highly flammable, corrosive, Refer to CLEAPSS® Hazcard 76) Magnesium (Highly flammable, Refer to CLEAPSS® Hazcard 59A) (see note 6) Calcium (Highly flammable, Refer to CLEAPSS® Hazcard 59A) Aluminium foil (Low hazard, Refer to CLEAPSS® Hazcard 1) Carbon (Low hazard, Refer to CLEAPSS® Hazcard 21) Phosphorous Red (Highly flammable, Refer to CLEAPSS® Hazcard 73A) Sulfur (Low Hazard, Refer to CLEAPSS® Hazcard (96A)

Technical notes 1 It is recommended that these experiment are done by the demonstrator before demonstrating to pupils, in order to gain experience if they have not done it before. 2 A steady evolution of chlorine gas can be obtained by dripping concentrated hydrochloric acid onto solid potassium manganate(VII). Chlorine should be generated in small amounts at a time. 3 Sodium thiosulfate solution rapidly reduces chlorine gas to harmless chloride ions. Excess chlorine can be bubbled into small quantities of sodium thiosulfate solution in a beaker, as necessary. The other elements 1 Group 1 metals are stored under oil; this can be removed by using tissue paper. 2 Cut pieces of Group 1 metals into cubes no bigger than 3mm. 3 For the other elements 0.1g is sufficient to see a spectacular reaction.

tap funnel

potassium metal 3mm cube

reaction tube

Concentrated HCI

Clamp at end

chlorine gas generated

Bunsen burner potassium manganate(VII)

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Procedure

••(

)

HEALTH & SAFETY: Wear eye protection and gloves at all times. Chlorine gas will react with rings etc. a The apparatus is assembled in a fume cupboard, according to the diagram. b A reaction tube is clamped horizontally close to the bung end. c The tubes are used to connect the gas generator to the delivery tube in the bung. d A piece of the element is placed near the bottom of the test tube. e The bung is pushed gently into the reaction tube. f Start the chlorine gas generation by running hydrochloric acid onto the potassium manganate(VII) solid. g Heat the solid element using a Bunsen burner with a blue flame. h Add more hydrochloric acid slowly i Remove the Bunsen burner as the element catches fire

Teaching notes The synthesis reactions of these binary compounds (two elements) will catch the attention of students because they are spectacular. Done in a series of reactions, students begin to appreciate ‘periodicity’ and other important aspects of chemistry. Oxygen and chlorine comparison – Students see that the reactions with chlorine initiate more easily than reactions with oxygen (they infer this from the heating time). This relates to the much weaker single bond of the chlorine molecule compared to the stronger double bond between the oxygen atoms in the oxygen molecule. Group 1 and 2 – Students see the increase in reactivity going down the group (inferred from the heating time required). They will see the reaction start when the metal begins to melt. Melting allows fresh metal to flow out from under the oxide coating which inevitably forms on the surface of metals. Students understand why metals start to react vigorously with gases when they melt. They will not appreciate that most combustion reactions do not involve solids. The temperature of reaction and the melting points of elements can be related to their structure and bonding. Silicon burns in the solid state, unlike the other elements. Carbon does not react with chlorine directly. The melting points, reaction with water and pH of the solutions of these compounds are all logical developments of this topic.

References This experiment was written by Mike Thompson on behalf of the RSC

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26

Identifying the products of combustion In this demonstration a solid hydrocarbon burns and a pump is used to draw the gaseous combustion products over a piece of cobalt chloride paper and through limewater to show the presence of water and carbon dioxide respectively.

Lesson organisation With this demonstration, the apparatus can be left running for some time and students can file past in small groups to see it more closely. Alternatively a flexicamera can be used linked to a projector. If students are not familiar with the cobalt chloride paper and limewater tests, either demonstrate these separately or allow students to try the tests themselves. Assuming everything is already set up, this demonstration takes only a few minutes.

Apparatus and chemicals Eye protection Glass funnel, about 6 cm in diameter Boiling tubes, 2 Two-holed rubber bungs, 2, to fit the boiling tubes, and fitted with one long and one short piece of glass tubing (see diagram) Pump, see standard procedure: Filter pumps1 Glass or plastic tubing for connections (see note 3) Candle

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Piece of blue cobalt chloride paper (Toxic) (see note 1) About 20 cm3 of limewater (treat as Irritant) (see note 2) to pump

funnel

cobalt chloride paper

limewater

‘tea light’ or ‘night light’

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Technical notes Cobalt chloride/cobalt chloride paper2 (Toxic, Danger to the environment) (see note 1) Refer to CLEAPSS® Hazcard 25 and CLEAPSS® Recipe card 46 Calcium hydroxide solution, ‘limewater’ (treat as Irritant) (see note 2) Refer to CLEAPSS® Hazcard 18 and CLEAPSS® Recipe card 15 1 Cobalt chloride paper can be stored in a desiccator. Minimise handling of cobalt chloride paper (Sensitiser) and wash hands after use (cobalt chloride is a category 2 carcinogen).

See standard procedure: make your own cobalt chloride paper2

2 Ideally, the limewater (a saturated solution of calcium hydroxide, Ca(OH)2) should be made fresh on the day. 3 Care should be taken with the right-angle bend connected to the funnel. If this is made of flexible tubing, it can get hot and melt. Ideally, the glass stem of the funnel should be bent into a right-angle. Alternatively, join a standard funnel onto a right angled piece of glass tubing using epoxy resin.

glass right angle bend flexible rubber or plastic hose

funnel

A more temporary arrangement is to slide one arm of a right-angled piece of glass tubing inside the stem of the funnel and seal the join on the outside with a piece of flexible tubing. 4 A ‘tealight’ or ‘nightlight’ is more squat and so is more stable than a table candle.

Procedure

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HEALTH & SAFETY: Wear eye protection a Before the demonstration, assemble the apparatus as shown in first diagram. Ensure that the connections to the boiling tubes are the correct way round. b Place a piece of blue cobalt chloride paper into the first boiling tube and half-fill the second boiling-tube with limewater. c At the start of the demonstration, turn on the pump so that a gentle stream of air is drawn through the apparatus. d Light the candle and leave for a few minutes until the cobalt chloride paper turns pink (from blue) and the limewater goes milky. This indicates the presence of water and carbon dioxide respectively.

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Teaching notes Some students will know that air contains both water vapour and carbon dioxide. To show that the changes observed are not due to these alone, repeat the experiment without the candle and note how much longer it takes for any changes to be observed. Understanding the process of burning is important at all levels of chemistry. Emphasise that burning in air is a reaction with oxygen. The elements hydrogen and carbon are present in hydrocarbons, such as candle wax. Students will quite readily appreciate that carbon reacts with oxygen to form carbon dioxide, but often need help to grasp that hydrogen combines with oxygen to form water. The production of carbon dioxide could lead to discussion of the role of this gas in the greenhouse effect. The experiment could be extended to burning alcohols with a spirit burner.

References This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/chemicals-from-oil/identifyingthe-products-of-combustion,43,EX.html

Standard procedures (standard procedures can be found by clicking on the link at www.practicalchemistry.org) 1

Filter pumps (http://www.practicalchemistry.org/standard-techniques/filtering,51,AR.html)

2

ake your own cobalt chloride paper (http://www.practicalchemistry.org/standardM techniques/preparing-and-using-cobalt-chloride-indicator-papers,43,AR.html)


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27

The ‘Whoosh’ bottle demonstration A mixture of alcohol and air in a large polycarbonate bottle is ignited. The resulting rapid combustion reaction, often accompanied by a dramatic ‘whoosh’ sound and flames, demonstrates the large amount of energy released in the combustion of alcohols.

Lesson organisation This demonstration requires careful preparation, with strict adherence to the conditions required by the risk assessment provided. Schools are advised not to deviate from the details described in this risk assessment. If any variation is necessary, members should contact CLEAPSS® for a Special Risk Assessment. A single demonstration will take 5 – 10 minutes. It is recommended that any repeat demonstrations use spare dry reaction vessels. Under no circumstances should the reaction vessel be flushed out with pure oxygen in order to be quickly reused.


•

Eye protection Reaction vessel, 1 or more (see note 1) Rubber stopper or plastic cap (to fit the reaction vessel) Beaker (250 cm3), 1 for each alcohol used Wooden splints, as needed (see note 3) Metre rule One or more of the following alcohols, 40 cm3 of each one used: Methanol (Highly flammable, Toxic) Ethanol (IDA, Industrial Denatured Alcohol) (Highly flammable, Harmful) Propan-1-ol (Highly flammable, Irritant) Propan-2-ol (Highly flammable, Irritant)

Technical notes Methanol (Highly flammable, Toxic) Refer to CLEAPSS® Hazcard 40B Ethanol (IDA, Industrial Denatured Alcohol) (Highly flammable, Harmful) Refer to CLEAPSS® Hazcard 40A Propan-1-ol (Highly flammable, Irritant) Refer to CLEAPSS® Hazcard 84A Propan-2-ol (Highly flammable, Irritant) Refer to CLEAPSS® Hazcard 84A 1 The reaction vessel consists of a large polycarbonate bottle, as used in workplace water dispensers. These have a volume of 16 - 20 dm3. A clean, dry bottle is required for each demonstration. It takes time to clean and dry once it has been used for a demonstration. For this reason, up to 4 bottles may be required. The bottle must be made of polycarbonate (marked PC) and of no other material. If using empty but wet water cooler containers, stand them inverted to allow any remaining water to drain and then leave upright for several days until completely dry. 2 Select a safe, level place for the demonstration, with at least 2.5 m clearance above the top of the vessel to the ceiling above, and no flammable materials above it. If the laboratory bench does not allow for this, 4 stable laboratory stools supporting a large wooden tray may give sufficient clearance and stability.

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3 Attach a wooden splint to the end of the metre rule or stick using adhesive tape, angling the splint so that when the metre rule is horizontal, the splint is sloping downwards. Provide a lighter or matches well away from the alcohol bottles. 4 Set out the bottles containing the alcohols and the beakers at least 1 m away from the demonstration. No flames within 1 m. Students at least 4 m away.

Procedure HEALTH & SAFETY: Both demonstrator and class should be wearing eye protection. a Pour about 40 cm3 of the selected alcohol into a beaker and then transfer into the reaction vessel.

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b Insert the rubber stopper and roll the reaction vessel on its side for 10 seconds, to and fro, allowing the alcohol to vaporise and the vapour to fill the vessel. Do not warm the alcohol. c Pour surplus liquid alcohol back into the beaker, draining the vessel as completely as possible, and move the beaker back to the rest of the alcohol stock, away from any risk of catching fire. Surplus liquid left in the vessel may ignite and set fire to the vessel as well. d Stand the reaction vessel securely inside the safety screens and remove the stopper. Light the wooden splint, and apply the lighted end of the splint to the open neck of the vessel. Do not lean over the screens to apply the ignition. It is dangerous to ignite by dropping a lighted match into the vessel when using ethanol or methanol. For both propanols, this method may be used providing the neck of the bottle is above head height e The alcohol vapour should ignite with a loud ‘whoosh’, with flames shooting out of the top of the vessel.

Teaching notes This demonstration is the subject of a Supplementary Risk Assessment by CLEAPSS®, SRA06, which members of CLEAPSS® are able to consult on the Science Publications CD-ROM, updated and re-issued annually to all members. Others will, of course, have to consult their own employer's Risk Assessment. The experiment demonstrates dramatically just how much chemical energy is released from such a small quantity of fuel. The flame colour varies with the proportion of carbon in the alcohol molecule. With methanol and ethanol there is a very quick ‘whoosh’ sound and a blue flame shoots out of the bottle. With propan-1-ol and propan-2-ol, the sound is similar but the reaction is slightly slower, easier to observe, and blue and yellow flames may be observed ‘dancing’ in the bottle. The presence of water reduces the likelihood of dancing flames.

References This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/energy-changes-and-fuels/thewhoosh-bottle-demonstration,240,EX.html

Useful resource A wide-ranging and up-to-date review of the production and use of alcohols for vehicle fuels, with links to a variety of related sites, can be found at: http://en.wikipedia.org/wiki/Alcohol_fuel (Last accessed December 2009) Health & Safety checked, June 2008 Updated 29 Oct 2008

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28

Fat-pan fire! The context of a fat-pan (chip-pan) catching fire is used to demonstrate the conditions required to start combustion, and how to put such a fire out safely.

Lesson organisation For school use, this must only be done as a demonstration. The demonstrator must have practised the demonstration beforehand until confident that the procedure can be done safely in front of a class, when it should take 10-15 minutes.


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The teacher will require: Face shield Heat-resistant gloves Crucible (25 mm diameter), nickel or steel (see note 1) Pipeclay triangle to support crucible (see note 1) Tripod Bunsen burner Heat resistant mats (see note 2) Safety screens (see note 3) A small square of hardboard or aluminium Test-tube, fixed securely to end of 1 metre pole (see note 4) Cooking oil, 3 cm3 per demonstration The class will require: Eye protection

Technical notes 1 Wedge the nickel crucible firmly and upright in the pipe-clay triangle on the tripod. It must not tip over when the flame is smothered by the demonstrator. The wires of the triangle may need to be bent over the tripod. 2 Protect the demonstration bench from hot burning fat by covering with an array of heat resistant mats. 3 Arrange safety screens and secure in place so they protect both demonstrator and the class. 4 The test-tube should be held firmly, e.g. in a test-tube holder, and strapped to the end of a long pole (a metre rule will suffice) so that the tube will not fall off when engulfed in flame during the demonstration.

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Procedure HEALTH & SAFETY: Teacher to wear face shield and heat resistant gloves. Class to wear eye protection and must be kept not less than 4 metres back. Safety screens must be positioned and secured to protect both students and the demonstrator. The experiment must not be sited below a light fitting.

•

•

25mm diameter tall nickel crucible cooking oil

pipe clay triangle


heat resistant mat

a Place about 5 cm3 of water in the test-tube ready for use during the demonstration. b Pour 3 cm3 of cooking oil into the crucible and place a lighted Bunsen burner beneath it. c Once the oil catches fire, switch off the gas supply to the Bunsen burner and extinguish the flame by placing a small square of hardboard or aluminium over the crucible to simulate placing a tray over a burning chip pan to remove the oxygen from the fire. d Explain that a damp tea-towel would also extinguish the fat-pan fire. In this demonstration this method is unsuitable, as it could knock the apparatus off the tripod, but for a real fat-pan fire it is a good method. e Remove the square and light the Bunsen burner again until the cooking oil re-ignites. f Switch off the gas supply to the Bunsen burner, and holding the pole with the testtube containing water attached at arm’s length, add the water to the burning oil. This will cause a ball of fire to rise about a metre, effectively demonstrating the hazard of attempting to put out a fat-pan fire with water.

Teaching notes The procedure described is hazardous and risks are made acceptable only by adherence to the restrictions and precautions advocated. In addition to the precautions described above: the demonstration must NOT be done in a fume cupboard,

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the quantities prescribed must NOT be exceeded; do NOT be tempted to use more cooking oil'

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a squat crucible must NOT be used as it ejects the hot fat sideways,

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a porcelain crucible is NOT safe as it is liable to break.

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This demonstration can be linked to the teaching of the ‘fire triangle’ as well as to the more general aspects of assessing risks and taking action to reduce risks to themselves and others. Chip pan fires cause one-fifth of all accidental dwelling fires in the UK each year. As well as the damage they can cause to people’s homes, these fires also injure around 4,000 people every year.

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Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/energy-changes-and-fuels/fatpan-fire,228,EX.html This in turn has been reproduced from CLEAPSS® Guide, L195: 'Safer Chemicals, Safer Reactions', section 9.5 by permission of CLEAPSS®

Useful resource The West Sussex Fire and Rescue Service outlines the dangers and provides advice on how to avoid accidents. http://www.westsussex.gov.uk/ccm/content/emergency-services/west-sussex-fire-and-rescueservice/safety-at-home/chip-pan-safety.en (Website accessed December 2009)

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Money to burn

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A piece of paper (or a £5 or £10 note) soaked in a mixture of ethanol and water is ignited. The ethanol burns but the paper does not.

Lesson organisation This is a demonstration experiment which can either be used for fun as part of a public event or in a class to stimulate discussion of the conditions required for combustion.

Apparatus and chemicals The quantities given are for one demonstration. The teacher requires:

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Eye protection Bunsen burner Pair of tongs Heat-resistant mats, 2 Beakers (250 cm3), 3 Paper, e.g. filter paper (see note 1) Access to: Ethanol (Highly flammable) or Industrial Denatured Alcohol (IDA) (Highly flammable, Harmful), 75 cm3 Sodium chloride, NaCl (common salt) (Low hazard), about 1 g

Technical notes Ethanol (Highly flammable) Refer to CLEAPSS® Hazcard 40 Industrial Denatured Alcohol (IDA) (Harmful, Highly flammable) Refer to CLEAPSS® Hazcard 40 Sodium chloride (Low hazard) Refer to CLEAPSS® Hazcard 47B 1 Prepare some pieces of absorbent paper, e.g. filter paper, about the size of a £5 note. 2 Place about 50 cm3 of water in one beaker, a similar volume of ethanol in a second beaker, and a mixture of 25 cm3 water and 25 cm3 ethanol in the third beaker. Add a little (about 1 g) of sodium chloride to the third beaker and stir until it has all dissolved. Label the beakers.

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Procedure HEALTH & SAFETY: Wear eye protection a Label the beakers. b Place the Bunsen burner on the heat-resistant mat and adjust it to give a yellow flame. Ensure that the beakers of ethanol, water, and the ethanol-water mixture are a safe distance (2 m) away from the Bunsen burner. c Using the tongs, soak one piece of paper in the water in the first beaker. Allow the paper to drain. Try to ignite it by holding it in the Bunsen flame for a few seconds. It does not ignite. d Soak a second piece of paper in ethanol and use the tongs to hold it in the Bunsen flame just long enough for it to ignite. Take care to drip as little alcohol as possible on the bench between the beaker and the Bunsen burner. The alcohol on the paper ignites easily and sets fire to the paper, which burns away. (If the alcohol in the beaker does ignite by accident during the demonstration, it can be easily and safely extinguished by covering the beaker with a heat-resistant mat.) e Soak the third piece of paper in the alcohol-water mixture and use the tongs to hold it in the Bunsen flame just long enough for it to ignite. Swiftly remove the paper from the Bunsen flame and observe as the alcohol burns with a yellow flame, but the paper does not burn. The paper will still be wet with water after the alcohol has burnt away.

Teaching notes A wealthy and/or confident demonstrator can start this experiment with a £5, or even higher value, note and the alcohol-water mixture! More amusement can be added if a member of the audience ‘with money to burn’ can be persuaded to part with the money. It is important to use a yellow Bunsen flame, and to only hold the paper in the flame long enough for it to ignite, to prevent the note from burning. The demonstrations with ordinary paper and the other liquids could then follow to provide an explanation. The water in the alcohol-water mixture evaporates as the alcohol burns, keeping the temperature of the paper below its ignition temperature (approximately 230°C). Converting this temperature to the Fahrenheit scale might remind some science fiction fans in the audience of the film ‘Fahrenheit 451’.1 The flame from the paper soaked in alcohol alone should be visible but the flame from a burning alcohol-water mixture is often difficult to see. This is why the sodium chloride is added, to give an orange-yellow colour to the flame. The demonstration looks even more impressive in subdued lighting.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/money-to-burn,49,EX.html

Useful resources 1

Film: 'Farenheit 451: http://en.wikipedia.org/wiki/Fahrenheit_451

To view a video clip of this demonstration experiment, visit: http://media.rsc.org/videoclips/demos/nonburningnote.mpg (Websites accessed December 2009)

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The methane rocket

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Description A strong plastic bottle is filled with a 2 : 1 ratio of oxygen to methane and the mixture ignited with the bottle standing on a suitable ‘launch pad’. The mixture ignites with a loud bang and the bottle flies several metres. This experiment takes around ten minutes.

Apparatus and chemicals Eye protection Ear protection

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A carbonated soft drink bottle of between 300 cm3 and 500 cm3 capacity. A rubber bung to fit the bottle. A large trough or washing up bowl. Measuring cylinder, 500 cm3. Rubber tubing to fit the gas tap Access to an oxygen cylinder or other source of oxygen. Canisters of oxygen can be bought on the internet.

Procedure Health & Safety: Both demonstrator and audience should wear eye protection. The demonstrator should wear ear protectors and the audience should be advised to cover their ears. Use each bottle for one demonstration only.

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Your employer’s risk assessment should be consulted before carrying out this activity. This activity is covered by model (general) risk assessments widely adopted for use in UK schools such as those provided by CLEAPSS®, SSERC and ASE. Bear in mind, however, that these may need some modification to suit local conditions. Before the demonstration a Select a suitable place to fire the bottle. If launched approximately horizontally it may well fly the whole length of a typical laboratory so make sure that there are no obstacles (such as reagent bottles) that it might hit – a corridor might be a better choice than a laboratory. b Also prepare a launch pad. For instance, open a fairly heavy paperback book in the middle and place it, covers down, on a table. The bottle can be placed in the V formed between the left hand and right hand pages and this can be pointed in a suitable direction to aim the rocket when it is ignited.

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The demonstration c Fill a plastic carbonated drinks bottle of between 300 cm3 and 500 cm3 capacity with water and pour the water into a measuring cylinder to determine its total volume. d Pour one-third of the bottle’s volume of water back into the bottle and mark the level with a waterproof pen. It is important that a carbonated drink bottle is used; bottles used to contain still drinks may not be strong enough. Each bottle should be used for one demonstration only as it may be weakened by the explosion. e Next completely fill the bottle and invert it in a trough or washing-up bowl of water. f Place the end of a rubber tube connected to the gas tap under the neck of the bottle. and fill the bottle to the marked level with methane (natural gas) from the gas tap. Remember to turn the tap on for a few seconds to allow air in the tube to be displaced before starting to fill the bottle. Now fill the rest of the bottle with oxygen from the chosen source, again remembering to displace air in the delivery tube for a few seconds before starting to fill. The bottle now contains a 2 : 1 mixture of oxygen and methane by volume. This is the stoichiometric mixture. g Stopper the bottle with a rubber bung and place the bottle on the launch pad. Check the aiming of the rocket and ensure than none of the audience is near the flight path. h Wear eye and ear protection and advise the audience to cover their ears. Remove the bung and ignite the gas mixture by applying a lighted splint to the neck of the bottle. The rocket will take off with a loud bang and fly for several metres.

If a second flight is to be done, use a new bottle.

Teaching notes After firing, the rocket can be recovered and shown to the audience to point out that it is covered on the inside by condensation – droplets of water formed in the reaction.

Theory The reaction is CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) ΔH = -890 kJ mol-1 The gases react in a 2 : 1 ratio and the reaction is strongly exothermic. Note that there are three moles of gas on both sides of the equation so all the force that propels the rocket comes from the expansion of the gases as they are heated by the energy given out by the reaction, rather than by the production of extra molecules of gas. Please note: You can get students to balance the equation and try to work out for themselves what the ratio should be, then fill the bottle and launch. The best bang will be from a stoichiometric mix.

References This experiment has been reproduced from the RSC video material for teachers of chemistry http://www.rsc.org/education/teachers/learnnet/videoclips.htm

Useful resources A similar experiment, the hydrogen rocket, is described in T. Lister, Classic Chemistry Demonstrations, London: Royal Society of Chemistry, 1995. This describes ignition using an electric spark, a method which could be adapted for this experiment. To view a video clip of this experiment visit http://www.rsc.org/education/teachers/learnnet/videoclips.htm

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The thermal decomposition of nitrates – ‘writing with fire’

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A message is written on filter paper with a solution of sodium nitrate and is then dried, rendering it invisible. Applying a glowing splint to the start of the message makes the treated paper smoulder and the message is revealed as the glow spreads its way through the treated paper.

Lesson organisation The demonstration takes about 10 -15 minutes. It could be a student activity, but with a large class it will need a well ventilated laboratory . The message drawn on the paper should be such that when the treated areas burn through, the letters, and the sheet of paper as a whole, remain intact.

Apparatus and chemicals Eye protection Filter or blotting paper sheets – as large as possible Wooden splints, Hot-air blower, e.g. Hair dryer (see note 1) Small paint brush Beaker (100 cm3) Stirring rod

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Sodium nitrate(V) (Oxidising, Harmful), about 10 g

Technical notes Sodium nitrate (Oxidising, Harmful) Refer to CLEAPSS® Hazcard 82 1 Make sure the hot-air blower is electrical safety tested. If a hot-air blower is not available, judicious use of a Bunsen flame or an oven provides an alternative method for drying the paper.

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Procedure

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HEALTH & SAFETY: wear eye protection

Before the demonstration Make a saturated solution of sodium nitrate by adding about 10 g of solid to 10 cm3 of water and stirring.

The demonstration a Using a small paintbrush (or a length of wooden splint), write a message on the absorbent paper. Use joined up writing! Design the message so that the sheet of paper will remain in one piece after the message has burnt through. b Thoroughly dry the message using a hot-air blower, or by holding the paper well above a Bunsen flame. The message will be virtually invisible, so mark the start of it with a light pencil mark. c Fix the paper where the audience can see it easily, and away from combustible material. d Apply a glowing splint to the start of the message until the treated paper starts to glow and char. e Remove the splint and watch as the glow and charring work their way along the message, leaving the untreated paper untouched.

Teaching notes If lesson time is limited, the writing of the message and the drying process could be carried out before the demonstration begins. This experiment could be used to introduce the fire triangle: fuel, oxygen and energy. With older students, the demonstration could be used to revise the equations for the decomposition of nitrates. In this particular case, sodium nitrate decomposes to give sodium nitrite (sodium nitrate(III)) and oxygen, and it is the oxygen released which helps to promote the burning process and produce the glow and charring: 2NaNO3(s) → 2NaNO2(s) + O2(g) Most other nitrates will also produce a similar effect, but potassium nitrate is less effective because it is less soluble and some other nitrates may give off very toxic nitrogen dioxide when they decompose.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/energy-changes-and-fuels/thethermal-decomposition-of-nitrates,280,EX.html

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Iron and sulfur reaction

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This demonstration or class experiment shows the exothermic reaction of two elements, iron and sulfur, to form the compound, iron sulfide. The two solids are mixed and heated in a test-tube (or ignition tube). The reaction can be used to illustrate elements, mixtures and compounds.

Lesson organisation This reaction can be carried out as a demonstration or class experiment in a well-ventilated laboratory provided that the instructions provided here are strictly adhered to. The reaction can be carried out in borosilicate glass test-tubes as a demonstration or in smaller (ignition) tubes by students. The reaction provides an opportunity to show that the properties of a compound are different from its constituent elements. The reaction must not be carried out on tin lids in the open laboratory as is suggested in some sources. The sulfur may boil or burn releasing sulfur dioxide which is a Toxic and Corrosive gas and may trigger an asthmatic attack.

Apparatus and chemicals Eye protection Balance (1 or 2 d.p.)

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For the demonstration the teacher will need: Test-tube made from borosilicate glass (e.g. Pyrex) Bunsen burner Heat resistant mat Clamp stand and clamp Spatulas, 2 Small bar magnet Watch glass Filter paper, 2 pieces (or use 2 weighing boats) Mineral wool (or mineral fibre) Iron powder (potential Irritant) (see note 1) Sulfur – finely powdered roll or flowers (Low hazard) (see note 2) For the class practical each group of students will need: Prepared ignition tube (see note 3) Bunsen burner Heat resistant mat Test-tube tongs

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Technical notes Iron powder – this can cause severe irritation in eyes because the iron oxidises rapidly in the saline environment. Refer to CLEAPSS® Hazcard 55A. Sulfur is Low hazard. Refer to CLEAPSS® Hazcard 96A. Sulfur dioxide is formed if the sulfur catches fire. This is Toxic by inhalation and Corrosive. Refer to CLEAPSS® Hazcard 97. 1 Iron powder is preferred to iron filings. If fine sulfur powder is mixed with iron filings, it is difficult to obtain a consistent mix, because the two solids can separate. 2 Roll sulfur or flowers of sulfur should be finely powdered using a pestle and mortar. 3 Ignition tubes (75 mm x 10 mm test-tubes) should be filled to no more than one-quarter full with the iron – sulfur powder mix (see Procedure note a). 0.2 g of the mixture is sufficient for the effect to be seen. Place a small plug of mineral wool in the mouth of each ignition tube. After the experiment, the iron(II) sulfide is low hazard and can be discarded into the refuse.

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Procedure HEALTH & SAFETY: Wear eye protection throughout and ensure that the lab is wellventilated.

Demonstration a Prepare a mixture containing iron powder and sulfur powder in the ratio 7:4 by mass. Do this by weighing out 7 g of iron powder and 4 g of finely powdered sulfur onto separate pieces of filter paper (or use weighing boats). Mix the two powders by pouring repeatedly from one piece of paper to the other until a homogeneous mixture (by appearance) is obtained. b Note the appearance of the pure elements and the mixture. Demonstrate that iron can be separated from the mixture by physical means. Do this by wrapping the end of a small bar magnet in a paper tissue or cling film, and dipping it into a teaspoon-sized heap of the mixture on a watch glass. The iron will be attracted, but the sulfur remains on the watch glass. c Place about 2 g of the mixture into a borosilicate test-tube. d Insert a plug of mineral wool (mineral fibre) into the mouth of the test-tube. Clamp the test-tube as shown in the diagram. e Heat the powder mixture at the base of the testtube – gently at first and then more strongly (use a blue flame throughout). Heat until an orange glow is seen inside the test-tube. Immediately stop heating. Let the students see that the glow continues and moves steadily through the mixture.

clamp 2g of iron/sulfur mix

mineral wool plug

f Allow the tube to cool down. At this point the students could carry out their own small-scale version of the reaction. g Once cool, it is possible to break open the test-tube to show the appearance of the product, iron(II) sulfide. The test-tube can be broken open using a pestle and mortar. It is advisable to wear protective gloves.

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h It may be possible to show that the product, iron(II) sulfide is non-magnetic. However, this is not always successful. It has been suggested that using a very weak magnet is advisable.

Class practical a Students should be provided with pre-prepared ignition tubes containing the iron–sulfur mixture and a mineral wool plug. b Using suitable tongs or test-tube holders, the iron-sulfur mix in the tube should be heated until the mixture just starts to glow. Bunsen burners should then be turned off. c The reaction tubes should be left to cool on the heat resistant mat. It may be sensible to get the students to place all their used reaction tubes onto one heat resistant mat set aside for this purpose (e.g. on the teacher’s desk or in a fume cupboard).

Teaching notes On heating the reaction mixture, the sulfur melts and reacts with the iron exothermically to form iron(II) sulfide. The mineral wool plug in the mouth of the test-tube prevents sulfur vapour escaping and possibly catching fire. If, despite all precautions, the sulfur vapour does ignite, students must be trained to extinguish it by placing a damp rag firmly over the mouth of the tube. The signs that a chemical reaction occurs are: the glow, and the fact that a new substance (black iron sulfide) is formed which cannot be separated by using a magnet. Although see note h in the Procedure section. This may be an opportunity to introduce or reinforce the ‘rule’ that if only two elements are combined together, the name of the compound ends in ‘-ide’.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/elements-compounds-andmixtures/iron-and-sulfur-reaction-demonstration-and-class-experiment,88,EX.html

Useful resource Clip on the Demonstrating Chemistry - Exciting Elements 1 video, free to download from http://www.chemsoc.org/networks/learnnet/videoclips.htm.


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The reaction between zinc powder and sulfur A reaction between zinc and sulfur can be used to demonstrate that chemical changes are often accompanied by a large change in energy

Introduction The reaction between iron and sulfur is often used to demonstrate that the properties of the products of a chemical reaction are quite different from the reactants, are difficult to separate to form the reactants, unlike mixtures, and there is often a large change in energy involved in the formation of the product. The reaction between iron and sulfur is suitable for a class practical. It often helps to reinforce the ideas by demonstrating the reaction between zinc and sulfur.

Lesson organisation This experiment works well as a class demonstration. The demonstration takes about 5 minutes.


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Eye protection Access to a fume cupboard Test tube Pyrex (or boiling tube) Metal test tube holder Bunsen burner Weighing boat Spatula (2) Top pan balance (1 dp) 0.1 g Zinc powder,(Highly flammable, Refer to CLEAPSS® Hazcard 107) (see note 1) 0.1 g Sulfur powder (Low hazard, Refer to CLEAPSS® Hazcard 86A) (see note 2) 10 g Mineral wool

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Technical notes 1 Zinc powder or dust can be very reactive. It may be supplied in different states of fineness, and it may have become oxidised and be mainly zinc oxide. For that reason the reactivity seen from any given sample can be very different. 2 Sulfur may be supplied as crushed roll sulfur, flowers of sulfur, precipitated sulfur or resublimed sulfur. All are suitable, but resublimed sulfur seems to react more vigorously

Procedure HEALTH & SAFETY: Wear eye protection, do reaction in a fume cupboard

The demonstration

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a Measure out 0.1 g of zinc powder into a weighing boat. b Measure out 0.1 g sulfur powder into the weighing boat. c Mix the two powders to form a uniform mix. d Put the powder into a Pyrex test tube. e Fit a mineral wool plug to the top of the test tube. f Light the Bunsen and adjust to a blue working flame. g Holding the tube with the test tube holders, heat the mixed powders and direct the mouth of the tube towards the inner corner of the fume cupboard, until the reaction occurs.

Teaching notes The reaction between fresh zinc powder and sulfur can give a very bright flash. On this scale it is harmless, but makes an impressive comparison. If the reaction is not impressive, the zinc has oxidised. The reaction between magnesium powder and sulfur can be explosive.



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Reaction between aluminium and iodine The reaction between aluminium and iodine is catalysed by water. This is a spectacular demonstration as clouds of purple iodine vapour are produced.

Lesson organisation This experiment must be done as a demonstration, in a fume cupboard. The experiment takes only 5 minutes if the iodine is ground and weighed in advance. You should try the experiment in advance if you have not done it before.


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Eye protection Access to a fume cupboard (see note 2) Each demonstration requires: Mortar and pestle Tin lid (or other metal container) (see note 1) Heat resistant mat Teat pipette Aluminium powder (fine) (Highly flammable), about 3 g Iodine (Harmful, Danger to the environment), about 5 g Warm water

Technical notes Aluminium powder (fine) (Highly flammable) Refer to CLEAPSS® Hazcard 1 Iodine (Harmful, Danger to the environment) Refer to CLEAPSS® Hazcard 54A 1 A tin lid is an excellent piece of apparatus for this reaction, but any other piece of flat metal can be used. The reaction produces a lot of heat, so protecting the floor of the fume cupboard is essential. 2 Check that the fume cupboard works before starting. Tape a length of tissue paper or ribbon onto the sash to see that air is drawn in.

Procedure

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HEALTH & SAFETY: Wear eye protection. The demonstration must be done in a fume cupboard a Finely grind about 5 g of iodine in the mortar. b Carefully mix the iodine with approximately the same volume of aluminium powder and place the mixture on the tin lid as a mound. c Put one or two drops of warm water onto the top of the mound using the teat pipette. There can be an induction period before the reaction starts but if there appears to be nothing happening add another one or two drops of water. A little detergent in the water assists wetting.

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d When the reaction starts, clouds of purple iodine vapour are released as heat is generated. At this point the fume cupboard should be switched on, as iodine vapour is toxic. The mixture then bursts into flame, producing a white smoke together with the iodine vapour, and leaving a glowing, white residue of aluminium iodide.

Teaching notes It is important to try this experiment before doing it as a demonstration, as different samples of aluminium powder can react differently. The induction period for some samples can be quite long. However, this is an impressive and spectacular demonstration, proving that water can be a catalyst. It also shows that aluminium is a very reactive metal, and that its usual unreactive nature is due to the surface oxide layer. The chemical properties of iodine are very similar to those of bromine and chlorine. However, its reactions are far less vigorous. It can also act as an oxidant for a number of elements such as phosphorus, aluminium, zinc and iron, although increased temperatures are generally required. Oxidation of finely dispersed aluminium with iodine can be initiated using drops of water. The reaction is strongly exothermic, and the excess iodine vaporises, forming a deep violet vapour. The reaction is: 2Al(s) + 3I2(s) → Al2I6(s) Anhydrous aluminium halides, such as the aluminium iodide produced here, react vigorously with water, sometimes violently if freshly prepared and still hot, releasing fumes of corresponding hydrogen halide. The residue should be disposed of in a fume cupboard, after allowing it to cool completely, by adding small amounts to 1 mol dm-3 sodium carbonate solution, allowing the reaction to subside between additions. The resulting slurry can then be disposed of with plenty of water.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/enhancement/spectacular-demonstrations/ reaction-between-aluminium-and-iodine,121,EX.html

Useful resource There are a number of websites with information on this reaction and these can found using a Google search.

Health & Safety checked, February 2008 Updated 30 Aug 2009

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Reaction of zinc with iodine This experiment involves the synthesis of a metal salt by direct reaction of a metal and a non-metal. Zinc powder is added to a solution of iodine in ethanol. An exothermic redox reaction occurs, forming zinc iodide, which can be obtained by evaporating the solvent. Zn + I2 → ZnI2 The experiment can be extended to show the decomposition of a compound into its elements by electrolysing the zinc iodide.

Lesson organisation This experiment can be used to illustrate the differences between metallic and non-metallic elements and their reaction to form a compound – a metal salt – with new properties. The reaction can be easily reversed using electrolysis to decompose the compound back into its elements. These are easily recognisable from their distinctive appearances. Both parts of the experiment can be done either as demonstrations or as class experiments. Each part should take about 10 mins as a demonstration; longer as a class experiment.

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Apparatus and chemicals Eye protection Each group (or demonstration) requires: Test-tubes (100 x 16 mm), 3 Test-tube bung Test-tube rack Measuring cylinder (10 cm3) Small filter funnel Filter paper Teat pipette Thermometer (0–100 °C) Spatula Watchglass Weighing boat or suitable container for zinc powder Chemicals are for one demonstration or one group of students. Solid iodine (Harmful, Dangerous for the Environment), about 0.5 g (see note 1) Zinc powder (Highly Flammable. Contact with water can also release a flammable gas, dangerous for the environment), about 0.5 g (see note 2) Ethanol (Highly Flammable) or IDA (Industrial Denatured Alcohol) (Highly Flammable, Harmful) about 5 cm3

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For the extension work Beaker (100 cm3) Pair of graphite electrodes mounted in a rubber bung Electrical leads and crocodile clips Source of 3–6 V DC, either battery or power supply Torch bulb in a suitable holder Distilled water, about 20 cm3 Spatula Access to a little dilute (about 1 M) hydrochloric acid (Low Hazard at this concentration) or sulfuric acid (Irritant at this concentration)

– +

zinc iodide solution

graphite electrodes

Technical notes Solid iodine (Harmful, Dangerous for the Environment) Refer to CLEAPSS® Hazcard 54A Zinc powder (Highly Flammable, Contact with water can also release a flammable gas, Dangerous for the environment) Refer to CLEAPSS® Hazcard 107 Ethanol (Highly Flammable) Refer to CLEAPSS® Hazcard 40A or IDA (Highly Flammable, Harmful) Refer to CLEAPSS® Hazcard 40A Zinc iodide (Irritant, Dangerous for the environment) No CLEAPSS® Hazcard but similar to 108A Hydrochloric acid (Low Hazard at concentration used) Refer to CLEAPSS® Hazcard 47A and Recipe card 31 Sulfuric acid (Irritant at concentration used) Refer to CLEAPSS® Hazcard 98A, Recipe card 69 and L195 Safer chemicals, safer reactions 1 The solid iodine should be powdered by grinding in a mortar in a fume cupboard. For a class experiment a stoppered test-tube containing 0.5 g of powdered iodine should be supplied to each group of students. 2 For a class experiment each group of students should be supplied with a pre-weighed sample of 0.5 g zinc powder in a weighing boat or a test-tube.


Synthesis of zinc iodide

•

a Measure out 5 cm3 of ethanol using a measuring cylinder. Place a thermometer in the ethanol and record the temperature. b Add the ethanol to 0.5 g of powdered iodine in a test-tube. Stir carefully, using the thermometer, to dissolve the iodine. The solution should be dark brown. Note the temperature. c When all the iodine has dissolved, slowly add the zinc powder using a spatula and stir the mixture with the thermometer. The temperature should rise, indicating an exothermic reaction. When the reaction is finished, the colour of the iodine should have faded and excess zinc will be left. If not, add further small amounts of zinc powder and stir until the brown colour due to iodine has gone. d Filter the solution into another test-tube. Using a teat pipette, transfer a few drops of the filtrate on to a watchglass and allow the solvent to evaporate. This can be speeded up by placing the watchglass on a beaker containing some hot water. Zinc iodide will be left as a white solid.

105

Decomposition a Pour the remainder of the solution containing the zinc iodide into a 100 cm3 beaker. Add about 20 cm3 of distilled water and stir to mix. b Clamp the bung carrying the two graphite electrodes over the beaker, so that the bottoms of the electrodes are immersed as far as possible in the solution. It may be easier just to rest the bung in the beaker so that the electrodes touch the bottom. c Using the leads and crocodile clips, connect the electrodes and the bulb in series and then to the power supply as shown in the diagram on this page. The bulb should glow to show that the circuit is complete, and that electrolysis is occurring. d If the bulb does not glow, raise the bung out of the solution and check the connections by touching both electrodes at once with a metal spatula. If the bulb lights up, put the electrodes back into the solution. If there is still no indication of electrolysis, add a small amount of zinc iodide from the watchglass to the solution and stir. Repeat until the bulb starts to glow. e Allow electrolysis to continue for a few minutes. Note any changes occurring around the electrodes in the solution – a brown colour (due to iodine) should develop in the solution around the positive electrode. There may be some effervescence at the negative electrode. f Disconnect the power supply. Lift the electrodes out of the solution. Wash them under a tap. The bottom of the negative electrode should be covered with a silver-grey layer of zinc metal. The zinc deposit can be tested (and removed) by immersing the tip of the electrode in a little dilute acid. It reacts, giving off a colourless gas (hydrogen).

Teaching notes This reaction shows the synthesis of a compound from two elements, each with their own distinctive appearance and properties. (A practical worksheet could involve drawing up a table of properties (type of element, appearance, and so on) for each of the elements and the compound formed.) The reaction can also be used to illustrate the direct reaction of a typical metal and nonmetal. It is one of the few reactions of the halogens (Group 7) with a metal that students can do safely themselves. A useful extension of this experiment is the decomposition, by electrolysis, of the compound formed back into its elements.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/elements-compounds-andmixtures/reaction-of-zinc-with-iodine,119,EX.html

Useful resource At this website, you can find images and a movie showing this experiment: http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA1/R1MAIN/CD1R1260.HTM#1320

106

Health & Safety checked, October 2007 Updated 29 Oct 2008

36

The combustion of iron wool Iron wool is heated in air on a simple ‘see-saw’ balance. The increase in mass is seen clearly.

Lesson organisation This demonstration takes around 5 minutes once it has been set up.


•

For one demonstration: Eye protection Bunsen burner Heat resistant mat Wooden metre rule (see note 1) Aluminium cooking foil, about 10 cm x 10 cm Retort stand, boss and clamp Plasticine, few grams Knife edge, triangular block or something similar Steel wool (Low hazard), about 4g

Technical notes Steel wool (Low hazard) Refer to CLEAPSS® Hazcard 55A 1 A shallow groove cut across the width of the ruler at the 50 cm mark will help when balancing it on the knife edge. Cover the end of the meter ruler with foil to protect it from the Bunsen burner.

Procedure a Cover one end of the meter ruler with foil to protect it from the Bunsen burner. Take about 4 g of steel wool and tease it out so that the air can get around it easily. Use a few of the strands to attach it to the end of the ruler.

•

b Balance the ruler on a knife edge or triangular block at the 50 cm mark. Weight the empty end with plasticine until this end is just down (see the diagram). This part is critical. steel wool

Plasticine

before

after

foil to protect ruler

knife edge

c Place a heat resistant mat underneath the steel wool. d Wear eye protection. Light the Bunsen burner and heat the steel wool from the top with a roaring flame. It will glow and some pieces of burning wool will drop onto the heat resistant mat. Heat for about a minute by which time the meter ruler will have overbalanced so that the iron wool side is down.

107

Teaching notes As you are setting up, ask the students whether they think the iron wool will go up, down or remain the same. Many will predict a weight loss. If fine steel or iron wool is used then it may be possible to light it using a splint.

Equation: Iron + oxygen → iron oxide 2Fe(s) + 3/2 O2(g) → Fe2O3(s) Please note: In some circumstances the steel wool has been known to fall off the ruler during the practical – to avoid this a small tray (around 15 cm2) can be made from the foil used to protect the ruler.

Reference This experiment has been adapted from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/elements-compounds-andmixtures/the-combustion-of-iron-wool,206,EX.html

Useful resource See also experiment 37: The change in mass when magnesium burns.

108

Health & Safety checked, April 2008 Updated 10 Apr 2008

The change in mass when magnesium burns

37

Magnesium is weighed and then heated in a crucible. It reacts with oxygen to produce the oxide. It can be shown that there has been an increase in mass. The results can be used to find the formula of magnesium oxide and two methods are described for doing so.

Lesson organisation The practical activity takes around 30-45 minutes, depending on the competence of the class. Students should all be standing and should wear eye protection. Students with long hair should tie it back. It is a good idea for students to practice lifting the lid on and off the crucible and the crucible off the pipe clay triangle before they start. This has the added bonus of checking that all the tongs are functioning correctly. To enable students to light their Bunsen burners they will need access to matches or lighters. Alternatively, light one or two Bunsens around the room and students can light their own using a splint. The most significant hazard in this experiment is the hot apparatus. Warn students that it will take some time to cool down. For classes with shorter attention spans, the final step of heating to constant mass could be omitted.

Apparatus and chemicals Per pair or group of students: Eye protection

•

Crucible with lid Tongs Pipe clay triangle Bunsen burner Tripod Heat resistant mat Emery paper (optional) Magnesium ribbon (Low hazard), about 10-15 cm (see note 1) Access to: Balance (2 d.p.)

Technical notes Magnesium ribbon (Low hazard) Refer to CLEAPSS® Hazcard 59A 1 Fresh, clean magnesium is best for this experiment. If the magnesium is tarnished then emery or sand paper will be required to clean it.

109

Procedure

•

HEALTH & SAFETY: Wear eye protection a Cut a piece of magnesium about 10-15 cm long. If it is looking tarnished or black then clean it using the emery paper. Twist it into a loose coil. b Weigh the crucible with the lid (mass 1) and then the magnesium inside the crucible with the lid (mass 2). c Set up the Bunsen burner on the heat resistant mat with the tripod. Place the pipe clay triangle over the tripod in a ‘star of David’ formation, ensuring that it is secure. Place the crucible containing the magnesium in the pipe clay triangle and put the lid on. crucible magnesium

pipe clay triangle


heat resistant mat

d Light the Bunsen burner and begin to heat the crucible. It is best to start with a gentle blue flame, but you will need to use a roaring flame (with the air hole fully open) to get the reaction to go. e Once the crucible is hot, gently lift the lid with the tongs a little to allow some oxygen to get in. You may see the magnesium begin to flare up. If the lid is off for too long then the magnesium oxide product will begin to escape. Don't let this happen. f Keep heating and lifting the lid until you see no further reaction. At this point, remove the lid and heat for another couple of minutes. Replace the lid if it appears that you are losing some product. g Turn off the Bunsen burner and allow the apparatus to cool. h Re-weigh the crucible with lid containing the product (mass 3). i Heat the crucible again for a couple of minutes and once again allow to cool. Repeat this step until the mass readings are consistent. This is called ‘heating to constant mass.’

110

Teaching notes Students should have recorded the following masses: (mass 1) Crucible + lid (mass 2) Crucible + lid + magnesium (mass 3) Crucible + lid + product This should allow them to calculate the mass of the mass of the magnesium (mass 2) - (mass 1) and the mass of the product (mass 3) - (mass 1). They could also calculate the increase in mass (mass 3) - (mass 2), which corresponds to the mass of oxygen. The equation is: Magnesium + oxygen → magnesium oxide 2Mg + O2 → 2MgO. Students sometimes get unconvincing results to this experiment. It is worth evaluating what they have done as there are several reasons why their results may be disappointing: the magnesium oxide product may escape as they lift the lid

●

not all the magnesium may have reacted (the product may still look a bit grey rather than white)

●

they may have prodded the product with their splint so not all of it got weighed (more common than you might expect!)

●

not tareing the balance correctly for one of the weighings

●

having the magnesium coiled too tightly so that not all of it reacts.

●

Finding the formula of magnesium oxide Method 1 To find the formula of magnesium oxide, students will need the mass of the magnesium and the mass of the oxygen. They will also require the relative atomic masses. Magnesium is 24 and oxygen is 16. They should divide mass by the atomic mass for each element. The gives the number of moles of each. Having done this for both elements, they should find the ratio between the two by dividing them both by the smallest number. The ratio should be close to 1:1 as the formula of magnesium oxide is MgO. Example calculation Mass magnesium = 2.39 g Mass magnesium oxide = 3.78 g So mass oxygen = 1.39 g Number moles Mg = 2.39/24 = 0.0995 Number moles O = 1.39/16 = 0.0868 Divide by the smallest to give the ratio aprox. 1 Mg: 1 O This would suggest a formula of MgO, which is the correct formula.

111

Method 2 Students will need the mass of the magnesium and the mass of oxygen which has combined with it. You will need a copy of the graph below for the class. All students plot their masses of magnesium and oxygen onto the graph. The majority of the class’ results should fall on or near the line representing the formula MgO, a 1:1 ratio. This helps to show clearly any anomolous results and should help to convince students who are disappointed by a 1:1.25 ratio, for instance, that the correct formula really is MgO. 0.6

Mass of oxygen (g)

0.5 MgO2

0.4

MgO

0.3 Mg2O

0.2 0.1 0 0

0.1

0.2

0.3 0.4 0.5 Mass of magnesium (g)

0.6

0.7

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/elements-compounds-andmixtures/the-change-in-mass-when-magnesium-burns,207,EX.html

Useful resource You may also wish to look at experiment 36: The combustion of iron wool.

112


Competition for oxygen

38

Description Mixtures of metals and metal oxides are heated over a Bunsen burner flame. Students observe the reactions and decide if a reaction occurs. Students should understand the idea of competition for oxygen. This experiment can be used as an introduction to the reactivity series but it can also be used when students know how to predict the outcome of the reaction from the reactivity series. This experiment should take around 60 minutes.

Apparatus and chemicals Per group of students: Eye protection

•

Bunsen burner Tripod Pipe clay triangle Crucible Tongs 0.5 g magnesium oxide (Low hazard) and 0.5g iron (Low hazard) mixture 0.5 g lead(II) oxide (Toxic and Dangerous for the Environment) and 0.5g iron (Low hazard) mixture 0.5g lead(II) oxide (Toxic and Dangerous for the Environment) and 0.5g zinc (Highly flammable and Dangerous for the Environment) mixture 0.5g copper(II) oxide zinc (Harmful and Dangerous for the Environment) and 0.5g zinc (Highly flammable and Dangerous for the Environment) mixture

Technical notes Magnesium oxide (Low hazard) Refer to CLEAPSS® Hazcard 59B Lead(II) oxide (Toxic and Dangerous for the Environment) Refer to CLEAPSS® Hazcard 56 Copper(II) oxide (Harmful and Dangerous for the Environment) Refer to CLEAPSS® Hazcard 26 Zinc (Highly flammable and Dangerous for the Environment) Refer to CLEAPSS® Hazcard 107 Iron (Low hazard) Refer to CLEAPSS® Hazcard 55A If iron powder is used this is highly flammable. Refer to CLEAPSS® Hazcard 55A 1 Under no circumstances should a mixture of copper oxide and magnesium be heated by students (see CLEAPSS® Hazcard 59A). 2 In the reaction of copper(II) oxide and zinc, students should be told not to stir the mixture with a metal spatula. 3 This experiment uses lots of crucibles, which a school may not have. An alternative is to use beer bottle tops. The plastic insert of the tops should first be removed by heating them strongly in a fume cupboard. 4 Some teachers recommend using ceramic paper rather than crucibles, as some products are difficult to remove from the crucible after reaction. One consideration is that using ceramic paper causes more mess and the Bunsen burners become clogged from spilt powders. 5 Some teachers like to heat the mixture directly from above. Care is needed not to let powder be sprayed by the flame.

113

Procedure

•

Health & safety: Students must wear eye protection. Students must not lean over the reaction mixture. Some of the reactions may be unexpectedly violent. Ensure the room is well ventilated. If the mixtures are not given to the students pre-mixed, then students should be told to place the chemicals on a piece of paper and pass them back and forth to another piece of paper until they are well mixed. The mixture can then be placed in the crucible (or on the beer bottle top). See student worksheet s38 for the diagram and steps in the procedure.

Answers 1

Reaction mixture

Does this mixture react?

Magnesium oxide and iron

No

Lead oxide and iron

Yes

Lead oxide and zinc

Yes

Iron oxide and zinc

Yes

2 Lead oxide + iron → iron oxide + lead Lead oxide + zinc → zinc oxide + lead Iron oxide + zinc → zinc oxide + iron

Reference This experiment has been adapted from Classic Chemistry Experiments, Royal Society of Chemistry, London, p.79-82

114


Student worksheet Competition for oxygen

s38

Introduction This experiment involves the reaction of a metal with the oxide of another metal. When reactions like these occur, the two metals compete for the oxygen. The more reactive metal finishes up with the oxygen (as a metal oxide). If the more reactive metal starts as the oxide then no reaction takes place.

What to record Decide whether a reaction takes place in each case.

Procedure Health & safety: Wear eye protection. Do not lean over the crucible. a Set up the apparatus as shown in the diagram.

•

crucible metal and metal oxide mixture

pipe clay triangle


heat resistant mat

b Place one spatula measure of one of the reaction mixtures into the crucible. c Heat the mixture gently at first and then more strongly. Watch carefully to see what happens but do not lean over the crucible. d Allow the mixture to cool. Look for evidence that a reaction has taken place. e Use your observations to decide which of the two metals has ‘won’ the competition for oxygen – which is more reactive? f Choose another mixture and repeat the experiment.

Questions 1 Complete the following table. Reaction mixture

Does this mixture react?

Magnesium oxide and iron Lead oxide and iron Lead oxide and zinc Iron oxide and zinc

2 Write word equations for any reactions that occur.

115

39

Displacement reactions between metals and their salts Some metals are more reactive than others. In this experiment, a strip of metal is added to a solution of a compound of another metal. A more reactive metal displaces (pushes out) a less reactive metal from its compound. In carrying out the experiment, students investigate competition reactions of metals and arrive at a reactivity series of the four metals they use.

Lesson organisation There are many ways of carrying out this series of reactions. The one described here uses a spotting tile but the same procedure could be adapted for use with test-tubes. The advantages of the spotting tile method include: very small quantities of chemicals are used

●

the whole set of experiments is displayed together, making comparison easier

●

clearing-up afterwards is simple and avoids metal deposits being left in sinks.

●

Careful thought needs to be given to distribution of the chemicals to the class. Solutions could be distributed in test-tubes, or in small bottles fitted with droppers for sharing between several pairs of students. Metals could be issued in sets. The teacher should keep control of the magnesium ribbon, dispensing short lengths when required. There should be no flames alight so that students are not tempted to burn pieces of magnesium and the teacher should be alert to the possibility of pieces of magnesium being removed from the laboratory. The experiment should take about 30 minutes.

•

Apparatus and chemicals Eye protection Each student or pair of students will require: Spotting tile, with at least 16 depressions (or two smaller tiles) Dropping (teat) pipette Beaker (100 cm3) Felt tip pen or other means of labelling Access to about 5 cm3 each of the following 0.1 mol dm-3 metal salt solutions: Zinc sulfate (Low Hazard at this concentration), Magnesium sulfate (Low hazard) Copper(II) sulfate (Low Hazard at this concentration) Lead(II) nitrate (Toxic, Dangerous for the environment) Five samples, approximately 1 cm lengths or squares, of the following metals. The metals, except lead, present are low hazard as used here. Zinc foil Magnesium ribbon Copper foil Lead foil (Toxic, Dangerous for environment)

116

Technical notes Zinc sulfate (Harmful, Oxidising) Refer to CLEAPSS® Hazcard 108. Magnesium sulfate (Low Hazard) Refer to CLEAPSS® Hazcard 59B. Copper(II) sulfate (Harmful) Refer to CLEAPSS® Hazcard 27B. Lead nitrate (Toxic, Dangerous for the environment) Refer to CLEAPSS® Hazcard 57A. Zinc foil (Low Hazard) Refer to CLEAPSS® Hazcard 107. Magnesium ribbon (Low Hazard) Refer to CLEAPSS® Hazcard 59A. Copper foil (Low Hazard) Refer to CLEAPSS® Hazcard 26. Lead foil (Toxic, Dangerous for Environment) Refer to CLEAPSS® Hazcard 56. 1 Solutions may be dispensed in 5 cm3 beakers to each pair of students or in small bottles fitted with droppers to groups of students. 2 Metals should be approximately 1 cm lengths or squares of ribbon or foil cleaned with emery cloth and as similar in size as possible.

Procedure a Using a dropping pipette, put a little of the zinc nitrate solution in four of the depressions in the spotting tile, using the following illustration as a guide. Label this row with the name of the solution. Rinse the pipette well with water afterwards. magnesium lead zinc copper

zinc nitrate magnesium nitrate copper nitrate lead nitrate

spotting tile

b Do this for each solution in turn , rinsing the pipette when you change solution. c Put a piece of each metal in each of the solutions, using the illustration as a guide. d Over the next few minutes observe which mixtures have reacted and which have not.

What to record Record which metals react with the solutions. A table may be useful. Use a ✓ to show reactivity and a ✗ to show no reaction. Solution / Metal

Zinc

Magnesium

Copper

Lead

Zinc nitrate Magnesium nitrate Copper nitrate Lead nitrate

117

Teaching notes Remind the class that they are looking for cases where one metal displaces another. Some of the solutions are slightly acidic so that bubbles of hydrogen are sometimes seen. Explain that this does not count as displacement of one metal by another. It might be best to get the class to tell you what they think the order of reactivity is while they still have the evidence in front of them, so that apparent discrepancies can be resolved.

Reference This experiment has been adapted from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/metals/displacement-reactionsbetween-metals-and-their-salts,304,EX.html

Useful resource A number of other experiments in this book also illustrate the competition principle. Examples include: Experiment 40: Extracting metals with charcoal Experiment 42: The reaction between zinc and copper oxide Experiment 43: The Thermite reaction

118

Health & Safety checked, March 2009 Updated 19 Aug 2009

Extracting metals with charcoal

40

In each of the two experiments, illustrating the idea of competition between metals and carbon, students heat a metal oxide with powdered charcoal. If the carbon is more reactive than the metal it will remove the oxygen from the metal oxide and leave traces of the metal in the reaction vessel. The first experiment uses lead(II) oxide; the second modifies the technique slightly and uses copper(II) oxide.

Lesson organisation The solids may be dispensed in plastic weighing dishes. It is best not to issue all three solids at the same time because students may confuse the charcoal and the copper oxide. The experiment should take about 30 minutes.

Apparatus and chemicals Eye protection Each student or pair of students will require:

•

Small, hard glass test-tubes (ignition) tubes, 3 (see note 1) Test-tube holder Test-tube rack Spatula Plastic weighing dish (boat) Heat resistant mat Access to: Lead(II) oxide (Toxic, Dangerous for the environment), about 1 g (see note 2) Powdered charcoal, about 2 g Copper(II) oxide (Harmful, Dangerous for the environment), about 1 g

Technical notes Lead(II) oxide (Toxic, Dangerous for the environment) Refer to CLEAPSS® Hazcard 56 Copper(II) oxide (Harmful, Dangerous for the environment) Refer to CLEAPSS® Hazcard 26 1 Test-tubes made of heat resistant borosilicate glass (Pyrex or similar) must be used. Testtubes with a capacity of about 10 cm3 are ideal. It is important that the test-tubes are dry. Heating lead and its compounds strongly in glass often results in the lead compounds fusing into the glass, rendering the test-tube impossible to re-use. If this is an issue, old but unstained test-tubes can be used and discarded after this use. 2 The three solids may be dispensed in separate, labelled, plastic weighing dishes. It is wise to withhold the copper(II) oxide until it is required to avoid the students confusing it with the charcoal.

119

Procedure HEALTH & SAFETY: Wear eye protection throughout. The room should be well ventilated.

Experiment 1 a Transfer one small spatula measure of lead(II) oxide to the empty weighing dish. b Add one spatula measure of charcoal powder. c Mix the two powders together using a spatula. d Transfer the mixture into a hard-glass test tube and strongly heat this mixture for five minutes in a Bunsen flame. e Allow the test-tube to cool in its holder on a heat resistant mat. f Tip the cooled mixture out onto the heat resistant mat.

ignition tube

Experiment 2 a Transfer one spatula measure of copper(II) oxide to a hard glass test-tube. b Carefully add one spatula of charcoal powder on top of the copper oxide without any mixing. c Strongly heat these two layers for five minutes in a Bunsen flame.

charcoal copper oxide

Bunsen burner

d Allow to cool and then look closely at where the powders meet in the test-tube.

heat resistant mat

Teaching notes In each case the students should look for signs that reaction has occurred producing the metal. The copper should be obvious from its colour. The lead may be less obvious; it may appear as globules or as grey powder. The reactions confirm the place of carbon in the reactivity series, above lead and copper, as it reduces the metal oxides to the metals and is itself oxidised to carbon dioxide: 2CuO(s) + C(s) → 2Cu(s) + CO2(g) and 2PbO(s) + C(s) → 2Pb(s) + CO2(g)

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/metals/extracting-metals-withcharcoal,305,EX.html

Useful resource Several other experiments in this book also illustrate the competition principle. Examples include: Experiment 39: Displacement reactions between metals and their salts Experiment 42: The reaction between zinc and copper oxide Experiment 43: The thermite reaction

120

Updated 22 Apr 2009

Extraction of iron on a match head!

41

Introduction Students reduce iron(III) oxide with carbon on a match head to produce iron in this small scale example of metal extraction. The experiment can be used to highlight aspects of the reactivity series.

Lesson organisation This experiment can easily be carried out on an individual basis by students. The experiment itself is very quick to do provided that the apparatus and chemicals are already set out around the laboratory.

Apparatus and chemicals Eye protection Each student (or pair of students) will need:

•

Match (non-safety) (see note 1) Tongs (crucible tongs) Weighing boat (small white plastic ones are ideal) Spatula Students will also need access to: Bunsen burner Heat resistant mat Magnet (e.g. bar magnet) Iron(III) oxide powder (Low hazard) (see note 2) Sodium carbonate powder (Irritant) (see note 2) Water (see note 2)

Technical notes Iron(III) oxide powder (Low hazard). Refer to CLEAPSS® Hazcard 55A. Sodium carbonate (Irritant). Refer to CLEAPSS® Hazcard 95A. 1 The experiment works best with non-safety matches. These are often referred to as 'strike anywhere' matches and have a pinkish-red head. 2 Small amounts (a few spatula measures are sufficient) of each of the powders can be provided in, for example, Petri dishes or watch glasses. Groups of students can share the chemicals. The water can be provided in a small beaker.

121

Procedure

•

HEALTH & SAFETY: Wear eye protection a Dip the head of a match in water to moisten it. b Roll the damp match head first in sodium carbonate powder, then in iron(III) oxide powder. c Hold the match in a pair of tongs. Put the head of the match into a blue Bunsen flame (air-hole open). The match will flare and burn. Do not allow the match to burn more than half way along its length. d Allow the match to cool for about 30 seconds. e Use a spatula to crush the charred part of the match into a small plastic weighing boat. f Move a magnet around under the weighing boat – some of the small particles will move around in the weighing boat following the track of the magnet. Do not dip the magnet into the particles directly, unless you have first wrapped the magnet in cling film – any pieces of iron will stick to the magnet and will be difficult to clean off.

Teaching notes A simple equation for the reaction would be: Iron(III) oxide + carbon → iron + carbon dioxide 2Fe2O3(s) + 3C(s) → 4Fe(s) + 3CO2(g) Carbon is more reactive than iron. The iron oxide is reduced by the carbon (the oxygen is removed) to form metallic iron. The sodium carbonate fuses easily and brings the iron oxide into close contact with the carbon.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/elements-compounds-andmixtures/extraction-of-iron-on-a-match-head,99,EX.html

Useful resource Teachers.tv. KS3/4 Science - Banging Chemistry: Fast and Furious http://www.teachers.tv/video/20295 (Website accessed December 2009)

122

Health & Safety checked, June 2007 Updated 21 Feb 2008

The reaction between zinc and copper oxide

42

In this experiment copper(II) oxide and zinc metal are reacted together. The reaction is exothermic and the products can be clearly identified. The experiment illustrates the difference in reactivity between zinc and copper and hence the idea of competition reactions.

Lesson organisation This is best done as a demonstration. The reaction itself takes only three or four minutes but the class will almost certainly want to see it a second time. The necessary preparation can usefully be accompanied by a question and answer session. The zinc and copper oxide can be weighed out beforehand but should be mixed in front of the class. If a video camera is available, linked to a TV screen, the ‘action’ can be made more dramatic.

Apparatus and chemicals Eye protection Bunsen burner Heat resistant mat Tin lid Beaker (100 cm3) Circuit tester (battery, bulb and leads) (Optional) Safety screens (Optional) Test-tubes, 2 (Optional – see Procedure g) Test-tube rack

•

Access to a balance weighing to the nearest 0.1 g The quantities given are for one demonstration. Copper(II) oxide powder (Harmful, Dangerous for the environment), 4 g Zinc powder (Highly flammable, Dangerous for the environment), 1.6 g Dilute hydrochloric acid, approx. 2 mol dm-3 (Irritant), 20 cm3 Zinc oxide (Dangerous for the environment), a few grams Copper powder (Low hazard), a few grams. Concentrated nitric acid (Corrosive, Oxidising), 5 cm3 (Optional – see Procedure g)

Technical notes Copper(II) oxide (Harmful, Dangerous for the environment) Refer to CLEAPSS® Hazcard 26. Zinc powder (Highly flammable, Dangerous for the environment) Refer to CLEAPSS® Hazcard 107 Dilute hydrochloric acid (Irritant at concentration used) Refer to CLEAPSS® Hazcard 47A and Recipe Card 31 Concentrated nitric acid (Corrosive, Oxidising) Refer to CLEAPSS® Hazcard 67 Copper powder (Low hazard) Refer to CLEAPSS® Hazcard 26 Zinc oxide (Dangerous for the environment) Refer to CLEAPSS® Hazcard 108.

Procedure HEALTH & SAFETY: Wear eye protection throughout. Consider placing safety screens around the experiment (Samples of zinc can vary considerably in reactivity, depending on particle size and the state of surface oxidation.)

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a Weigh out 2 g (0.025 mol) of copper oxide and 1.6 g (0.025 mol) of zinc powder. b Mix thoroughly to give a uniformly grey powder. c Pour the mixture in the shape of a ‘sausage’ about 5 cm long onto a heat resistant mat or clean tin lid. d Heat one end of the ‘sausage’ from above with a roaring Bunsen flame until it begins to glow, then remove the flame. A glow will spread along the ‘sausage’ until it has all reacted. A white/grey mixture will remain. e Heat this to show that the white powder (zinc oxide) is yellow when hot and white when cool. f Pour the cool residue into a 100 cm3 beaker and add a little dilute hydrochloric acid to dissolve the zinc oxide (and also any unreacted zinc and copper oxide), warming if necessary. Red-brown copper will be left. This can be rinsed with water and passed around the class for observation. Show that the powder conducts electricity using a circuit tester. g If further confirmation of identity is required, treat a small amount of the red-brown powder with a few drops of concentrated nitric acid in a test-tube in a fume cupboard. A brown gas (NO2, Very Toxic) is given off as the copper reacts and dissolves. After the reaction adding a little water makes the blue solution of copper(II) nitrate visible.

Teaching notes The depth of discussion depends on the level of the students involved. Essentially it is a competition between metal(1) and metal(2) for oxygen in a reaction represented by: Metal(1) + Metal(2) oxide → Metal(1) oxide + Metal(2) The more reactive metal displaces the less reactive metal from its oxide, as in the case of zinc and copper(II) oxide, for example: Zn(s) + CuO(s) → ZnO(s) + Cu(s) Demonstrate that zinc oxide goes yellow when heated and returns to white when cool to help confirm the identity of this product. (This phenomenon is caused by a change in crystal structure - a genuine example of a 'physical change'.) Where appropriate, it could be pointed out that these reactions are redox reactions, the more reactive metal behaving as a reducing agent, and the metal oxide acting as an oxidising agent. This could be extended to consider these redox reactions in terms of the loss and gain of electrons by the metals. Other metals can be used, but take care to compare like with like. Coarse magnesium powder, for example, gives a less vigorous reaction than powdered zinc. Finely powdered magnesium gives a very vigorous reaction and should only be attempted with great care. The reaction between aluminium powder and copper oxide is almost explosive and must not be attempted.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/metals/the-reaction-between-zincand-copper-oxide,303,EX.html

Useful resource A number of other experiments in this book explore the competition principle. Examples include: Experiment 39: Displacement reactions between metals and their salts Experiment 40: Extracting metals with charcoal Experiment 43: The thermite reaction

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Health & Safety checked, March 2009 Updated 24 Apr 2009

The thermite reaction

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This demonstration shows the highly exothermic reaction between aluminium and iron(III) oxide that produces molten iron. This is a competition reaction, showing aluminium to be a more reactive metal than iron. A redox reaction takes place.

Lesson organisation The reaction is violent but safe provided the procedures are followed exactly. Some teachers have had accidents when performing the procedure outside in a strong breeze; the powders blew into the flame, caught fire and caused burns to the hand and/or face. Siting the demonstration in a fume cupboard has caused damage to the cupboard. The method described here is performed on a laboratory bench and does not produce many fumes. Do NOT do this demonstration in a fume cupboard or out of doors. It produces a result within seconds of setting it off because the water cools the iron down very quickly. A rehearsal is essential if this experiment has not been done before. There have been occasional reported explosions when using methods similar to this. It is essential not to exceed the stated quantities and that the demonstrator and students are protected by safety screens. The bench should be clear of combustible materials and protected with a sheet of hardboard or heat resistant mats. Pupils should not look directly at the glare of the burning magnesium but cover their eyes with their fingers slightly apart. The demonstrator must have room to move quickly away to a safe distance. The demonstration takes about 10 minutes to carry out if the apparatus is set up and the solid reagents are weighed in advance.

Apparatus and chemicals Eye protection: Safety glasses for observers, goggles or face shield for the demonstrator. The quantities given are for one demonstration

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Filter papers, 12 cm diameter, 2 Pipe-clay triangle (or similar) Tripod Beaker, thick-walled (1 dm3) Dry sand (see diagram) Heat resistant mats Safety screens Small bar magnet Thermite mixture: Iron(lIl) oxide (Low hazard), 9 g (see notes 1 and 2) Aluminium powder (medium grade) (Highly flammable), 3 g (see note 3) Igniter mixture: Magnesium powder (Flammable), 0.2 g Barium nitrate (Harmful, Oxidising), 2 g (see note 4) Magnesium ribbon (Flammable), 10 cm length

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Technical notes Aluminium powder (Highly flammable) Refer to CLEAPSS® Hazcard 1 Magnesium powder (Highly flammable) Refer to CLEAPSS® Hazcard 59A Barium nitrate (Harmful, Oxidising) Refer to CLEAPSS® Hazcard 11 Magnesium ribbon (Low hazard) Refer to CLEAPSS® Hazcard 59A 1 It is important that the iron(III) oxide used in this demonstration is absolutely dry. An hour or so in a warm oven, or heating in an evaporating dish over a Bunsen flame, should suffice. The oxide should be allowed to cool completely before mixing. 2 The weighed quantities of iron(III) oxide (9 g) and aluminium (3 g) may be thoroughly mixed beforehand by repeatedly pouring the mixture to-and-fro between two pieces of scrap paper, and then stored for the demonstration in a container labelled ‘Thermite mixture’. 3 The demonstrator may wish (or be persuaded by the audience) to do a repeat demonstration. In this event it is important to keep the second set of materials well away from the first demonstration site. 4 The weighed quantities of magnesium powder (0.2 g) and barium nitrate (2 g) may also be thoroughly mixed beforehand as indicated in note 2, and then stored for the demonstration in a container labelled ‘Igniter mixture’.

Procedure

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HEALTH & SAFETY: A face shield or goggles and a laboratory coat (it can become messy at the end) should be worn by the demonstrator. Students should stand further than 4 m from the reaction and wear eye protection. Safety screens must be used to surround the apparatus. magnesium ribbon igniter mixture thermite mixture

fluted filter paper pipe clay triangle

tripod water layer sand layer 1 litre beaker heat resistant mat

a Fold two 12 cm diameter circles of filter paper into fluted cones and place one inside the other. b Into a 1 dm3, thick-walled beaker, pour dry sand until it is one-third full and then add water until it is two-thirds full. c Cover an area of the bench with several heat resistant mats and place the beaker in the centre. Set up the equipment as shown in the diagram above and surround it with safety screens. Add the Thermite mixture (see note 2) to the fluted filter paper cone sitting in the pipe clay triangle.

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d Make a depression in the Thermite mixture with a spatula and place the igniter mixture (see note 4) into it. e Insert a magnesium ribbon fuse upright into the igniter mixture. It must extend above the fluted filter paper. Light the magnesium fuse with a Bunsen burner flame and retreat to a safe distance behind the safety screens. A very vigorous reaction should follow, with some sparks flying upwards. The very hot residue containing molten iron will fall through into the water. f Once the reaction has stopped, remove the beaker and decant the water into the sink. Retrieve the iron formed with a magnet. Wash the iron under running water.

Teaching notes The reaction is: iron(III) oxide + aluminium → aluminium oxide + iron This shows that aluminium is above iron in the reactivity series. The ‘Thermite’ mixture is stable until strong heating is applied, hence the need for an initiating reaction between the barium nitrate and magnesium powder. Once underway, the reaction is highly exothermic, rapidly reaching temperatures as high as 2000 °C, well in excess of the melting point of iron (1535 °C). The practical use of this reaction to weld railways lines together should be mentioned – see web link below. Do not use potassium manganate(VII) and hot glycerol as an alternative to initiate the reaction in this version because the filter papers catch fire. Do not use any other metal oxides, such as copper oxides, chromium(VI) oxide, lead oxides or manganese(IV) oxide. However, chromium(III) oxide and Mn3O4 can be used.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/metals/the-thermitereaction,172,EX.html

Useful resource There are several experiments in this book which use the competition principle. Examples include: Experiment 39: Displacement reactions between metals and their salts Experiment 40: Extracting metals with charcoal Experiment 42: The reaction between zinc and copper oxide There are many video clips of Thermite reactions on the internet, some carried out on a scale and in a manner which is extremely hazardous. Two clips of reactions carried out safely using a different procedure to that outlined here can be found at: http://genchem.chem.wisc.edu/demonstrations/Gen_Chem_Pages/06thermopage/thermite_ reaction.htm Note that in the following reaction a much coarser mixture of the solids, as in commercial Thermite charges, is used. Using powdered solids on this scale would be extremely hazardous. http://www.davidavery.co.uk/thermite/index.htm Details and pictures of the thermite welding of railway tracks can be found at: www.northeast.railfan.net/high_iron.html#thermite (Last accessed December 2009)


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Making silicon and silanes from sand Description Magnesium and sand are heated together and silicon is produced by an exothermic reaction. The product is placed in acid to remove magnesium oxide and unreacted magnesium. Small amounts of silanes are produced by the reaction of magnesium silicide (a side product) with the acid. These react spontaneously with air to give spectacular but harmless small explosions. This experiment should take around 5-10 minutes.


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Eye protection Safety screen One pyrex test-tube, approximately 150 mm x 17 mm Clamp and stand Bunsen burner One 250 cm3 beaker One 250 cm3 conical flask Filter funnel and filter paper. Access to oven Desiccator Access to top pan balance. The quantities given are for one demonstration. 1 g of dry magnesium powder (Highly flammable) 1 g of dry silver sand About 50 cm3 of approximately 2 mol dm–3 hydrochloric acid (Irritant)

Technical notes Magnesium powder (Highly flammable) Refer to CLEAPSS® Hazcard 59A Dilute hydrochloric acid (Irritant at concentration used) Refer to CLEAPSS® Hazcard 47A and Recipe Card 31

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Procedure Health & Safety: Wear safety goggles. Use a safety screen between the apparatus and the audience. Magnesium powder burns vigorously in air. The dust from magnesium powder may be hazardous. Ensure that the mixed powders are absolutely dry before the reaction. It is the responsibility of teachers doing this demonstration to carry out an appropriate risk assessment.

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Before the demonstration a It is important that the reactants are dry. Dry the magnesium powder and the sand for a few hours in an oven at about 100 °C. Store them in the desiccator until ready to use them. Ensure that the test-tube is dry.

The demonstration b Weigh 1 g of silver sand and 1 g of magnesium powder and mix them thoroughly. This mixture has a small excess of magnesium over the stoichiometric masses (1 g of sand to 0.8 g of magnesium) because some magnesium will inevitably react with air. Spread the mixture along the bottom of a test-tube that is clamped almost horizontally. Place a safety screen between the tube and the audience if the spectators are close. c Heat one end of the mixture with a roaring Bunsen flame, holding the burner by hand. After a few seconds the mixture will start to glow. This glow can be ‘chased’ along the tube with the flame until all the mixture has reacted. The tube will blacken and partly melt. If the two powders are not dry, some magnesium will react with the steam and the resulting hydrogen will pop. This can be disconcerting if it is not expected. d When the reaction is complete, allow the mixture to cool (about five minutes) and with the aid of a spatula pour the products into about 50 cm3 of 2 mol dm–3 hydrochloric acid. The solid will contain silicon, magnesium oxide (the main products), magnesium silicide formed from the reaction of excess magnesium with silicon, unreacted magnesium and possibly a little unreacted sand. The mixture will fizz as excess magnesium reacts with the acid. There will also be pops accompanied by small yellow flames. These are caused by silanes that are formed from the reaction of magnesium silicide with acid. Silanes inflame spontaneously in air. Magnesium oxide will react with the acid to form a solution of magnesium chloride. e After a few minutes the pops will cease and grey silicon powder, possibly with a little unreacted sand, will be left on the bottom of the beaker. Pour off the acid, wash the solid a few times with water and filter off the silicon. It can be passed around the class to show its slightly metallic silver-grey colour. If desired show that it does not react with alkalis (or acids).

Visual tips Make sure the safety screen is clean.

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Teaching notes There are many interesting contrasts to be drawn between silicon compounds and their carbon analogues. Silicon dioxide is a solid with a giant structure, while carbon dioxide is molecular. Silanes react spontaneously with air at room temperature while alkanes are stable. These differences can be explained by considering the relevant bond energies and availability of d-orbitals in silicon but not in carbon. Bond energies in kJ mol–1: Si=O 638; Si–O 466; C– O 336; C=O 805; Si–H 318; C– H 413.

Theory The reactions are: SiO2(s) + 2Mg(s) → 2MgO(s) + Si(s) 2Mg(s) + Si(s) → Mg2Si(s) MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) Mg2Si(s) + 4HCl(aq) → 2MgCl2(aq) + SiH4(g) (Higher silanes such as Si2H6 may also be produced.) SiH4(g) + 2O2(g) → SiO2(s) + 2H2O(l)

Further details One teacher reported that the ‘pops’ continued while the silicon dried on the filter paper. Silicon is extracted from sand industrially by reduction with carbon.

Reference This experiment has been adapted from Classic Chemistry Demonstrations, Royal Society of Chemistry, London, p.127-129

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Reduction of copper(II) oxide with methane

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Copper metal of high purity is an essential of our way of life. It is used in all electrical apparatus from cell phones to aircraft and in many plumbing applications, such as pipes and tanks.

Lesson organisation This experiment works well as a class practical for students working in pairs and involves heating a small quantity of copper(II) oxide in an atmosphere of methane. The reaction is rapid. Educational benefit can be obtained from allowing air to come into contact with the copper metal when it is still hot. This can be used to introduce reversible reactions and allows a discussion on rates of reaction. This practical is suitable for observing the reaction qualitatively, or for the determination of the formula of copper(II) oxide.

Apparatus and chemicals Eye protection Reaction tube (see technical note 2) 1-hole bung to fit reaction tube Rubber tubing and adaptors Bunsen burner Clamp stand Boss head Clamp Wire test tube holders

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Copper(II) Oxide (Harmful, Refer to CLEAPSS® Hazcard 26) (See technical note 1)

Technical notes 1 Wire form copper(II) oxide gives the best results. Copper(II) oxide powder can be used as an alternative, as can copper(II) carbonate. If wire form copper(II) oxide is available use a pea-size amount. If the powder form only is available, use about 10 pin-heads equivalent, spread out. The wire form yields the best results for gravimetric experiments. 1g is a suitable amount. Methane gas can flow around and react with the wire form copper(II) oxide which it cannot do with a pile of copper(II) oxide powder. 2 Make the reaction tube as in experiment 14

Procedure HEALTH & SAFETY: Wear eye protection. Teachers must supervise lighting the burning of excess methane see f below. There is the potential for a very large flame.

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a Clamp the reaction tube as close to the open end as possible, so it is supported by the stand, as in the diagram. b Place the copper(II) oxide ¾ of the way down the tube and spread it out. c Fit the bung and use the tubing and adaptors to connect to a methane gas supply. d Open the gas supply to the tube for 10 seconds to flush out any air. (There is potential for an explosion if the air inside the apparatus is not flushed out with methane before the methane is lit). Then close off the gas supply. e Light a Bunsen burner and close the air hole.

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f Holding the Bunsen flame near the hole in the reaction tube, open the gas tap very cautiously. Adjust the flame coming out of the reaction tube to about 5 cm.

excess methane burned off copper (II) oxide methane supply via bung

g Adjust the Bunsen flame by opening the air hole fully. h Adjust the height of the reaction tube so it corresponds to the hottest part of a blue Bunsen flame

reaction tube

i Heat the copper(II) oxide until it is reduced (copper coloured). It may be necessary to move the heating Bunsen burner in order to reduce all the copper(II) oxide.

Bunsen burner

j Turn off the gas supply to the Bunsen burner. k Allow the apparatus to cool while the methane flame from the tube is still burning. l When the apparatus is cool, close the gas tap. m Examine the copper formed.

Teaching notes At high temperatures methane reduces copper(II) oxide, 4CuO(s) + CH4(g) → 4Cu(s) +2H2O(g) + CO2(g). The gaseous product is carried off with the excess methane. If the experiment is not to be done with the intention of subsequent qualitative work, it is worth considering turning off both gas supplies when the copper formed is still hot and removing the bung. Oxygen from the air instantly oxidises the copper metal back to copper(II) oxide. Students are more likely to remember the reduction process if they have to do it for a second time and it also introduces the concepts of reversible reactions and the effect of heat on rates of reactions. The cooling of the reaction tube after the experiment can be encouraged by using wire type sprung test tube holders. These should be alternately clipped to the tube until they get hot and the clamp stand until they get cold. This makes the cooling process quicker and occupies students, getting them thinking about heat transfer and temperature. They can estimate the temperature of the tube (and hence when it is cool enough to touch) by touching the test tube holder which has just been removed from the reaction tube. Copper(II) carbonate can be used for this experiment. Copper(II) carbonate (green) decomposes to form copper(II)oxide (black) on heating. CuCO3(s) → CuO(s) + CO2(g). The copper metal that is formed in this experiment is similar to the ‘blistered’ copper metal that is formed by the first stage of the industrial extraction of copper. It is not sufficiently pure for the uses in electronics and plumbing for which copper is used. A second electrolytic purification process is necessary.


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Finding the formula of copper oxide

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Students heat copper(II) oxide in a glass tube while passing methane over it. The copper(II) oxide is reduced to copper. If the reactants and products are weighed carefully the formula of the copper oxide can be deduced. This could also be used simply as an example of reduction.

Lesson organisation This experiment is likely to take up to an hour, perhaps more to analyse the results. Students who have not carried out this type of reaction before may find it helpful to have the techniques demonstrated first. It is not really suitable for a class practical for students under the age of 14, but could be a useful demonstration. Each pair or group of students will need access to 2 gas taps. The students will need access to matches or lighters to light their Bunsen burners. Alternatively, light a few around the room and students can light their own using a splint.

Apparatus and chemicals Per pair or group of students: Eye protection

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Reaction tube, (hard glass test-tube with small hole near closed end: see note 2) 1-hole bung with glass tube to fit the reduction tube Rubber tubing Retort stand Boss Clamp Bunsen burner Heat resistant mat Spatula Copper(II) oxide (Harmful, Dangerous for the environment), 2 spatulas (see note 1) Access to: Balance – must be accurate to at least 0.01g

Technical notes Copper(II) oxide (Harmful, Dangerous for the environment) Refer to CLEAPSS® Hazcard 26 1 For best results use wire form copper(II) oxide. Alternatively, use analytical grade copper(II) oxide which has been dried by heating in an open dish at 300–400 °C for 10 min and then stored in a dessicator. It is also worth referring to the CLEAPSS® Laboratory Handbook Section 13.2.3 for further information about this experiment. 2 Hard glass test-tube with small hole near closed end. See experiment 14: making a reaction tube.

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Procedure

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HEALTH & SAFETY: Wear eye protection. a Weigh the test tube with the bung in (mass 1). Put 2 spatulas of copper oxide into the tube and spread it out as much as possible.

excess methane burned off copper (II) oxide from gas tap

b Weigh the tube again, with the copper oxide in it (mass 2). c Assemble the apparatus as shown in the diagram, but do not place the Bunsen burner underneath yet. Clamp the test tube as near to the bung as possible.

reaction tube Bunsen burner

d Turn on the gas tap attached to the test tube about half way to get a steady flow of gas. This will pass methane through the apparatus. e Wait for a few moments, until you think that all the air will have flushed out of the tube and then light the gas coming out of the hole at the end of the tube. If this experiment is a student activity, a teacher should supervise this step. Take care not to lean over the tube as you light the gas. Adjust the gas tap so that the flame is about 3 cm high. f Light the Bunsen burner and begin to heat the copper oxide in the tube. You will need to use a roaring flame (air hole fully open). You will need to pick up the Bunsen burner and move the flame around to heat every bit of the copper oxide. Ensure that the hottest part of the Bunsen burner flame (the top of the inner cone) is being used for heating. If there are parts which look unreacted, gently shake the tube – it will be very hot so do so by gently shaking the clamp stand. g When all the copper oxide looks like it has reacted (it will look like copper), keep heating for a minute or two and then turn off the Bunsen burner. h Keep the methane passing over the product as it cools down to prevent it from reacting with any oxygen present and turning back into copper oxide. When the tube is cool, switch off the gas. i Weigh the test tube with the bung and the product (mass 3).

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Teaching notes Students should have recorded the following masses: (mass 1) Test tube + bung (mass 2) Test tube + bung + copper oxide (mass 3) Test tube + bung + copper (product) This should allow them to calculate the mass of the mass of the copper oxide (mass 2) - (mass 1) and the mass of the copper (mass 3) - (mass 1). They should also calculate the decrease in mass (mass 3) - (mass 2), which corresponds to the mass of oxygen. With this information they can calculate the formula of the copper oxide. Students will also need the relative atomic masses. Copper is 63.5 and oxygen is 16. They should divide mass by the atomic mass for each element. This will give the number of moles of each. Having done this for both elements, they should find the ratio between the two by dividing them both by the smallest number. The ratio should be close to 1:1 as the formula of copper oxide is CuO.

Example calculation Mass copper oxide = 1.76 g Mass copper = 1.43 g So mass oxygen = 0.33 g Number moles Cu = 1.43/63.5 = 0.02251 Number moles O = 0.33/16 = 0.020625 Divide by the smallest to give the ratio aprox. 1 Cu: 1 O This would suggest a formula of CuO, which is the correct formula.

Reference This experiment has been adapted from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/elements-compounds-andmixtures/finding-the-formula-of-copper-oxide,210,EX.html


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The reaction of magnesium with steam Burning magnesium ribbon is plunged into the steam above boiling water in a conical flask. In the first method, the hydrogen that is formed is allowed to burn at the mouth of the flask. In the second method, the hydrogen is collected over water and tested with a lighted spill.


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Eye protection Method 1 Bunsen burner, tripod and gauze Tongs One 250 cm3 conical flask Method 2 Bunsen burner, tripod and gauze. One 1 dm3 conical flask with a one-holed rubber bung to fit. Glass trough or washing up bowl. One boiling tube. One short length of glass tube of approximately 1 cm diameter. About half a metre of rubber tubing. Wooden spills. Each method needs the following chemicals. The quantities given are for one demonstration. About 45 cm of magnesium ribbon (Flammable) A little Universal indicator solution (Highly flammable) with appropriate colour chart.

Technical notes Magnesium ribbon (Low hazard) Refer to CLEAPSS® Hazcard 59A Universal indicator solution (Highly flammable) Refer to CLEAPSS® Hazcard 32 and 40A, and CLEAPSS® Recipe Card 36.

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Procedure

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Health & Safety: Wear safety goggles.

Before the demonstration For method 2 a Enlarge the hole in the rubber bung so that it will take a piece of glass tubing of diameter about 1 cm. Attach about half a metre of rubber delivery tube to this glass tube. This will be of similar bore to the tubing used for a Bunsen burner. The reason for this unusually wide tubing is so that it can cope with the rapid evolution of hydrogen that occurs in this demonstration.

The demonstration Method 1 a Stand the 250 cm3 conical flask on the tripod and clamp its neck to steady it. Place about 50 cm3 of water in the flask. Bring this to the boil and allow it to boil for at least five minutes to displace all the air from the flask and replace it with steam. Take three 15 cm lengths of magnesium ribbon and twist them together to form a length of plaited ribbon of the same length. This is more rigid than a single strand and can therefore be manoeuvred more easily when held in a pair of tongs. Take care that the ribbon does not break during plaiting. Leave the Bunsen burner on, boiling the water.

plaited Mg ribbon hydrogen flame tongs

steam

burning Mg

boiling water

Bunsen burner

b Holding the plaited magnesium ribbon in tongs by one end, light the other end in the Bunsen flame (a second Bunsen burner may be helpful) and hold the burning end in the steam inside the flask. Avoid looking directly at the burning ribbon. The ribbon will continue to glow brightly, forming hydrogen by reaction with steam. This ignites and burns at the mouth of the flask with a slightly yellowish flame. The magnesium oxide falls into the water and a little dissolves. Turn off the Bunsen burner and add a few drops of Universal indicator to the water. It will be significantly alkaline due to dissolved magnesium hydroxide.

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Method 2 b Stand the 1 dm3 conical flask on the tripod and clamp its neck to steady it. Place about 200 cm3 of water in the flask. Bring this to the boil and allow it to boil for at least five minutes to displace all the air from the flask and replace it with steam. Plait the magnesium as described above and attach it to the underside of the bung on the wide bore delivery tube. The easiest way to do this is to cut a small slit in the rubber with a scalpel and insert one end of the plaited ribbon into the slit. c Fill a trough with water and clamp a boiling tube full of water in an inverted position with its mouth under water. Place the free end of the rubber delivery tube in the mouth of the boiling tube. Clamp the delivery tube if necessary to prevent it coming out of the mouth of the boiling tube as the other end, attached to the bung, is moved (see diagram). wide bore glass tube (should protrude from underside of stopper)

rubber delivery tube

rubber stopper

boiling tube full of water steam boiling water

burning ‘plait’ of Mg ribbon

Bunsen burner

d Leave the Bunsen burner on, boiling the water. Light the end of the plaited magnesium ribbon and lower it into the steam in the flask until the bung is fitted into the mouth of the flask. The magnesium will continue to glow brightly in the steam, forming hydrogen. This will be forced along the delivery tube and some will be collected in the boiling tube, although much will overflow. Remove the bung and delivery tube from the flask to prevent suck-back and test the gas in the boiling tube with a lighted spill. It will ‘pop’ showing it to be hydrogen. The magnesium oxide will have fallen into the water and a little will have dissolved. e Turn off the Bunsen burner and add a few drops of Universal indicator to the water. It will be significantly alkaline due to dissolved magnesium hydroxide.

Visual tips The hydrogen flame in method 1 would be more easily seen in a slightly darkened room.

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Teaching notes Do not allow the burning magnesium to touch the side of the flask. This can be a difficult task if you are dazzled by its flame. Wearing sunglasses might help. Compare the reaction of magnesium with steam with its reaction with cold water using the apparatus shown in the diagram. Very small bubbles will be seen on the surface of the magnesium but it will take several days before a significant volume can be collected. collected hydrogen

water beaker

pieces of Mg ribbon

Theory The reaction is Mg(s) + H2O(g) → MgO(s) + H2(g) Followed by MgO(s) + H2O(l) → Mg(OH)2(aq)


Useful resource Another method for this reaction (see diagram) is described in Nuffield combined science teachers’ guide II, Sections 6 – 10. London: Longman/Penguin, 1970, 62. This may be suitable as a class experiment. The steam is generated by heating mineral wool soaked with water. However, the test-tubes (which must be of borosilicate glass) often crack and are ruined by the reaction of the hot magnesium with the glass. magnesium ribbon mineral wool soaked in water

rubber delivery tube

Bunsen burner


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The ‘blue bottle’ experiment A colourless solution in a flask is shaken. It turns blue and then gradually back to colourless. The cycle can be repeated many times.

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Apparatus and chemicals Eye protection One 1 dm3 conical flask with stopper Access to a nitrogen cylinder (optional) Access to a fume cupboard (optional) The quantities of chemical given are for one demonstration. 8 g of potassium hydroxide (Corrosive) or 6 g of sodium hydroxide (Corrosive) 10 g of glucose (dextrose) 0.05 g of methylene blue (Harmful) 50 cm3 of ethanol (IDA, Industrial Denatured Alcohol) (Highly flammable, Harmful)

Technical notes 8 g of potassium hydroxide (Corrosive) or 6 g of sodium hydroxide (Corrosive) Refer to CLEAPSS® Hazcard 91 0.05 g of methylene blue (Harmful) Refer to CLEAPSS® Hazcard 32 Ethanol (IDA, Industrial Denatured Alcohol) (Highly flammable, Harmful) Refer to CLEAPSS® Hazcard 40A

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Procedure Health & Safety: Wear safety goggles.

Before the demonstration a Make a solution of 0.05 g of methylene blue in 50 cm3 of ethanol (0.1 %). b Weigh 8 g of potassium hydroxide or 6 g of sodium hydroxide into a 1 dm3 conical flask. Add 300 cm3 of water and 10 g of glucose and swirl until the solids are dissolved. Add 5 cm3 of the methylene blue solution. None of the quantities is critical. The resulting blue solution will turn colourless after about one minute. c Stopper the flask.

The demonstration Shake the flask vigorously so that air dissolves in the solution. The colour will change to blue. This will fade back to colourless over about 30 seconds. The more shaking, the longer the blue colour will take to fade. The process can be repeated for over 20 cycles. After some hours, the solution will turn yellow and the colour changes will fail to occur.

Visual tips A white laboratory coat provides the ideal background.

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Teaching notes On a cold day, it may be necessary to warm the solution to 25–30 °C or the colour changes will be very slow. The demonstration can be used to start a discussion on what is causing the colour changes. Students’ suggestions can be tried out as far as is practicable.

Theory Glucose is a reducing agent and in alkaline solution will reduce methylene blue to a colourless form. Shaking the solution admits oxygen which will re-oxidise the methylene blue back to the blue form.

Extensions To confirm that oxygen is responsible for the colour change, nitrogen can be bubbled through the solution for a couple of minutes to displace air from the solution and the flask. If the stopper is now replaced and the bottle shaken, no colour change will occur. Reintroducing the air by pouring the solution into another flask and shaking will restore the system. Natural gas can be used (in a fume cupboard) if nitrogen is not available. Some teachers may wish to present this experiment as a magic trick. The colour change can be brought about by simply pouring the solution from a sufficient height into a large beaker. This experiment can be a popular open-day activity. If visitors are to be allowed to shake the bottle themselves it might be wise to use a plastic screw-top pop bottle to eliminate the risk of the stopper coming off or the bottle being dropped and broken. The solution does not appear to interact with the plastic over a period of a day but it would be wise to try out the bottle you intend to use. Redox indicators other than methylene blue can be used. In each case add the stated amount of indicator to the basic recipe of 10 g of glucose and 8 g of potassium hydroxide in 300 cm3 of water. 1 Phenosafranine. This is red when oxidised and colourless when reduced. Use about 6 drops of a 0.2 % solution in water for a bottle that goes pink on shaking and colourless on standing. The initial pink colour takes some time to turn colourless at first. A mixture of phenosafranine (6 drops) and methylene blue (about 20 drops of the 0.1 % solution in ethanol) gives a bottle which will turn pink on gentle shaking through purple with more shaking and eventually blue. It will reverse the sequence on standing. 2 Indigo carmine. Use 4 cm3 of a 1 % solution in water. The mixture will turn from yellow to red-brown with gentle shaking and to pale green with more vigorous shaking. The changes reverse on standing. 3 Resazurin. Use about 4 drops of a 1 % solution in water. This goes from pale blue to a purple-pink colour on shaking and reverses on standing. On first adding the dye, the solution is dark blue. This fades after about one minute. Mixtures of the above dyes can also be used.


Useful resource A. G. Cook, R. M. Tolliver and J. E. Williams, J. Chem. Ed., 1994, 71, 160. The article The blue bottle experiment revisited gives some details of the reaction mechanism and alternative dyes. Health & Safety checked, December 2009

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Turning copper coins into 'silver' and 'gold' A ‘copper’ coin is dipped into a solution of sodium zincate in contact with zinc metal. The coin is plated with zinc and appears silver in colour. The plated coin is held in a Bunsen flame for a few seconds and the zinc and copper form an alloy of brass. The coin now appears gold.

Lesson organisation A simple demonstration involving electroplating and the chemistry of alloys, this is suitable for any age group depending on the sophistication of the theoretical treatment used - if any. The demonstration takes about 10 -15 minutes.

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Apparatus and chemicals For each demonstration: Eye protection: goggles Disposable gloves (preferably nitrile) Beaker (250 cm3) Bunsen burner (see note 1) Tripod and gauze Pair of tongs or forceps Glass stirring rod Access to a top-pan balance Zinc powder (Highly flammable), 5 g Sodium hydroxide pellets (Corrosive), 24 g Steel wool (see note 2) Deionised or distilled water, 100 cm3 Copper coins (see note 3)

Technical notes Zinc powder (Highly flammable) Refer to CLEAPSS® Hazcard 107 Sodium hydroxide (Corrosive) Refer to CLEAPSS® Hazcard 91 Hydrogen (Extremely flammable) Refer to CLEAPSS® Hazcard 48 1 Since hydrogen is evolved from a hot solution of zinc in sodium hydroxide, an alternative source of heating is to be preferred, e.g. an electric heating plate. If a Bunsen burner is to be used then it should be turned off before the zinc is added. 2 If steel wool isn't available a proprietary mild abrasive material (for example, ‘Brillo’ soap pads) can be used instead. 3 Copper foil could be used instead, but coins are better since they are everyday articles, and there are bound to be requests from the audience to turn copper into 'gold'. Strictly speaking it is illegal to "deface coins of the realm", so the law-abiding teacher might prefer to use foreign coins instead. It would be wise under these circumstances to ensure that the plating works, since many other alloys are used in foreign coinage. 4 Any remaining finely powdered zinc should not be left to dry because it can ignite spontaneously. Dispose of it by rinsing with water, dissolving in excess dilute sulfuric acid and washing the resulting zinc sulfate solution down the sink.

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Procedure HEALTH & SAFETY: Wear goggles and disposable gloves.

Before the demonstration

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a Dissolve 24 g of sodium hydroxide in 100 cm3 of deionised/distilled water in a 250 cm3 beaker, stirring continuously. The solution will get warm and is corrosive. b Heat the solution to boiling point on a Bunsen burner (caution: the hot solution is Highly Corrosive). c Turn the Bunsen off. d Add 5 g of zinc powder carefully. The solution will fizz as some of the zinc dissolves forming sodium zincate and giving off hydrogen. e Clean a ‘copper’ coin with steel wool until it is shiny.

The demonstration a Drop the cleaned coin into the hot solution containing sodium zincate and the remaining zinc powder. b The coin must make contact with the powdered zinc at the bottom of the solution. If necessary use a glass rod to move the coin until this is so. c Leave the coin until it is plated with a shiny coat of zinc. This will take about 2-3 minutes. Leaving the coin too long may cause lumps of zinc to stick to it. d Remove the plated coin with tongs or forceps and rinse it under running tap water to remove traces of sodium hydroxide and sodium zincate. e Show the 'silver' coin to the audience. f Using tongs or forceps, hold the plated coin in the upper part of a roaring Bunsen flame for a few seconds until the surface turns gold. Turn the coin so that both sides are heated equally. Overheating will cause the coin to tarnish. g Allow the coin to cool and show it to the audience.

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Teaching notes It may be sensible to carry out a trial experiment before performing the demonstration in front of an audience. If the mixture of sodium zincate solution and zinc is cloudy, allow to cool, and then filter off the zinc to leave a clear filtrate. Place a small piece of zinc foil in the liquid as a substitute for the powder. Younger students might want to have their own coins plated. The theory is as follows: The reaction between zinc and sodium hydroxide to form sodium zincate is as follows: Zn(s) + 2NaOH(aq) + 2H2O(l) → Na2[Zn(OH)4](aq) + H2(g) The plating reaction involves an electrochemical cell; it will not take place unless the copper and the zinc are in contact, either directly (as here) or by means of a wire. The electrode reactions are: At the zinc electrode: Zn(s) → Zn2+(aq) + 2e– followed by complexing of the zinc ions as [Zn(OH)4]2–(aq) At the copper electrode: [Zn(OH)4]2–(aq) + 2e– → Zn(s) + 4OH–(aq) The coating of zinc gives the impression that the coin is now coated with silver. On heating the coin in the Bunsen flame, brass is formed by the zinc migrating into the surface layer of the copper. This gives a gold appearance to the coin. Brass is an alloy of copper containing between 18% and 40% of zinc. A similar zinc plating process is used industrially, but with cyanide ions rather than hydroxide ions as the complexing agent.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/metals/turning-copper-coins-intosilver-and-gold,275,EX.html

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Alkali metals

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These demonstrations show the similarity of the physical and chemical properties of the alkali metals and the trend in reactivity down Group 1 of the Periodic Table.

Lesson organisation These experiments must be done as a demonstration at all levels of school chemistry. The experiments take about 10 – 20 mins if everything is prepared in advance. Advance preparation includes cutting pieces of alkali metals to the recommended size, filling water troughs and setting up safety screens. You should try the experiments in advance if you have not done them before.

Apparatus and chemicals Goggles or a face shield for the demonstrator, eye protection for the audience All experiments

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Tweezers or forceps Filter paper Beaker (150 cm3) Ceramic tile Scalpel or sharp knife to cut the metals Small bottles of oil containing small pieces of: Lithium (Highly flammable, Corrosive), 5 mm cubes (see note 1) Sodium (Highly flammable, Corrosive), 4 mm cubes (see note 1) Potassium (Highly flammable, Corrosive), 3 mm cubes (see note 1) Ethanol (Highly flammable) or Industrial denatured alcohol (IDA) (Highly flammable, Harmful), 150 cm3 (see note 2) 2-methylpropan-2-ol (Highly flammable, Harmful), 150 cm3 (see note 2) Experiment 1 Protective gloves (preferably nitrile) Small iron nail Power supply (~6 V) Light bulb in holder (for testing conductivity) Electrical leads Petri dishes, with lids Adhesive tape (to seal petri dishes) Experiment 2 One or more large glass troughs (5 dm3 capacity) Safety screens, at least 2 Glass or Perspex sheets to cover troughs (optional) Detergent, 1 drop Universal indicator solution (Highly flammable)

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Technical notes Lithium (Highly flammable, Corrosive) Refer to CLEAPSS® Hazcard 58 Sodium (Highly flammable, Corrosive) Refer to CLEAPSS® Hazcard 88 Potassium (Highly flammable, Corrosive) Refer to CLEAPSS® Hazcard 76 Ethanol (Highly flammable) Refer to CLEAPSS® Hazcard 40A Industrial denatured alcohol (Highly flammable, Harmful) Refer to CLEAPSS® Hazcard 40A 2-methylpropan-2-ol (Highly flammable, Harmful) Refer to CLEAPSS® Hazcard 84B Universal indicator solution (Highly flammable) Refer to CLEAPSS® Hazcard 32 and Recipe card 36 1 A technician should prepare the pieces of metal and store them under oil. Using the tweezers, remove a large piece of the alkali metal from the oil. Ensure that conditions are dry. Place the metal on a tile and, using a scalpel or sharp knife, cut pieces of lithium (5 mm cubes), sodium (4 mm cubes) and potassium (3 mm cubes). Place the small pieces in separate bottles of oil, labelled with the metal name and the hazard symbol. Cut each alkali metal separately and return the larger piece to its bottle before starting the next one. 2 Place any apparatus used to cut (and later handle) the metal (filter paper, scalpels etc) in a trough of water after use. Small pieces of alkali metal for disposal should be allowed to react fully with ethanol (for lithium and sodium) or 2-methylpropan-2-ol (for potassium) until fizzing stops, before washing away with water.

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Procedure HEALTH & SAFETY: Demonstrator to wear goggles or a face shield. Pupils to be 2 – 3 m away and wearing eye protection.

Experiment 1: Physical properties Do the following with a small piece of each metal, lithium, sodium and potassium a Remove the metal from the oil bottle with tweezers. b Cut the metal with a scalpel (lithium is the hardest to cut). Place any pieces that will not be used back into the oil. c Wearing protective gloves, squeeze the remaining metal in your gloved hand to show its softness. Lithium is the hardest to mould. d Drop each metal from a height of a few centimetres on to a piece of filter paper on the bench. Compare with a piece of iron of about the same size. Note the gentle impacts that reveal that the metals have a low density. e Use the circuit to show that the metals conduct electricity well. f Place the piece of metal in a petri dish and pass around the class. They must not touch the piece of metal – tape the lid shut if necessary. g Dispose of the lithium and sodium in ethanol, and dispose of the potassium in 2-methylpropan-2-ol.

Experiment 2: Reaction with water a Fill the trough(s) about half-full of water. Add a drop of detergent (to stop the metals sticking to the side). Place enough Universal indicator solution into each, with stirring, until the colour is clearly visible. b At least two safety screens should be used, as close to the troughs as possible. Alternatively use glass plates to cover the trough(s). c Ensure that stock bottles are closed (and no metal comes into contact with splashed water) before the reaction begins.

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d Ensure that the students cannot pick up pieces of the alkali metals.

e Using dry tweezers, remove a small piece of lithium from the stock bottle, place the lithium on a filter paper and close the bottle. Use the filter paper to wipe off the oil. f Using tweezers drop the piece of metal on to the water surface in a trough (and place the cover over the trough). g Repeat with a piece of filter paper floated on the surface of the water, placing another piece of lithium on the paper. h In both cases, the lithium should float and fizz – giving off hydrogen. The water in the trough turns alkaline. The lithium does not catch fire. i Use a fresh trough (or fresh water) for the next metal. Note that the reaction heats up the water. j Repeat with sodium. This time the piece on the water floats, moves around more vigorously and fizzes. It may catch fire if it sticks to the side of the trough. On the filter paper it will only catch fire. It burns with a yellow flame and gives off a white smoke. CARE – when sodium and potassium catch fire, they give off corrosive oxides as smokes. The students must be far enough away not to breathe these (or the troughs should be covered). k Replace the water and repeat with potassium. Do not use the filter paper. Be careful as potassium is very reactive. The potassium moves around vigorously, melts, fizzes and catches fire, burning with a lilac flame. l Be careful to ensure that no traces of the metals remain on the tweezers, filter paper etc. The simplest way is to place them in one of the troughs of water to react. Small pieces of metals can be disposed of in ethanol (for lithium and sodium) or 2-methylpropan-2-ol (for potassium) and the residue flushed down the sink with when all reaction has ceased.

Teaching notes The reaction with water can be done with the trough on an overhead projector. The projector should first be focussed on a matchstick floating in the water. Alternatively use a flexicam linked to a projector or a TV screen. The reactions are: 2M(s) + 2H2O(l) → 2MOH(aq) + H2(g) where M represents the alkali metal. The solutions of the hydroxides formed are alkaline. The reactions clearly show that the reactivity sequence is lithium< sodium < potassium

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/periodic-table/alkalimetals,155,EX.html

Useful resource www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/alkali-reac.html explains simply order of reactivity (Website accessed December 2009)

Health & Safety checked, October 2007 Updated 29 Oct 2008

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Heating Group 1 metals in air and in chlorine This is a demonstration that shows the reactions of Group 1 metals in air and in chlorine. It does not clearly show the trends in reactivity of Group 1 metals, which is better demonstrated by the reactions in water, which follow on well from this demonstration.

Lesson organisation This experiment must be done as a demonstration. If you have not attempted this experiment before, it is strongly advised that you try it before performing the demonstration in front of students. The first step is to generate chlorine, which can be done in advance, and requires a fume cupboard. The rest of the demonstration can be done in a well-ventilated laboratory. Goggles should be worn during the chlorine generation and the demonstration. The class should also wear eye protection during the demonstration. How long the demonstration takes depends largely on how much talking you do between each part of the experiment. To speed things up a bit, the metals can be pre-cut into appropriately sized pieces, but they should be returned to the oil until just before they are used. For some classes it may be appropriate to do just one part of the experiment and heat the metals in either air or chlorine.


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Goggles for the demonstrator, eye protection for the audience Fume cupboard (only for generating the chlorine) Clean, dry bricks with at least one flat surface, 3 (see note 1) Gas jars with lids, 3 (see note 1) Bunsen burner Heat resistant mat Scalpel Forceps or tweezers Tile Filter paper Universal Indicator paper Chlorine generator1 (Toxic, Dangerous for the environment) (see note 2) Lithium (Highly flammable, Corrosive) (see note 3 and 4) Sodium (Highly flammable, Corrosive) (see note 3) Potassium (Highly flammable, Corrosive) (see note 3) Sodium chlorate(I) solution, 10-14% (w/v) (Corrosive), fresh (see note 2) Hydrochloric acid, 5 mol dm-3 (Irritant at this concentration) (see note 2)

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Technical notes Chlorine (Toxic, Dangerous for the environment) Refer to CLEAPSS® Hazcard 22A and Recipe card 26 Lithium (Highly flammable, Corrosive) Refer to CLEAPSS® Hazcard 58 Sodium (Highly flammable, Corrosive) Refer to CLEAPSS® Hazcard 88 Potassium (Highly flammable, Corrosive) Refer to CLEAPSS® Hazcard 76 Sodium chlorate(I) (Corrosive) Refer to CLEAPSS® Hazcard 89 Hydrochloric acid (Irritant at concentration used) Refer to CLEAPSS® Hazcard 47A and Recipe card 31 1 The mouth of the gas jar must be narrower than the brick, to reduce the amount of gas escaping during the demonstration. 2 There are two methods given in the standard techniques for generating chlorine. The method that uses sodium chlorate(I) is safer than the method that uses potassium manganate(VII), but will not work well if the sodium chlorate(I) is an old sample as the concentration will be too low. Note that sodium chlorate(I), NaOCl, is NOT the same as chlorate(V), NaClO3. 3 It is very helpful to have the 3 mm cubes of lithium, sodium and potassium cut ready to use. These should still be kept under oil until they are required. 4 Do not heat lithium in crucibles or other porcelain material – explosions have occurred.

5M HCI

card cover

gas jar

NaOCl solution

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Procedure

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HEALTH & SAFETY: Demonstrator to wear goggles or face shield, class to wear eye protection.

Heating the metals in air a Starting with lithium, use the tweezers to pick up a small piece of metal and place it on a tile. Use a scalpel to cut a small cube with an edge of about 3 mm. Show the students the freshly cut surface which soon tarnishes, showing that the metal reacts quickly with oxygen. Blot off the oil using the filter paper, and place it onto the flat surface of the brick. b Heat the metal from above using the hottest part of a roaring Bunsen flame just beyond the blue cone. Once the metal is on fire, remove the Bunsen flame. You should be able to observe the classic red of a lithium flame. (You may initially see a yellow flame, but this is the burning of any oil which was not removed.) c Once the metal has stopped burning, test the residue with damp indicator paper and show that it is alkaline. d Repeat for sodium and potassium.

Heating the metals in chlorine a Prepare gas jars on chlorine in advance using a chlorine generator.1 b Check that the mouths of the gas jars of chlorine are narrower than the brick to reduce the amount of escaping gas, and that the colour of the gas in the jar is green. If it is not then there is not enough chlorine present for the demonstration to be successful. c Starting with lithium, cut a small cube with an edge of about 3 mm. Blot off any excess oil. Place it on the clean, dry brick. d Heat the piece of metal from above using the Bunsen burner (see diagram below). When the metal is burning, take away the Bunsen, invert the gas jar, remove the lid and immediately place over the burning metal. It helps to have a second pair of hands to do this. The metal continues to burn, producing fumes of white chloride. This method avoids producing FeCl3 or CuCl2, which can occur when a combustion spoon, or deflagration spoon made of iron or brass, is used. e Repeat for sodium and potassium. inverted gas jar of chlorine

gas jar coated with sodium chloride

clean sodium gas jar lid

sodium burns intensely brick

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Teaching notes When heating in both air and chlorine, the expected pattern of lithium being the least reactive through to potassium being the most reactive may not be observed as it is hard to see potassium burning without the Bunsen flame. This may well be due to the potassium reacting faster than the others and an oxide coating being formed almost as soon as you begin to heat it. In air: 4Li(s) + O2(g) → 2Li2O(s) Sodium and potassium produce a mixture of oxides, peroxides and superoxides. In chlorine: 2Na(s) + Cl2(g) → 2NaCl(s) and similarly for the others. The typical flame colours for lithium (red) and sodium (yellow) can usually be seen and sometimes the lilac of the potassium flame.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/elements-compounds-andmixtures/heating-group-1-metals-in-air-and-in-chlorine,127,EX.html 2

iagram: heating sodium in chlorine Reproduced from CLEAPSS® Guide: L195 'Safer D Chemicals, Safer Reactions', section 9.3, by permission of CLEAPSS®

Standard techniques (standard procedures can be found by clicking on the link at www.practicalchemistry.org 1

Generating, collecting and testing gases: See experiment 12

Useful resource Inspirational chemistry on Learnnet has more information: www.chemsoc.org/networks/learnnet/inspirchem.htm http://media.rsc.org/videoclips/demos/Reactionsgroup1metals.wmv is a useful video showing how to carry out this demonstration. (Websites accessed December 2009)


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Reactions of aqueous solutions of the halogens This activity compares the colours of three halogens in aqueous solution and in a non-polar solvent. These halogens also react to a small extent with water, forming acidic solutions with bleaching properties. Halogens undergo redox reactions with metal halides in solution, displacing less reactive halogens from their compounds. These displacement reactions are used to establish an order of reactivity down Group 7 of the Periodic Table.

Lesson organisation This series of simple experiments illustrates some of the chemical properties of the halogens following an introduction to the physical properties of the Group 7 elements. It can be done as a demonstration or as a class experiment. Investigating the solubility of the halogens in a non-polar solvent can be left out, or only shown as a demonstration. If the activity is done as a demonstration it should take around 15 minutes. If it is done as a class experiment you should allow 30 minutes.

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Apparatus and chemicals Eye protection One demonstration or one group of students requires: Test-tube rack, to hold 10 test-tubes Test-tubes, 10 Cork or rubber bungs to fit, 4 Plastic dropping pipettes, 6 White spotting tile White tile Glass rod Paper towel or tissue Universal Indicator paper (about 2 cm strips), 3 About 10 cm3 of each of the following halogen solutions in stoppered test-tubes (see notes 1 and 2): Chlorine water, 0.1% (w/v) (The gas is Toxic, Irritant, Dangerous for the environment but the solution is Low Hazard) Bromine water, 0.3% (w/v) (Toxic, Irritant, Dangerous for the environment at this concentration) Iodine solution, 0.5% (w/v) (Dangerous for the environment at this concentration) Half a test-tube of each of the following solutions (see note 3): Potassium (or sodium) chloride solution, about 0.1 mol dm-3 (Low hazard) Potassium (or sodium) bromide solution, about 0.1 mol dm-3 (Low hazard) Potassium (or sodium) iodide solution, about 0.1 mol dm-3 (Low hazard) Optional: Cyclohexane (Highly flammable, Harmful, Danger for the Environment) or other suitable non-polar solvent, about 10 cm3 (see note 1)

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Technical notes Chlorine water (Chlorine gas escapes, which is Toxic, Irritant, Dangerous for the environment but the solution is Low Hazard at the concentration used) Refer to CLEAPSS® Hazcard 22B, Recipe card 28 Bromine water (Toxic, Irritant, Dangerous for the environment at concentration used) Refer to CLEAPSS® Hazcard 15B, Recipe card 28 Iodine solution (Dangerous to the environment at concentration used) Refer to CLEAPSS® Hazcard 54, Recipe card 39 Potassium chloride solution (Low hazard) Refer to CLEAPSS® Hazcard 47B, Recipe card 51 Sodium chloride solution (Low hazard) Refer to CLEAPSS® Hazcard 47B, Recipe card 63 Potassium iodide solution (Low hazard) Refer to CLEAPSS® Hazcard 47B, Recipe card 55 Cyclohexane (Highly flammable, Harmful, Danger for the Environment) Refer to CLEAPSS® Hazcard 45B 1 Each group of students should be supplied with stoppered test-tubes containing about 10 cm3 of each of the aqueous solutions of the halogens and one of cyclohexane (optional). 2 The halogen solutions can be diluted further to minimise the amount of chlorine or bromine fumes given off but should not be so dilute that their distinctive colours are not clearly visible in the test-tubes (a white background may be needed for chlorine water). 3 The concentration of the potassium (or sodium) iodide should be adjusted so that it gives a light brown solution on adding the chlorine water. If these reagents are too concentrated, a black precipitate of iodine often results instead of a brown solution. 4 At the end of the experiments all mixtures and solutions should be returned to a suitable waste container in a fume cupboard for safe disposal. 5 It is essential for the acidity test that the chlorine water is just that. Some samples of ‘chlorine water' that can be purchased from suppliers are actually chlorine in sodium hydroxide solution: this can give unexpected results in the test for pH.


The halogens in water and a hydrocarbon solvent (optional)

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a Pour about 2 cm3 of each of the aqueous halogen solutions into separate test-tubes. Add equal volumes of hydrocarbon solvent to each tube, stopper the tube and, holding your thumb over the bung, shake the mixture by inverting the test-tube a few times. b Allow the two layers to settle. Observe and record the colour of each layer. It may be necessary to shake the test-tubes again to transfer more of the halogen from the water to the hydrocarbon layer.

Acidic and bleaching properties of halogen solutions a Place a piece of Universal Indicator paper on a white tile. Transfer a drop of chlorine water onto the paper using a glass rod. Observe and record the colour of the paper. b Wipe the glass rod and the tile clean with a paper towel or tissue. Place a fresh piece of indicator paper on the tile and transfer a drop of bromine water onto it using the glass rod. Observe the colour of the paper. c Repeat b, using the iodine solution.

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Displacement reactions a Using a plastic pipette put two drops of chlorine solution in each of three dimples in the spotting tile, as shown below. In the same way and using a clean plastic pipette for each solution, add bromine water, and iodine solution to the spotting tile.

1

2

3 chlorine water bromine water iodine solution

b Add two drops of potassium chloride solution to each of the three dimples in column 1 of the tile. Observe and record any colour changes that take place. c Add two drops of potassium bromide solution to each of the three dimples in column 2 of the tile. Observe and record any colour changes that take place. d Add two drops of potassium iodide solution to each of the three dimples in column 3 of the tile. Observe and record any colour changes that take place. e (Optional) For reactions in which bromine or iodine are suspected to have formed, the reaction could be repeated with 2 cm3 of each solution in a test tube, and hexane could then be added to confirm the presence of bromine or iodine.

Teaching notes A results table similar to the one below could be used for the recording of results. It has been completed with expected observations.

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Colour after shaking with hydrocarbon solvent

Effect on indicator paper

Reaction with potassium chloride solution

Reaction with potassium bromide solution

Reaction with potassium iodide solution

Chlorine water

Aqueous layer: pale yellow-green to colourless Hydrocarbon layer: colourless to pale yellowgreen

Turns red, then rapidly bleaches white

No reaction

Yellow-orange colour of bromine appears

Brown colour of iodine appears

Bromine water

Aqueous layer: yellow-orange to colourless Hydrocarbon layer: colourless to yelloworange

Turns red, then slowly bleaches white

No reaction

No reaction

Colour darkens from yelloworange to brown

Iodine solution

Aqueous layer: brown to colourless Hydrocarbon layer: colourless to purple

Paper stained brown

No reaction

No reaction

No reaction

The halogens are more soluble in the hydrocarbon and move to this top layer, when shaken with a hydrocarbon solvent. For chlorine and bromine the colour does not change. You might need a white background to see the colour of the chlorine solution. However, for iodine there is a colour change, from brown in water to purple in the hydrocarbon layer. Where no displacement reaction takes place between a halogen solution and a halide solution, it may be that some lightening in the colour of the solution is observed and this can be explained by the effect of dilution. Take care to limit students’ exposure to chlorine and bromine water fumes. Some students with respiratory problems can show an allergic reaction to chlorine, the onset of which may be delayed. Iodine is the least soluble of the halogens in water. It is more soluble in potassium iodide solution, so the ‘iodine solution’ here is actually iodine in potassium iodide solution. Draw the students’ attention to the similarity between the colour of iodine vapour and its colour in a non-polar solvent. Polar water molecules interact with iodine molecules, altering the wavelengths of light they absorb. All three halogens react with water to produce a strong acid (HX), and a weak acid (HOX), which has bleaching properties and is an oxidising agent. X2(aq) + H2O(l) → HX(aq) + HOX(aq) The extent of reaction decreases down the Group. With iodine it is so small that the acidic and bleaching properties of the solution are not seen in this experiment. In the displacement reactions chlorine displaces both bromine and iodine from their compounds and bromine displaces iodine – for example: Cl2(aq) + 2KI(aq) → I2(aq) + 2KCl(aq) The order of reactivity is therefore chlorine > bromine > iodine. A more advanced treatment identifies the halogens as oxidising agents, accepting an electron to form halide ions: Cl2(aq) + 2I-(aq) → I2(aq) + 2Cl-(aq) Contrary to belief among many students, the reaction has nothing to do with the reactivity of potassium ‘grabbing’ the chlorine. Potassium is only present here as very unreactive potassium ions in solution!

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/periodic-table/reactions-ofaqueous-solutions-of-the-halogens,136,EX.html


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Halogen reactions with iron In this demonstration experiment, iron wool is heated in the presence of chlorine gas and the vapours of bromine and iodine. Exothermic redox reactions occur, causing the iron wool to glow. Iron(III) halides (FeX3) are formed as coloured solids. The vigour of the reactions corresponds to the order chlorine> bromine> iodine, showing the trend of decreasing reactivity of the elements down Group 7.

Lesson organisation These experiments must be carried out in a fume cupboard as both the reactants and products are hazardous. Teachers attempting this demonstration for the first time are strongly advised to carry out a trial run before doing it in front of a class. Time allowed should be at least 20 min, depending on the amount of discussion and testing of the products between each experiment. In addition to using this demonstration to show the relative reactivity of the halogens, the reaction of chlorine or bromine with iron could be used on its own to show the reaction between a reactive non-metallic element and a metal.


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Eye protection for teacher and students, protective gloves for teacher Access to a fume cupboard Apparatus to set up a chlorine generator Boiling-tubes, 2 Reduction tube (see note 1) Beakers (100 cm3), 3 Tweezers Teat pipette (preferably glass, with a narrow tip) Test-tubes, 3 small, and a test-tube rack Bunsen burner and heat-proof mat Bosses, clamps and stands The approximate quantities of chemicals below are sufficient for one demonstration. Iron wool, 3 tufts about 1 g mass each (see note 2) Liquid bromine (Corrosive, Very toxic, Danger to the environment), 0.5 cm3 (see note 3) Sodium thiosulfate solution (Low hazard), 500 cm3 at 1 mol dm-3 (see note 3) Iodine solid (Harmful, Danger to the environment), 0.5 g. Hexane (Highly flammable, Harmful), 100 cm3 (see note 2) Silver nitrate solution, 10 cm3 at approximately 0.1 mol dm-3. Care - stains skin and clothes. Chlorine generator (Toxic, Danger to the environment) (see note 4) Potassium manganate(VII) (Oxidising agent, Harmful, Danger to the environment), 10g (see note 4) Concentrated hydrochloric acid (Corrosive), 100 cm3 (see note 4) Deionised or distilled water

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Technical notes Liquid bromine (Corrosive, Very toxic, Danger to the environment). Refer to CLEAPSS® Hazcard 15 (2007: 15A). See Standard procedure: Liquid bromine1. Sodium thiosulfate solution (Low hazard). Iodine solid (Harmful, Danger to the environment). Refer to CLEAPSS® Hazcard 54. Hexane (Highly flammable, Harmful). Refer to CLEAPSS® Hazcard 45 (2007: 45A) Silver nitrate solution. Refer to CLEAPSS® Hazcard 87. Chlorine generator (Toxic, Danger to the environment). Refer to CLEAPSS® Hazcard 22. See Standard procedure: Generating, collecting, and testing gases2. Potassium manganate(VII) (Oxidising agent, Harmful, Danger to the environment). Refer to CLEAPSS® Hazcard 81 Concentrated hydrochloric acid (Corrosive). Refer to CLEAPSS® Hazcard 47 (2007: 47A) 1 The reduction tube should be fitted with a one-holed rubber stopper fitted with short length of glass tubing. Alternatively, an 8 to 10 cm length of wide-bore glass tubing with a stopper at each end fitted with a short length of glass tubing could be used. See diagram below.

dry chlorine gas

chlorine gas released into fume-cupboard iron wool

2 The finest grade iron wool is best since it provides the maximum surface area. Iron wool is often sold with a thin layer of grease on its surface to stop it rusting. Working in a fume cupboard, the layer of grease can be removed by dipping the iron wool in hexane (or alternative) a few times. The solvent must be allowed to completely evaporate. 3 Wear suitable protective gloves when handling bromine. Have at least 500 cm3 of 1M sodium thiosulfate readily available to treat any spillages. 4 Double-check that the acid you are using to generate chlorine is concentrated hydrochloric acid. Several accidents have occurred when teachers have inadvertently used concentrated sulfuric acid.

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Procedure

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HEALTH & SAFETY: Work in a fume cupboard throughout each stage of this demonstration. Wear suitable eye protection.

Chlorine a Place a 1 g tuft of cleaned iron wool in the reduction tube so that it is well spread out. Leave at least a 1 cm gap between the stopper and the iron wool. b Connect the reduction tube to the chlorine generator with a short length of rubber tubing. Clamp it in position over a Bunsen burner. c Pass a slow stream of chlorine over the iron wool from the chlorine gas generator. Do this by allowing the hydrochloric acid to drip slowly on to the potassium manganate(VII). After a few seconds, it should be possible to see the greenish colour of the chlorine gas filling the reduction tube, as all the air is expelled. d The iron wool may ignite without any heating. If not, gently heat at the end nearest to the chlorine generator until the wool does ignite (no further heating should be required). e A vigorous reaction will occur and the glow will spread along the wool in the tube, producing clouds of brown iron(III) chloride. Some of this may emerge as a smoke from the end of the reduction tube. f Continue passing chlorine over the iron wool until no further reaction occurs. Stop the chlorine supply and allow the tube to cool. g When cool, disconnect the reduction tube and rinse a little of the product into a clean beaker with some distilled water. Pour some of this solution into a clean test-tube and test with a few drops of silver nitrate solution. A white precipitate of silver chloride will form, confirming the presence of chloride ions.

Bromine See Standard procedure: Handling liquid bromine and preparing bromine water1 a Wear suitable protective gloves and take care to avoid spillage when handling liquid bromine. It is Corrosive and Very toxic. Transfer about 0.5 cm3 liquid bromine into one of the boiling tubes, using the teat pipette. Care is needed to avoid spillage - the density and volatility of the bromine cause it to drip very easily from the pipette. Keep the bromine container and the mouth of the test-tube close together. Replace the lid of the bromine container immediately. b Using tongs or tweezers, place a 1 g tuft of cleaned iron wool into the boiling-tube so that it is well spread out and almost fills the boiling-tube. Leave a 2 cm gap between the iron wool and the surface of the liquid bromine. c Clamp the test-tube near the top and at an angle of about 45° – see diagram.

fumes released into fume-cupboard

iron wool a few crystals of iodine or a few drops of bromine

d Heat the test-tube, gently at first, with a yellow-tipped blue flame (air hole on Bunsen burner slightly closed). Do this by moving the flame slowly between the bottom half of the iron wool and the bromine. As the bromine vapour starts to rise up into the iron wool, heat the wool more strongly.

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e Remove the heat when the wool starts to glow due to the heat of the reaction. Note the extra heating required to get this reaction started compared to the reaction involving chlorine. The iron will become coated with yellow-brown iron(III) bromide, and a brown ‘smoke’ may escape from the mouth of the test-tube. f When the reaction appears to be over, use tongs or tweezers to remove some of the remaining iron wool from the test-tube. g Rinse the iron wool in a few cm3 of deionised/distilled water in a small beaker. Pour out some of the resulting solution into a clean test-tube and test with a few drops of silver nitrate solution. Formation of a cream precipitate of silver bromide confirms that bromide ions are present.

Iodine a Transfer about 0.5 g of solid iodine (Harmful) into one of the boiling tubes. Place a 1 g tuft of cleaned iron wool in the test-tube and clamp it as before. b Working in a fume cupboard, heat the test-tube with a yellow-tipped blue flame (air hole on Bunsen burner slightly closed). Heat gently at first by moving the flame slowly between the bottom half of the iron wool and the iodine. c As the purple iodine vapour starts to rise up into the iron wool, heat the wool more strongly. Remove the heat when the reaction causes a dull glow – see Additional notes below. Some red-brown iron(III) iodide should form. d When the reaction appears to be over, remove some of the remaining iron wool from the test-tube with tweezers and rinse it in a few cm3 of deionised/distilled water in a small beaker. Pour some of the resulting solution into a clean test-tube and test it with a few drops of silver nitrate solution. Formation of a yellow precipitate of silver iodide confirms that iodide ions are present.

Additional teaching notes, hints and tips The order in which the experiments are done is a matter of choice, but it is probably best to leave the most reactive halogen (chlorine) to last, to end with a vigorous reaction - and confirm a class prediction? The reaction with iodine is much less vigorous than that with bromine and it may be difficult to see a glow at all. A couple of trial experiments beforehand may be necessary to get the right balance between heating the iodine and getting the iron hot enough for a reaction to start. If the iron is heated too vigorously, it may start to glow from reaction with the oxygen in any air that may still be present in the test-tube. The general equation for the reactions involved is: 2Fe(s) + 3X2(g) → 2FeX3(s) (X = Cl, Br and I)

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Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/periodic-table/halogen-reactionswith-iron,44,EX.html

Standard procedures Handling liquid bromine and preparing bromine water http://www.practicalchemistry.org/standard-techniques/handling-liquid-bromine-andpreparing-bromine-water,59,AR.html See also experiment 2 in this book.

1

2

Generating, collecting and testing gases http://www.practicalchemistry.org/standard-techniques/chlorine-generation,42,AR.html See also experiment 12 in this book.

Useful resource To view a video clip of these demonstration experiments, go to http://media.rsc.org/videoclips/clips/ReactionHalFeWool.mpg

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Health & Safety checked, November 2006 Updated 12 Feb 2009

Colourful electrolysis

54

An interesting introduction to the electrolysis of brine (sodium chloride solution). Students use Universal Indicator to help them follow what is happening during the reaction.

Lesson organisation This experiment works well if students are directed to make detailed observations and then attempt to explain for themselves what they think is happening. The main issue is likely to be the availability of sufficient U-shaped test tubes.

Apparatus and chemicals Eye protection Each working group will require:

•

U-shaped test tube Clamp and clamp stand Carbon electrodes and electrode holders, 2 of each Electrical leads, 2 Power pack Beaker (100 cm3) Spatula Stirring rod Students will need access to: Sodium chloride (salt) Universal indicator (Highly flammable) Distilled water

Technical notes Hydrogen (Highly flammable) Refer to CLEAPSS® Hazcard 48 Chlorine (Toxic, Dangerous for the environment) Refer to CLEAPSS® Hazcard 22A and 47B. Sodium hydroxide (Corrosive) Refer to CLEAPSS® Hazcard 91 Universal indicator solution (Highly flammable) Refer to CLEAPSS® Hazcard 32 and Recipe card 36 If electrode holders are not available, another suitable means of securing the electrodes could be used. Do not use bungs because the products are gases. If distilled water is a problem, then tap water could be used. But it may affect the colours produced, especially in areas with hard water.

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Procedure HEALTH & SAFETY: The products produced by this reaction are all more hazardous than the reactants. Hydrogen is Extremely flammable, chlorine is Toxic and Dangerous for the environment, and sodium hydroxide is Corrosive. Ensure that the current is turned off a soon as a trace of chlorine is detected. Chlorine (Toxic, Dangerous for the environment) can be a problem for asthmatic pupils. If the directions in the procedure notes are followed then very little chlorine is produced. Sodium hydroxide is Corrosive. Ensure that students wear eye protection, especially when they are clearing up the experiment. a Put about 75 cm3 distilled water into the beaker. Add about 2 heaped spatulas of sodium chloride. b Stir until the salt dissolves. Then add several drops of Universal Indicator solution. Stir to mix thoroughly. You need enough indicator to give the water a reasonable depth of green colour. c Pour coloured salt solution into the U-shaped test tube and clamp it as shown in the diagram.

– power supply + set to 10V

electrical leads with crocodile clips electrode holder graphite electrode clamp u-tube solution of salt and Universal indicator

d Wash the carbon electrodes carefully in distilled water and then fix them so that there is about 3 cm of electrode in each side of the U-tube – see diagram. This is most easily done using electrode holders. e Attach leads and connect to a power pack set to 10 V. f Turn on the power pack and observe closely what happens. A piece of white paper held behind the U-tube can help. Make sure the U-tube is kept very still during the experiment. g Turn off the power as soon as you notice any change at the positive electrode, or when you smell a ‘bleachy, swimming pool’ smell. This will probably take less than 5 minutes.

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Teaching notes This experiment is an interesting introduction to the electrolysis of brine. It is probably best not used as the first electrolysis that students encounter. They would really struggle to explain for themselves what is going on. It could be followed by the electrolysis of salt solution in industry. Students should be able to notice bubbles of gas at each electrode. At the positive electrode, the indicator turns red initially, and is then bleached to colourless. This indicates the presence of chlorine. At the negative electrode the indicator turns purple. The remainder of the solution stays green. The product at the negative electrode is hydrogen. This can be difficult for students to understand. Some of the water will ionise, that is, turn to hydrogen (H+) and hydroxide (OH-) ions. When the sodium chloride is dissolved in water, the ions forming the ionic solid separate out. This means that there are actually 4 ions present in the solution: H+, OH-, Na+ and Cl-. The negative ions are attracted to the positive electrode. The chloride ions are discharged (giving chlorine) in preference to the hydroxide ions. These are left behind in solution. At the negative electrode, the hydrogen ions are discharged (producing hydrogen gas) in preference to the sodium ions. These are also left behind in solution. Thus sodium hydroxide solution remains. This is the cause of the purple colour of the indicator at the negative electrode. In time, the green colour of the indicator in the middle would change too, as the ions diffuse through the resulting solution.

Equations: 2H+ + 2e– → H2 [negative electrode, cathode] 2Cl– → Cl2 + 2e– [positive electrode, anode] H2O → H+ + OH–

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/oxidation-and-reduction/ colourful-electrolysis,54,EX.html


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Electrolysis of zinc chloride This demonstration shows that an ionic salt will conduct electricity when molten but not when solid. Zinc chloride is used - this will melt at Bunsen burner temperatures.

Lesson organisation Zinc chloride offers a safer alternative to lead bromide for demonstrating the electrolysis of molten salts. Lead bromide decomposes to its elements just by heating without the need for electricity. The electrolysis of lead bromide must be carried out in a fume cupboard. The electrolysis of zinc chloride should be carried out in a fume cupboard. The chlorine produced at the positive electrode is Toxic and Dangerous for the environment. There are quite long periods of waiting, including at least 15 minutes for the electrolysis to take place so if you have access to a webcam, or video camera and a data projector, this would enable students to see what is going on inside the crucible. If not, bring students up in groups of 2 or 3 to view the experiment. They should note at which electrode bubbles are forming but must avoid smelling the bleachy smell ( be aware that many students are asthmatic). They should be able to see crystals of zinc around the negative electrode. It is worth having another, related, activity for the class to be getting on with.

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Apparatus and chemicals Eye protection Fume cupboard Low voltage (0-12 V) powerpack and electrical leads Graphite electrodes, 2, supported in an electrode holder or bung Ammeter and/or bulb (in holder) Circuit tester (optional) Bunsen burner, tripod and heat resistant mat Pipeclay triangle Crucible Clamp and stand Metal spatula Tongs Plastic beaker Filter paper and funnel Indicator paper and/or starch-iodide paper Solid zinc chloride (Corrosive, Danger to the environment) Distilled water

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Technical notes Zinc chloride (Corrosive, Dangerous for the environment) Refer to CLEAPSS® Hazcard 108 (2007: 108A). Chlorine (Toxic, Dangerous for the environment) Refer to CLEAPSS® Hazcard 22 See diagram below. ammeter 0-5A

bulb 12V, 5W

A

12V DC power supply

– + two-hole rubber bung

rheostat (optional)

graphite rods zinc chloride

crucible pipe clay triangle


heat resistant mat

Procedure HEALTH & SAFETY: Work in a fume cupboard. Wear eye protection.

Setting up the electrolysis

•

a Set up a heat-resistant mat, tripod, Bunsen burner and pipeclay triangle. Put the crucible onto the pipeclay triangle, ensuring that it is sitting firmly and is in no danger of falling through. b Set up the electric circuit with the power pack, ammeter and/or bulb and electrodes in series. Short the circuit at the electrodes with a key or the metal spatula. This is to satisfy yourself and the students that it is working. c Clamp the electrodes so that they almost touch the bottom of the crucible but do not touch each other. d Fill the crucible to within about 5 mm of the top with the powdered zinc chloride. As it melts the solid will shrink in volume as air escapes and it is important that the level of the molten salt does not drop below the level of the bottom of the electrodes. Ensure that the leads are well out of the way of the Bunsen flame. Using long electrodes can help with this.

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Showing that the solid zinc chloride does not conduct electricity a Begin to heat the crucible with a low to medium Bunsen flame. Watch the leads, and the bung if you are using one, to ensure that you are not over-heating them. b The zinc chloride takes about 3 or 4 minutes to melt. It may be tempting to use a roaring Bunsen flame to speed up the melting, but if you do so the zinc chloride can form a crust over the top. This will prevent students from seeing what is going on, and the liquid salt may boil. c As the salt melts, the bulb will light up and/or the ammeter will give a reading. Turn the Bunsen down a bit at this point. There will be some heating effect from the electric current which may be enough on its own to keep the zinc chloride molten (as in the industrial electrolysis of aluminium oxide.) d Bubbles of gas will be seen at the positive electrode. The gas can be confirmed as chlorine by holding moist indicator paper close to the bubbles - it will go red and the edges may start to bleach. A more convincing test is to use moist starch iodide paper which will go black. It is also possible to see crystals of zinc forming on the negative electrode. These can form a bridge across the electrodes, effectively shorting them. e Electrolyse the molten salt for about 15 minutes, with the current adjusted to about 0.5 A. Check every few minutes that the current remains roughly constant as there is a tendency for it to slowly increase. f After 15 minutes, turn off the power pack and Bunsen burner and remove the electrodes from the crucible. If this is not done while the salt is still molten the electrodes will stick. g Leave the crucible to cool for about 10 minutes. You may be able to see zinc crystals on both the electrode and on the surface of the mixture in the crucible. You could stop at this point, but to convince students that a metal really has been made you can separate the zinc from the remaining zinc chloride.

Separating the zinc a When the crucible is cool to the touch, put it into a beaker of distilled water. (If the water is at all basic like most hard tap waters, the zinc ions will flocculate forming large particles which are far harder to remove from the zinc metal.) The zinc chloride will dissolve (which may take some time) and can be decanted off. Swirl the beaker which will cause the zinc metal to concentrate in the centre of the beaker and decant off most of the liquid. b Filter the remainder and show students the shiny pieces of metal left on the filter paper. Dry the pieces of metal carefully between further sheets of filter paper and then test with a circuit tester to prove that you have a metallic product. Given that the starting material was zinc chloride and you have made chlorine, most students will have little difficulty in accepting that the metal is zinc.

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Additional teaching notes, hints and tips If the bulb does not light when testing the circuit, and the electrodes are mounted in a bung, check that the electrodes are not cracked. The boiling point of zinc chloride is about 730 °C. This can easily be reached by the combination of the heat from the Bunsen burner and the electric current. If the zinc chloride does begin to boil, it can boil over from the crucible and will also produce fumes of zinc chloride in the air. These rapidly turn back to the solid, forming a fine powder. It is possible to confuse the boiling bubbles with those of chlorine gas being formed. Therefore do not heat the molten zinc chloride too strongly. Do not try to remove the Bunsen and cool the salt while still electrolysing it, in order to show that the salt only conducts when molten. The heating effect of the electric current will keep the salt molten for several minutes, and when it does cool, a crust forms which is very difficult to re-melt.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/structure-and-bonding/ electrolysis-of-zinc-chloride,50,EX.html

Useful resource To view a video clip of this demonstration experiment, go to http://media.rsc.org/videoclips/demos/Electrolysisofmoltenzinc.mpg


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Identifying the products of electrolysis This experiment enables students to carry out the electrolysis of various solutions and to investigate the identity of the products formed at the electrodes. They should be able to link their practical experiences with theory and learn how to construct simple ionic equations.

Lesson organisation This class experiment is best done by students working in pairs or threes. There are plenty of tasks for each member of the team to complete. It should be carried out in a well-ventilated laboratory as significant amounts of toxic chlorine, bromine and iodine can be produced in some cases, as well as highly flammable hydrogen.

•

Apparatus and chemicals Eye protection Each working group requires: Electrolysis apparatus (see diagram) (see note 1) Graphite electrodes (about 5 mm diameter), 2 (see note 2) Large rubber bung to fit electrolysis cell, with holes to carry the graphite electrodes Small test-tubes (to fit over the electrodes), 2 DC power supply (6 V) Small light bulb in holder (6 V, 5 W) [optional] Leads and crocodile clips Wooden spills Small pieces of emery paper Strips of Universal indicator paper Diposable plastic gloves Clamp and stand Access to the following solutions (all approx. 0.5 mol dm-3 concentration) (see note 3) Aqueous potassium bromide (Low hazard) Aqueous sodium iodide (Low hazard) Aqueous calcium nitrate (Low hazard at this concentration) Aqueous zinc chloride (Irritant at this concentration) Aqueous copper nitrate (Low hazard at this concentration)

Technical notes Aqueous potassium bromide (Low hazard) Refer to CLEAPSS® Hazcard 47B Aqueous sodium iodide (Low hazard) Refer to CLEAPSS® Hazcard 47B Aqueous calcium nitrate (Low hazard at concentration used) Refer to CLEAPSS® Hazcard 19A Aqueous zinc chloride (Irritant at concentration used) Refer to Hazcard 108A Aqueous copper nitrate (Low hazard at concentration used) Refer to CLEAPSS® Hazcard 27B 50% Concentrated nitric acid – used to clean electrodes (Corrosive) Refer to CLEAPSS® Hazcard 67 and Recipe card 44 Chlorine (Toxic) Refer to CLEAPSS® Hazcards 22A & 22B Bromine (Toxic) Refer to CLEAPSS® Hazcards 15A & 15B Iodine (Toxic) Refer to CLEAPSS® Hazcards 54A & 54B

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1 The electrolysis apparatus shown below can be purchased ready-made. Alternatively, it can be made from thick glass tubing of 8 - 10 cm diameter, professionally cut into lengths of about 12 cm. A suitable glass bottle, with a wide-necked top and its base cut off, could be used instead (as shown in the diagram). The graphite rods should be well sealed into the holes, 2 - 3 cm apart, of the rubber bung, otherwise the electrolyte may leak onto the external wiring, causing it to corrode. Once made, this apparatus should last for several sessions, but the graphite rods tend to erode away quite quickly, particularly if students use larger than recommended voltages. The rods do eventually become thin and snap fairly easily, but they are cheap enough to replace.

electrolyte

electrolyte

–

+

6V DC 2 Once copper(II) nitrate has been power supply electrolysed (preferably last), a deposit of copper will have formed on the negative electrode (cathode). This has to be removed before the cells can be used again. Immersing the plated part of the electrode in a small quantity of 50% concentrated nitric acid (Corrosive) in a small beaker can be used to do this. Gloves and eye protection should be worn and the cleaning done in a fume cupboard by a suitably qualified person.

3 Depending on the volume of the electrolysis apparatus, each group of students needs enough solution to cover the electrodes plus about 2 cm to enable the full test-tubes of liquid to be inverted over the electrodes.

Procedure

•

HEALTH & SAFETY: Wear eye protection a Set up a table for results eg: Potassium bromide

Sodium iodide

Calcium nitrate

Zinc chloride

Copper nitrate

Lamp lights? Observations cathode (-) at anode (+) Test used for product at

cathode (-)

Identity of product formed at

cathode (-)

anode (+)

anode (+)

b Clamp the electrolysis cell and pour in enough of the first electrolyte so that the tops of the electrodes are covered with about 1–2 cm of liquid. Fill the two test-tubes with the same electrolyte. Wearing gloves, close the end of each test-tube in turn with a finger and invert it over an electrode, so that no air is allowed to enter (see diagram). During electrolysis it may be necessary to lift the test-tubes slightly to ensure that the electrodes are not completely enclosed, preventing the movement of ions to electrodes.

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c Connect the circuit, and mark the polarity of each electrode on the bung. The circuit should be checked before being switched on. d Switch on the circuit, then: ● observe whether or not the lamp lights up; ● look for the substances produced at each electrode – ie gaseous, solid or in solution; ● write down results after each observation, not when all the experiments are finished. e Only carry out the electrolysis for long enough to make the necessary observations. Prolonging the electrolyses unnecessarily causes toxic gases such as chlorine and bromine to be produced in unacceptably hazardous quantities. f After each electrolysis switch off the current and remove the test-tubes from the cell to test any gases present by lifting them slowly in turn to let any remaining solution drain out before closing the end with a finger. Carry out the tests on the gases as instructed. g (Optional) After removing the test-tubes from the cell, quickly pour the liquid down the sink with plenty of water. Wipe a piece of Universal indicator paper over each electrode and note any colour changes. h Wash the cell with plenty of water and dry the outside with a paper towel before fixing it back into position and re-connecting the power supply. It is important to connect the leads according to the polarities marked on the bung. i Repeat the experiment with each of the other four solutions, trying to keep to the order given in the table. Zinc chloride and copper nitrate should be the last electrolytes tested. This is because they deposit solids on the negative electrode (cathode). If zinc chloride is electrolysed first, the solid deposit on the negative electrode (cathode) can be easily removed with a piece of emery paper or dipping the end of the electrode in some dilute hydrochloric acid in a beaker.

Teaching notes The electrolysis of aqueous solutions, rather than molten salts, is easier and safer for students to do for themselves. Unfortunately the theory is more complicated, because the presence of water complicates what students may decide are the products formed at the electrodes. Ensure that students do not attempt to smell directly any of the halogen fumes produced. It is important that you are aware of any students who are asthmatic or who might have an allergic reaction to these toxic gases. In this context do not allow the electrolyses of the halide solutions to proceed any longer than is absolutely necessary. When testing for hydrogen or oxygen, the mouth of the test-tube can be closed with a gloved finger, and the test-tube transported to a central area, where a single naked flame has been set up, well away from the experiments. A supply of spills can also be kept in this area for the tests. For the hydrogen test, students may well ask why there is little or no ‘pop’ or ‘squeak’. Explain that pure hydrogen – rather than a mixture of hydrogen and air – is being tested if the testtube was full of gas before it was removed. For the oxygen test, care should be taken that the dampness at the mouth of the test-tube does not extinguish the ‘glow’, causing the test to fail. Once the electrolysis of zinc chloride or copper nitrate has been done, a deposit of metal will have formed on the cathode. This will have to be cleaned before the cell can be used again. These metal deposits can be removed using emery paper. Alternatively, small quantity of 50% concentrated nitric acid (Corrosive) in a small beaker can be used to remove the copper, providing gloves are worn and the operation is done in a fume cupboard by a suitably qualified person. Similarly dilute hydrochloric acid will remove the zinc.

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Results and conclusions Potassium bromide

Sodium iodide

Calcium nitrate

Zinc chloride

Copper nitrate

Yes

Yes

Yes

Yes

Yes

cathode (-)

Colourless gas

Colourless gas

Colourless gas

Whitish grey solid deposit

Reddish brown solid deposit

anode (+)

Orangebrown solution; maybe a little orangecoloured gas

Dark brown solution

Colourless gas

Tiny bubbles forming at electrode; very pale green gas, but most dissolves

Colourless gas

Gas ignited by burning spill; maybe a "squeak"

Gas ignited by burning spill; maybe a "squeak"

Appearance

Appearance

Lamp lights?

Observations at

Test used for product at

Gas ignited by burning cathode (-) spill; maybe a "squeak"

Appearance: Glowing (use of splint is starch – rekindled blue/black?)

Appearance: Glowing UI paper splint is turns red rekindled and is bleached

cathode (-) Hydrogen

Hydrogen

Hydrogen

Zinc

Copper

anode (+)

Iodine

Oxygen

Chlorine

Oxygen

anode (+)

Identity of product formed at

Appearance: UI paper turns red and is bleached

Bromine

Theory Reactions at negative electrode (cathode): KBr, NaI, Ca(NO3)2 : 2H2O(l) + 2e- → H2(g) + 2OH-(aq) ZnCl2, Cu(NO3)2 : M2+(aq) + 2e- → M(s) Reactions at positive electrode (anode): KBr, NaI, ZnCl2 : 2X-(aq) → X2(g/aq) + 2eCa(NO3)2, Cu(NO3)2 : 2H2O(l) → O2(g) + 4H+(aq) + 4eFor KBr, NaI and Ca(NO3)2 it is likely that students will ask why hydrogen is the gas evolved, rather than the metal. Students could then be asked to imagine what would happen if one of these metals is formed, given that this occurs in the presence of water. The ensuing reaction produces hydrogen as one of the products, and the metal hydroxide as the other. Students of higher ability could be introduced to the concept of electrode potentials, and be given details of the probable reaction occurring at the cathode, shown earlier. With chlorine in particular, and with bromine too, students will find that the indicator paper is bleached as well as showing signs of acidity. Iodine usually stains the paper brown. Some students may ask about the relative volumes of gases produced at the electrodes. While this practical is not designed to investigate this, they can be told the following. The volume ratio of hydrogen and chlorine gases produced during the electrolysis of NaCl is actually 1:1. Nothing like this is observed in practice, because chlorine is slightly soluble in the aqueous solutions, and the gas does not begin to collect until the electrolyte solution has become saturated with it.

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Less advanced students could be asked to concentrate on simple observations, e.g. Is a gas formed? What pH changes occur at the electrodes? The main principle to emphasise is that the conduction of electricity by aqueous solutions is due to the movement of ions (not electrons), and that these travel to the electrodes of opposite charge. Less advanced students should simply note that: ● the solution around the cathode tends to become alkaline; ● the solution around the anode tends to become acidic; ● metals low in the reactivity series appear to be deposited at the negative electrode; ● a gas is evolved at the negative electrode if the metal is high in the reactivity series. If appropriate, they can be told that this gas is hydrogen; ● non-metals are formed at the positive electrode: chloride ions produce gaseous chlorine, bromide and iodide ions form bromine and iodine respectively, which dissolve to form coloured solutions; and ● the electrolysis of copper nitrate produces a colourless gas at the positive electrode. If appropriate, students can be told that this is oxygen.

Reference This experiment has been adapted from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/electrolysis/identifying-theproducts-of-electrolysis,152,EX.html

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Electrolysis of potassium iodide solution

57

Filter paper soaked in potassium iodide solution which also contains starch and phenolphthalein is placed on an aluminium sheet which forms one electrode of an electric circuit. The other electrode is used as a ‘pen nib’ to ‘write’ on the filter paper. When this electrode is made positive and the aluminium sheet negative, the writing is blue/black and when the polarity is reversed, the writing is pink. This experiment should take around 20 minutes.

Apparatus and chemicals Eye protection Disposable gloves

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Aluminium (or other metal) sheet approximately 25 cm x 25 cm but the size is not critical DC power supply (0 - 12 V) Leads and crocodile clips to connect to the power pack Potassium iodide solution (Low hazard) (0.25 mol dm-3, but the concentration is not critical) (100 cm3) Starch solution (20 cm3) (see Technical notes below for how to prepare) Sodium thiosulfate solution (Low hazard) (approximately 1 mol dm-3, but the concentration is not critical) (a few drops) Phenolphthalein solution (20 cm3) (Highly flammable) (see Technical notes below for how to prepare) Filter papers, as large as possible to fit on the aluminium sheet

Technical notes Aqueous potassium iodide (Low hazard) Refer to CLEAPSS® Hazcard 47B Sodium thiosulfate solution (Low hazard) Phenolphthalein solution (Highly flammable) Refer to CLEAPSS® Hazcard 32 and Recipe card 36

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Procedure

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Health & safety: Both demonstrator and audience should wear eye protection. Wash hands after the experiment. Demonstrators with skin problems or cuts should wear disposable gloves. a Mix 40 cm3 of potassium iodide solution with 10 cm3 of starch solution. If the resulting solution has a blue colour (caused by contamination with iodine) add sodium thiosulfate solution dropwise until the solution becomes colourless. Then add 10 cm3 of phenolphthalein solution. If the resulting solution has a pink colour (caused by contamination with alkali) add dilute hydrochloric acid solution dropwise until the solution just becomes colourless. b Thoroughly moisten three sheets of filter paper in this prepared mixture and place the papers onto the aluminium sheet one on top of the other. Moistening may be done using a dropping pipette or wash bottle. Connect the aluminium sheet to the negative terminal of the power supply using a lead and crocodile clip. Connect a second lead to the positive terminal and switch on the power pack at between 6 V and 12 V. Now use the end of the positive lead to write or draw something on the top sheet of filter paper. The writing will appear blue/black, as iodine is discharged at the positive electrode and reacts with the starch to produce a blue/black complex. A corresponding pink line will appear on the lower filter paper in contact with the aluminium sheet but this will not be visible. This is caused by the discharge of H+ ions as hydrogen which leaves an excess of OH- ions in the solution. This alkaline solution turns the phenolphthalein pink. c Then switch the polarity of the electrodes so that the aluminium sheet becomes positive, and the free lead negative. The writing on the upper sheet of filter paper now becomes pink. (There will also be a corresponding blue/black line on the lower filter paper.) The reason for using three filter papers in a stack is that this blue/black line would be visible through a single moist filter paper and obscure the paler pink line. Some teachers may wish to draw attention to the lines underneath the filter papers, other may wish to ignore them.

Extensions If alternating current is used, a dotted line of alternating pink and blue/black is seen provided the lead is drawn over the filter paper quickly enough. This is not as spectacular as might be expected as the pink is much paler than the blue/black. Indicators other than phenolphthalein could be tried.

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Theory An aqueous solution of potassium iodide contains the following ions: K+(aq) and I-(aq) from the solute H+(aq) and OH-(aq) from dissociation of the water. At the positive electrode I-(aq) and OH-(aq) are attracted to the positive electrode (anode) where iodide ions are converted to iodine: 2I-(aq) – 2e- → I2(aq) (This occurs in preference to the thermodynamically favoured 4OH-(aq) – 4e- → O2(g) + 2H2O(l) because of the much higher concentration of I-(aq) than OH-(aq)). This iodine then forms a blue/black complex with the starch. At the negative electrode H+(aq) and K+(aq) are attracted to the negative electrode (cathode) where hydrogen ions are converted to hydrogen: 2H+(aq) + 2e- → H2(g) (This occurs in preference to K+(aq) + e- → K(s) because the discharge potential is more positive.) This leaves an excess of OH-(aq) ions around the cathode which turn phenolphthalein pink.

Teaching notes A simpler explanation as to why hydrogen is discharged at the cathode is that if potassium (the alternative product) were discharged it would immediately react with water to return to K+ ions, OH- ions and hydrogen. Teachers might wish to use this explanation with some students. A sheet of newspaper placed below the aluminium sheet will reduce mess if the filter papers have been over-enthusiastically moistened. This experiment is suitable for a class experiment or science club activity if sufficient apparatus is available - a biscuit tin lid (or almost any metal sheet) can be used as an alternative to the aluminium sheet. Attaching a graphite pencil sharpened at both ends to the second lead gives a good, narrow line and also nicely shows the conductivity of graphite at the same time.

Technical notes The starch solution must be prepared shortly before use – it will not keep. It is prepared by mixing 1 g of soluble starch with a little deionised water to form a thin paste then adding to this paste 80 cm3 of boiling water. Stir the mixture, allow it to cool and dilute to 100 cm3. To make phenolphthalein solution, dissolve 1 g solid phenolphthalein in 600 cm3 of industrial methylated spirits and make up to 1 dm3 with water.

Reference This experiment has been adapted by the RSC The RSC wishes to thank Dr Vladimir Volkovich formerly of Manchester University for translating an early draft of this demonstration from Russian. It also thanks Vanessa Byrne and Alison Oliver of North Leamington School for providing laboratory facilities and technical assistance. Dr Lynn Nickerson of Didcot Girls’ School trialled the experiment her Science Club, which made a number of helpful suggestions.


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Endothermic solid-solid reactions Solid hydrated barium hydroxide is mixed with solid ammonium chloride in a beaker. An endothermic reaction takes place to produce a liquid, with the evolution of ammonia. The temperature drops dramatically to about -20 °C.

Lesson organisation Although the experiment can be safely carried out as a class experiment (with GCSE or A-level candidates in mind), it lasts only about 5 minutes and is probably not worth the extra time spent by students setting up and clearing away. Therefore it is recommended as being more suitable as a teacher demonstration. Students could be allowed to feel the outside of the very cold container.

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)

Eye protection: goggles Fume cupboard (optional) One demonstration will require: Beaker (100 cm3) Watch-glass Thermometer, reading to -30 °C (see note 1) Top-pan balance Barium hydroxide-8-water (Corrosive), 32 g Ammonium chloride (Harmful), 10 g Concentrated hydrochloric acid (Corrosive) (see note 2) Universal indicator (or litmus) paper, 1 strip

Technical notes Barium hydroxide-8-water (Corrosive) Refer to CLEAPSS® Hazcard 10B Ammonium chloride (Harmful) Refer to CLEAPSS® Hazcard 9A Concentrated hydrochloric acid (Corrosive) Refer to CLEAPSS® Hazcard 47A 1 Consider using a thermocouple-type of thermometer which can be connected to a large display or computer monitor. 2 Small stock bottle, to provide fumes for ammonia test.

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Procedure

)

HEALTH & SAFETY: Wear goggles

Before the demonstration Weigh out separately the barium hydroxide and the ammonium chloride. Avoid lumps as far as possible.

The demonstration Work in a fume cupboard unless the room is well ventilated. a Stand the beaker on a watch-glass containing a few drops of water, so that the base of the beaker is touching the water.

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b Note the room temperature.

c Mix the two solids in the beaker and stir with the thermometer. The mixture becomes slushy as a liquid is formed, together with a white suspension. d The presence of ammonia can be detected by smell, and confirmed by blowing fumes from the hydrochloric acid bottle across the beaker’s mouth and by using moist indicator paper. e Observe the drop in temperature, which is confirmed by the fact that the beaker freezes to the watch-glass.

Teaching notes It helps to use a large thermometer display. The cold beaker can be passed around the class once the evolution of ammonia has stopped. It is not possible to determine easily the exact barium compound or compounds produced in this reaction but the equation is usually represented as: Ba(OH)2.8H2O(s) + 2NH4Cl(s) → 2NH3(g) + 10H2O(l) + BaCl2(s) or Ba(OH)2.8H2O(s) + 2NH4Cl(s) → 2NH3(g) + 8H2O(l) + BaCl2.2H2O(s) A-level students could be asked to calculate the value of the enthalpy and entropy changes for the reaction, using standard enthalpy changes of formation and standard entropy values obtained from a data book or from the table below. Compound

∆Hf°/kJ mol–1

S°/J mol–1 K–1

Ba(OH)2.8H2O(s)

–3345

427

NH4Cl(s)

–314

95

NH3(g)

–46

192

H2O(l)

–286

70

BaCl2(s)

–859

124

BaCl2.2H2O(s)

–1460

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An enthalpy change of +164 kJ mol–1 is obtained if the product is assumed to be BaCl2(s), and +135 kJ mol–1 if it is assumed to be BaCl2.2H2O(s). Students should be able to predict qualitatively that the entropy change for the system has a positive value because a gas and a liquid are formed from two solids. From the values above they could also be asked to calculate the actual entropy change for the system and the surroundings, and hence ∆Stotal or ∆G for the reaction and confirm that the process is spontaneous. A value of ∆Ssystem of +591 J mol–1 K–1 is obtained if the product is assumed to be BaCl2(s) and +530 J mol–1 K–1 if it is assumed to be BaCl2.2H2O(s).

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/energy-changes-and-fuels/ endothermic-solid-solid-reactions,277,EX.html

Useful resource This weblink shows a related endothermic reaction involving barium hydroxide: http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/MAIN/ENDO2/PAGE1.HTM (Website accessed December 2009)


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Exothermic or endothermic? This is a useful class practical to introduce energy changes in chemical reactions. The students measure the temperature changes in four reactions, and classify the reactions as exothermic or endothermic. The experiments can also be used to revise different types of chemical reaction and, with some classes, chemical formulae and equations.

Lesson organisation There are five solutions and three solids involved. Careful consideration will need to be given as to the most appropriate way to dispense these to the class. Special care should be taken with the magnesium ribbon and magnesium powder and, with some classes, teachers may prefer to dispense these materials directly. The length of time required for carrying out the actual reactions is around 30 minutes, but this will depend on the nature of the class and how the practical is organised.


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Eye protection Each group of students will need: Polystyrene cup (expanded polystyrene) Beaker (250 cm3) in which to stand the polystyrene cup for support (see note 1) Thermometer (–10 °C to 110 °C) Measuring cylinder (10 cm3), 2 Spatula Absorbent paper Access to the following solutions: (all at approx 0.4 mol dm–3 concentration); (see note 2) Copper(II) sulfate (Low hazard) Hydrochloric acid (Low hazard) Sodium hydrogencarbonate (Low hazard) Sodium hydroxide (Irritant) Sulfuric acid (Low hazard) Access to the following solids (see note 3): Magnesium ribbon (Highly flammable), cut into 3 cm lengths. Magnesium powder (Highly flammable). Citric acid (Irritant).

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Technical notes Copper(II) sulfate (Low hazard) Refer to CLEAPSS® Hazcard 27C and CLEAPSS® Recipe Card 19 Hydrochloric acid (Low hazard) Refer to CLEAPSS® Hazcard 47A and CLEAPSS® Recipe Card 31 Sodium hydrogencarbonate (Low hazard) Refer to CLEAPSS® Hazcard 95C and CLEAPSS® Recipe Card 64 Sodium hydroxide (Irritant) Refer to CLEAPSS® Hazcard 91 and CLEAPSS® Recipe Card 65 Sulfuric acid (Low hazard) Refer to CLEAPSS® Hazcard 98A and CLEAPSS® Recipe Card 69 Magnesium ribbon (Highly flammable) Refer to CLEAPSS® Hazcard 59A Magnesium powder (Highly flammable) Refer to CLEAPSS® Hazcard 59A Citric acid (Irritant) Refer to CLEAPSS® Hazcard 36C 1 Typical expanded polystyrene cups fit snugly into 250 cm3 squat form beakers. This provides a more stable reaction vessel and also prevents spillage if the polystyrene cup splits. 2 At the suggested concentrations, the solutions (except for sodium hydroxide) represent minimal hazards, although it is probably advisable to label them as Harmful. If the concentrations are increased then the solutions must be labelled with the correct hazard warning. The solutions could be provided in small (100 cm3) labelled conical flasks or beakers. 3 Small amounts of the solids can be provided in plastic weighing boats or similar. The teacher may prefer to keep the magnesium ribbon and powder under their immediate control and to dispense on an individual basis.

Procedure HEALTH & SAFETY: Wear eye protection throughout.

Reaction of sodium hydroxide solution and dilute hydrochloric acid

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a Stand the polystyrene cup in the beaker. b Use the measuring cylinder to measure out 10 cm3 of sodium hydroxide solution and pour it into the polystyrene cup. c Measure the initial temperature of the sodium hydroxide solution and record it in a suitable table. d Measure out 10 cm3 of hydrochloric acid and carefully add this to the sodium hydroxide solution in the polystyrene cup. Stir with the thermometer and record the maximum or minimum temperature reached. e Work out the temperature change and decide if the reaction is exothermic or endothermic. f Discard the mixture (in the sink with plenty of water). Rinse out and dry the polystyrene cup.

Reaction of sodium hydrogencarbonate solution and citric acid a Repeat steps a – c of the previous experiment, using sodium hydrogencarbonate solution in place of sodium hydroxide solution. b Add 4 small (not heaped) spatula measures of citric acid. Stir with the thermometer and record the maximum or minimum temperature reached. c Work out the temperature change and decide if the reaction is exothermic or endothermic. d Discard the mixture (in the sink with plenty of water). Rinse out and dry the polystyrene cup.

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Reaction of copper(II) sulfate solution and magnesium powder a Repeat steps a – c of the first experiment, using copper(II) sulfate solution in place of sodium hydroxide solution. b Add 1 small (not heaped) spatula measure of magnesium powder. Stir with the thermometer and record the maximum or minimum temperature reached. c Work out the temperature change and decide if the reaction is exothermic or endothermic. d Discard the mixture (in the sink with plenty of water). Rinse out and dry the polystyrene cup.

Reaction of sulfuric acid and magnesium ribbon a Repeat steps a – c of the first experiment, using sulfuric acid in place of sodium hydroxide solution. b Add one 3 cm piece of magnesium ribbon. Stir with the thermometer and record the maximum or minimum temperature reached. c Work out the temperature change and decide if the reaction is exothermic or endothermic. d Once all the magnesium ribbon has reacted, discard the mixture (in the sink with plenty of water). Rinse out and dry the polystyrene cup.

Teaching notes The reactions and types of reaction involved are: Sodium hydroxide + hydrochloric acid → sodium chloride + water (Neutralisation) NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) Copper(II) sulfate + magnesium → magnesium sulfate + copper (Displacement, Redox) CuSO4(aq) + Mg(s) → MgSO4(aq) + Cu(s) Sulfuric acid + magnesium → magnesium sulfate + hydrogen (Displacement, Redox) H2SO4(aq) + Mg(s) → MgSO4(aq) + H2(g) At this level the neutralisation reaction between sodium hydrogen carbonate and citric acid may be a bit complicated – it may be better to just use the word equation. More able students could use H+(aq) to represent the acid. Sodium hydrogencarbonate + citric acid → sodium citrate + water + carbon dioxide NaHCO3(aq) + H+(aq) → Na+(aq) + H2O(l) + CO2(g)

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/energy-changes-and-fuels/ exothermic-or-endothermic,72,EX.html

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Chemiluminescence – cold light

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A solution of sodium chlorate(I) oxidises an aqueous solution of luminol (3-aminophthalhydrazide). The reaction gives out a blue chemiluminescent glow without any increase in temperature of the mixture.

Lesson organisation This demonstration experiment shows that a chemical reaction can give out energy as light instead of heating up its surroundings. The demonstration can also be used to stimulate interest in chemistry at an Open Day or other public event.

Apparatus and chemicals Eye protection The teacher will require:

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Conical flasks with stoppers (1 dm3), 2 Beaker (2 dm3) Thermometer (0 – 100 °C) Balance (1 d.p.) Quantities of chemicals for one demonstration Luminol (3-aminophthalhydrazide), 0.4 g (Irritant) (see note 4) Sodium hydroxide pellets, 4.0 g (Corrosive) Household bleach, 100 cm3 (Irritant) (see notes 1, 2 and 3) Fluorescein (or sodium fluorescein) powder (optional)

Technical notes Luminol (Irritant) Refer to CLEAPSS® Hazcard 4B Fluorescein (Low hazard) Refer to CLEAPSS® Hazcard 32 Sodium hydroxide pellets (Corrosive) Refer to CLEAPSS® Hazcard 91 Sodium chlorate(I) solution (Corrosive) Refer to CLEAPSS® Hazcard 89 Household bleach (Irritant) Refer to CLEAPSS® Hazcard 89 1 Household bleach, which is typically a 5% 'available chlorine' solution, can be used. Make sure that the household bleach contains sodium chlorate(I) (sodium hypochlorite), NaOCl, and not hydrogen peroxide, as the bleaching agent. Many household bleaches nowadays also contain thickeners and/or detergents. Use ‘economy’ bleaches without any additives. 2 The sodium chlorate(I) solution sold by chemical suppliers contains up to 14% available chlorine. It has a limited shelf life. Adjust the volumes of bleach and water to make up the diluted bleach solution for the demonstration. See Procedure step a. 3 Tap water can be used for making up the solutions. The solutions are stable for over 12 hours and so can be made up well in advance. 4 The luminol does not always appear to dissolve completely, leaving a fine, greenish suspension.

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Procedure HEALTH & SAFETY: Wear eye protection.

Before the demonstration: a Add 100 cm3 of the household bleach solution to 900 cm3 of water in one of the flasks, mix well and stopper. Alternatively, add 50 cm3 of commercial NaOCl solution to 950 cm3 of water. See notes 1 to 3 above. b In the other flask put 0.4 g of luminol, 1 dm3 of water and 4.0 g of sodium hydroxide. Swirl to dissolve the chemicals and then stopper the flask. See notes 3 and 4 above.

The demonstration: c Take the temperature of the solutions. d Lower the room lights and slowly pour the two solutions at the same rate into the beaker so that they mix. A pale blue glow will be seen which will last for a few seconds. e Take the temperature of the mixture. It will be the same as that of the starting solutions.

Teaching notes For more dramatic effect, pour the solutions into a funnel attached to clear tubing bent into a variety of shapes, such as a spiral. In the reaction, luminol is oxidised by the bleach to the aminophthalate ion, which is produced in an electronic excited state. This gives out energy as light (fluorescence) when it decays to the ground state. O

O N H N H

NH2

+ 2OH– + 2NaOCl

oxidation

O

O– O– NH2

+ N2 + 2H2O + NaCl

O

aminophthalate ion

Adding a small amount of fluorescein to the luminol solution, just before the demonstration, will alter the glow to a yellow-green colour. Chemiluminescent ‘light sticks’ will be familiar nowadays to many students. A different reaction is used, involving the oxidation of a di-ester by hydrogen peroxide in an organic solvent. This reaction is much slower, the glow continuing for some hours. One solution is contained inside a glass phial inside a plastic tube containing the other solution. Bending the plastic tube breaks the glass phial, allowing the two reactants to mix. Colour effects are obtained by adding dyes to which the excitation energy is transferred, the excited dye molecules then emit light of different wavelengths. Can you slow the reaction down by placing the light stick in a freezer? Other recipes are given on CLEAPSS® Recipe Card 16.

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Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/energy-changes-and-fuels/ chemiluminescence-cold-light,62,EX.html

Useful resources Useful website: http://www.chm.bris.ac.uk/webprojects2002/fleming/intro.htm To view a video clip of this demonstration experiment, go to: http://media.rsc.org/videoclips/demos/Chemiluminescence.mpg


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Spontaneous exothermic reaction In this demonstration experiment, a mixture of glycerol (propane-1,2,3-triol) and potassium manganate(VII) crystals bursts into flame, giving off clouds of steam, after a short time lag.

Lesson organisation This reaction can be used as a fun demonstration to show a spontaneous reaction, or as an example of the redox reaction between a fuel and a powerful oxidising agent. The time lag illustrates the speeding up of an initially slow exothermic reaction as the energy given out raises the temperature of the mixture.


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Eye protection Safety screens The teacher requires: Clean metal lid from a tin can or jar (see note 1) Heat resistant mat The quantities of chemicals given are for one demonstration. Access to: Potassium manganate(Vll) (Oxidising agent, Harmful, Dangerous to the environment), 2-3 g in the form of fine crystals (see note 2) Glycerol (propane-1,2,3-triol) (Low hazard), about 1 cm3 in a test-tube (see note 3)

Technical notes Potassium manganate(VII) (Oxidising agent, Harmful, Dangerous for the environment) Refer to CLEAPSS® Hazcard 81 Glycerol (propane-1,2,3-triol) (Low hazard) Refer to CLEAPSS® Hazcard 37 1 If the lid from a jar has a plastic lining, this should be scraped off. Alternatively, use a small foil cake case which has been cleaned and dried. 2 Fine crystals of potassium manganate(VII) work much better than larger ones. Use a pestle to grind large crystals in a clean mortar, if necessary. 3 Old samples of glycerol are sometimes ineffective, possibly because of absorbed water from the air.

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Procedure HEALTH & SAFETY: Wear eye protection (teacher and students). Use safety screens. a Put 2-3 g of potassium manganate(VII) in a small pile on the tin lid standing on the heatproof mat. Make a small hollow in the centre of the pile.

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b Pour about 1 cm3 of glycerol into the hollow in the pile of potassium manganate(VII). It is sometimes better if the glycerol is warmed just before use. After about 20 seconds (but beware – it can be much longer), the mixture starts to give off steam. The glycerol in the mixture then ignites, burning with a bright, pinkish (lilac) flame for a few seconds more, leaving a dark brown or black residue.

Teaching notes Eye protection and safety screens are essential. Small particles of potassium manganate(VII) may fly out. A white background is useful. The reaction is even more spectacular in a darkened room. Point out that the pink (lilac) colour of the flame is characteristic of potassium salts.

Redox chemistry At advanced level, the redox nature of the reaction can be explored. Do this by allowing the residue to cool down and then dissolving it in water. This produces a green solution suggesting the presence of a Mn(Vl) species, as well as a brown solid, manganese(lV) oxide. This confirms the reduction of the manganate(Vll) ion; the glycerol has been oxidised to water (hence the steam) and carbon dioxide.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/oxidation-and-reduction/ spontaneous-exothermic-reaction,48,EX.html

Useful resource This website has a movie showing the experiment 'Oxidation of glycerine by potassium permanganate': http://www.jce.divched.org/JCESoft/CCA/CCA1/R1MAIN/CD1R1870.HTM#1920


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The effect of concentration on reaction rate Sodium thiosulfate solution is reacted with acid – a sulfur precipitate forms. The time taken for a certain amount of sulfur to form can be used to indicate the rate of the reaction. This experiment should take around 60 minutes.

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Apparatus and chemicals Eye protection Each group will need: 250 cm3 Conical flask 100 cm3 Measuring cylinder Sodium thiosulfate solution 50 g dm–3 (Low hazard) Hydrochloric acid 2 mol dm–3 (Irritant)

Technical notes Sodium thiosulfate solution 50 g dm–3 (Low hazard) Dilute hydrochloric acid (Irritant at concentration used) Refer to CLEAPSS® Hazcard 47A and Recipe Card 31

Teaching notes

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HEALTH & SAFETY: Wear eye protection. Sulfur dioxide (Toxic gas) forms as a by product. Ensure good ventilation. Warn asthmatics, who should preferably use a fume cupboard. As soon as the reaction is complete pour the solutions away, preferably into the fume cupboard sink. Wash away with plenty of water. See student worksheet s62. The method for this experiment is best understood when the teacher demonstrates it first. A light sensor can be used to monitor the precipitation on a computer. The result, in the form of graphs on the computer, can be analysed using data logging software. A light sensor clamped against a plastic cuvette filled with the reactants substitutes for a colorimeter. The data logging software shows the turbidity on a graph and this tends to yield more detail than the standard end-point approach. The rate of change can be measured using the slope of the graph or the time taken for a change to occur.

Background theory Basic collision theory.


Useful resource An alternative, microscale version of this experiment can be used to minimise the exposure to SO2. See CLEAPSS® Guide, L195 Safer chemicals, safe reactions p.43: A safer procedure for the thiosulfate/acid reaction.

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s62

Student worksheet The effect of concentration on a reaction rate Introduction In this experiment, the effect of the concentration of sodium thiosulfate on the rate of reaction is investigated.

initial view through conical flask from above

sodium thiosulfate and dilute hydrochloric acid

dilute hydrochloric acid

sodium thiosulfate

What to record 1 Complete the table: Volume of sodium thiosulfate solution/ cm3

Volume of water/cm3

50

0

40

10

30

20

20

30

10

40

Time taken for cross to disappear /s

Original concentration of sodium thiosulfate solution/g dm–3

1/time taken /s–1

50

What to do Health & safety: Wear eye protection. Take care not to inhale fumes. 1 Put 50 cm3 of sodium thiosulfate solution in a flask.

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2 Measure 5 cm3 of dilute hydrochloric acid in a small measuring cylinder. 3 Add the acid to the flask and immediately start the clock. Swirl the flask to mix the solutions and place it on a piece of paper marked with a cross. 4 Look down at the cross from above. When the cross disappears stop the clock and note the time. Record this in the table. 5 Repeat this using different concentrations of sodium thiosulfate solution. Make up 50 cm3 of each solution. Mix different volumes of the sodium thiosulfate solution with water as shown in the table. 6 As soon as possible, pour the solution down the sink (in the fume cupboard if possible) and wash away.

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Questions 1 Calculate the concentration of sodium thiosulfate in the flask at the start of each experiment. Record the results in the table. 2 For each set of results, calculate the value of 1/time. (This value can be taken as a measure of the rate of reaction). 3 Plot a graph of 1/time taken on the vertical (y) axis and concentration on the horizontal (x) axis.

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Iodine clock reaction

63

This is the hydrogen peroxide/ potassium iodide ‘clock’ reaction. A solution of hydrogen peroxide is mixed with one containing potassium iodide, starch and sodium thiosulfate. After a few seconds the colourless mixture suddenly turns dark blue. This is one of a number of reactions loosely called the iodine clock. It can be used as an introduction to experiments on rates / kinetics.

Lesson organisation This demonstration can be used at secondary level as an introduction to some of the ideas about kinetics. It can be used to stimulate discussion about what factors affect the rate of reaction. It also makes a useful starting-point for a student investigation. As described this is intended as a demonstration, best done on a large scale for the most visual impact. The demonstration itself takes less than 1 minute. For a student investigation, the quantities required would be smaller but volumes then need to be measured quite accurately with, for example, disposable plastic syringes. It also lends itself to a class competition aiming for a change at a teacher determined time.

Apparatus and chemicals Eye protection Balance (1 or 2 d.p.) Volumetric flasks (1 dm3), Beakers (100 cm3), 5 Beaker (250 cm3) Beaker (2 dm3) Boiling tubes, 5 Boiling tube rack Measuring cylinder (50 cm3) Measuring cylinders (100 cm3), 2 Stirring rod or magnetic stirrer and follower (optional) Stopclock/timer, 5

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0.2 g soluble starch 1M sulfuric acid (Irritant), 50 cm3 Potassium iodide (KI), 6.0 g. (Low hazard) Sodium thiosulfate-5-water (Na2S2O3.5H2O), 7.5 g (Low hazard) 20 volume hydrogen peroxide solution (H2O2(aq)), 100 cm3 (Irritant) Deionised/distilled water, 1 dm3.

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Technical notes 20 volume hydrogen peroxide is Irritant. Refer to CLEAPSS® Hazcard 50. 1M sulfuric acid is Corrosive. Refer to CLEAPSS® Hazcard 98A 1 Solution X and the starch solution should be made up before the demonstration. The solutions will keep overnight, but best results are obtained if the solutions are made up on the day. 2 The starch solution needs to be fresh.

Procedure

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HEALTH & SAFETY: Wear eye protection Solution X is made up as follows: a Dissolve 6.0 g of potassium iodide in approximately 800 cm3 of distilled water. b To the potassium iodide solution add 7.5 g of sodium thiosulfate and dissolve. c Transfer the solution to a 1 dm3 volumetric flask and make up the solution to 1 dm3 with distilled water. Ensure the solution is well mixed. Starch Solution is made up as follows: a Make a paste of 0.2 g of soluble starch with a few drops of water in a 250 cm3 beaker. Pour onto this approximately 100 cm3 of boiling water and stir. Both solutions are colourless although solution X will be slightly cloudy on storage.

The demonstration HEALTH & SAFETY: Wear eye protection a Stand five 100 cm3 beakers side by side on white paper or tiles. b To each beaker add 20 cm3 of solution X, 10 cm3 of 1M sulfuric acid and approximately 2 cm3 of starch solution. c Prepare five boiling tubes containing 30, 25, 20, 15, 10 cm3 respectively of 20 vols hydrogen peroxide. Maintain the overall volume by adding 0, 5, 10, 15, 20 cm3 of distilled water. d Add the contents of the five boiling tubes to the corresponding beakers at the same time whilst starting timing. Assistants will be required at this stage. e Record the time taken for the change in colour (blue/black) of each beaker.

Additional notes Hydrogen peroxide is capable of oxidising thiosulfate ions to tetrathionate ions but the reaction is too slow to affect this demonstration. The acid will react slowly with sodium thiosulfate and produce a cloudy suspension of sulfur and release sulfur dioxide which is TOXIC, therefore dispose of the mixture immediately after use.

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Teaching notes Visual tips: a white background will help so that the impact of the sudden and spectacular colour change is not lost. Scaling up the volumes of solution that are mixed may help in a large room. There is no warning of when the blue colour is about to appear. It may help understanding if the students are already familiar with the reactions of starch and iodine, and iodine and sodium thiosulfate, so it may be worth demonstrating these beforehand. The basic reaction is: H2O2(aq) + 2I–(aq) + 2H+(aq) → I2(aq) + 2H2O(l) [For more advanced discussions or investigations - this reaction is the rate determining step and is first order with respect to both H2O2 and I–.] As soon as the iodine is formed, it reacts with the thiosulfate to form tetrathionate ions and recycles the iodide ions by the fast reaction: 2S2O32–(aq) + I2(aq) → S4O62–(aq) + 2I–(aq) As soon as all the thiosulfate is used up, free iodine (or, strictly, I3- ions) remains in solution and reacts with the starch to form the familiar blue-black complex. The time for the blue colour to appear can be adjusted by varying the amount of thiosulfate in solution X so a ‘clock’ of any desired time interval can be produced. If the demonstration is being done for entertainment, the imaginative teacher will be able to think up some suitable patter.



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64

Rates and rhubarb In this experiment, rhubarb sticks, which contain oxalic acid, are used to reduce and decolourise potassium manganate(VII) solution. The experiment can be used to show how the rate of reaction is affected by surface area or concentration.

Lesson organisation This experiment is probably most suited to younger students or groups of students who do not need to be given the details of the reaction itself. It is difficult to relate the rate back to the equation. This is because the results can be complex, due to the competing reactions taking place. (See the additional teaching notes.) On a simple level the experiment works well, and makes an unusual alternative to hydrochloric acid and marble chips. All the practical work can easily be completed in a one-hour session, and graphs could be plotted of the results. Alternatively, the practical work and follow-up can be split over two separate sessions. You could also discuss with students in what ways this experiment is not a fair test. Warn students not to consume anything in the lab – they may be tempted to taste the rhubarb (although it doesn’t taste very good unsweetened). If you use rhubarb from home or someone’s garden, ensure that the leaves are removed before it is given to students. Rhubarb leaves contain far more oxalic acid than the stalk, and are harmful.

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Apparatus and chemicals Eye protection

Investigating the effect of surface area Each working group will require: Beakers (100 cm3), 2 or more Measuring cylinder (50 cm3) Timer White tile or piece of paper Students will need access to: Rhubarb stalks (frozen rhubarb also works if the pieces are long enough) (see note 3) Knives, 4 to 6 per class (ordinary table knives are probably most appropriate.) Acidified potassium manganate(VII) solution (Irritant) (see notes 1 and 2)

Investigating the effect of concentration: Additional requirements Each working group will require: Beaker (250 cm3) Bunsen burner Heat resistant mat Tripod Gauze Students will need access to: Filter funnels and filter paper or tea strainers

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Technical notes Potassium manganate(VII) solution (Irritant) Refer to CLEAPSS® Hazcard 81 and Recipe card 56 Sulfuric acid (Corrosive) Refer to CLEAPSS® Hazcard 98A Oxalic acid (Harmful) Refer to CLEAPSS® Hazcard 36A 1 Wear eye protection when making and using the solutions. When making up the potassium manganate(VII) solution, the sulfuric acid is 2 mol dm–3 and corrosive. Once diluted with the potassium manganate(VII) solution, the acid is approximately 1 mol dm–3 and an irritant. Each group of students will need approximately 300 cm3 of the solution in total (for both experiments). Adjust the volumes given below accordingly. 2 Put 4 or 5 crystals (or sufficient powder to just cover the tip of a spatula) of potassium manganate(VII) (Oxidising agent, Harmful, Dangerous for the environment) into a beaker with about 500 cm3 of distilled water. Stir until the crystals dissolve. Add about 500 cm3 of 2 mol dm-3 sulfuric acid (Corrosive). Stir to mix. The solution should be a light purple colour. If necessary, dilute further with a little more water. The exact concentration is not critical. 3 As the oxalic acid content of the rhubarb will vary, it is well worth checking that the solution of potassium manganate(VII) will give appropriate results with the rhubarb you are using.


Investigating the effect of surface area

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a Cut three 5 cm lengths of rhubarb. Leave one piece as it is, cut one piece in half lengthways, and cut the third piece into 4 evenly-sized pieces. b Measure 30 cm3 of acidified potassium manganate(VII) solution into a beaker. Pour the same quantity of water into another beaker. c Place the beakers on a white tile. Put the whole 5 cm long piece of rhubarb into the potassium manganate(VII) and start the timer. Stir the solution containing the rhubarb until the purple colour disappears. If you are not sure, briefly remove the rhubarb and compare the colour of the solution to the beaker of water. When they look the same, stop the timer. d Rinse out and dry the reaction beaker. e Repeat the experiment using the piece of rhubarb cut into 2 (use both halves). Rinse and dry the beaker. f Repeat the experiment again, this time using the piece of rhubarb cut into 4.

Investigating the effect of concentration a Cut the stick of rhubarb (widthways this time) into thin slices (about 0.5 cm) and put them into the 250 cm3 beaker. Cover the rhubarb with distilled water and heat gently. b Bring the rhubarb to the boil and continue heating gently until the rhubarb falls to pieces. This will take about 5 minutes. Turn off the Bunsen and leave the rhubarb mixture to cool. c When cool enough to handle, filter or strain the mixture and keep the filtrate (liquid). d Measure 30 cm3 of acidified potassium manganate(VII) solution into one of the 100 cm3 beakers and the same amount of water into the other. e Add one drop of the rhubarb filtrate to the potassium manganate(VII) solution and start the timer. Stop the timer when the colour disappears and is the same as the plain water. f Repeat the experiment for 2, 3, 4, 5, and 6 drops of the rhubarb extract.

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Additional teaching notes, hints and tips Oxalic acid Rhubarb contains oxalic acid (ethanedioic acid) which has the formula C2H2O4 Oxalic acid reacts with potassium manganate(VII) in acidic solutions and is oxidised to carbon dioxide and water: 2MnO4– + 5C2H2O4 + 6H3O+ → 2Mn2+ + 10CO2 + 14H2O

O HO

C C

OH

O

The potassium manganate(VII) decolourises which provides a convenient and easy-tomeasure end-point to the reaction. Aqueous solutions of Mn2+ are actually pale pink, but at these concentrations will appear almost colourless. Students should be able to observe that as the surface area of the rhubarb increases, so does the rate of the reaction. Likewise for increasing concentration of rhubarb juice. The concentration has been varied by putting in more drops of the rhubarb extract, so the total volume has been increased. You may like to discuss the implications of this with the students. It is worth noting that the reaction is autocatalysed (catalysed by a product of the reaction) by the Mn2+ ions. This could lead to some confusing patterns for students if the results are analysed too closely, and an attempt is made to link the results to the equation.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/rates-of-reaction/rates-andrhubarb,61,EX.html

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Catalysts for the decomposition of hydrogen peroxide

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Several measuring cylinders are set up each containing a little washing up liquid and a small amount of a catalyst for the decomposition of hydrogen peroxide. Hydrogen peroxide is poured into the cylinders and a foam rises up the cylinders at a rate that depends on the effectiveness of the catalyst.

Apparatus and chemicals Safety goggles and nitrile gloves must be worn by the demonstrator Several 250 cm3 measuring cylinders – one for each catalyst to be used. A large tray to catch any foam that spills over the top of the cylinders. Stopwatch or clock with second hand.

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The chemical quantities given are for one demonstration. 75 cm3 of 100 volume hydrogen peroxide solution (Harmful at this concentration) About 0.5 g of powdered manganese(IV) oxide (manganese dioxide, MnO2) (Harmful) About 0.5 g of lead(IV) oxide (lead dioxide, PbO2) (Toxic and dangerous for the environment) About 0.5 g of iron(III) oxide (red iron oxide, Fe2O3) (Low hazard) A small piece (about 1 cm3) of potato. A small piece (about 1 cm3) of liver.

Technical notes 75 cm3 of 100 volume hydrogen peroxide solution (Harmful at this concentration) Refer to CLEAPSS® Hazcard 50 About 0.5 g of powdered manganese(IV) oxide (manganese dioxide, MnO2) (Harmful at this concentration) Refer to CLEAPSS® Hazcard 60 About 0.5 g of lead(IV) oxide (lead dioxide, PbO2) (Toxic and dangerous for the environment) Refer to CLEAPSS® Hazcard 56 About 0.5 g of iron(III) oxide (red iron oxide, Fe2O3) (Low hazard). Refer to CLEAPSS® Hazcard 55A 1 Used liver should be wrapped up in paper and placed in the dustbin. 2 The use of the washing up liquid helps prevent the catalyst from going into the atmosphere but there may be some hydrogen peroxide aerosol. 3 To dispose of the lead(IV) oxide, filter it off from the solution, wash it and let it dry. It could be reused.

Procedure Health & Safety: The demonstrator should wear goggles and nitrile gloves (see CLEAPSS® Hazcard 50)

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Before the demonstration a Line up five 250 cm3 measuring cylinders in a tray. Add 75 cm3 of water to the 75 cm3 of 100 volume hydrogen peroxide solution to make 150 cm3 of 50 volume solution.

The demonstration

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a Place about 1 cm3 of washing up liquid into each of the measuring cylinders. To each one add the amount of catalyst specified above. Then add 25 cm3 of 50 volume hydrogen peroxide solution to each cylinder. The addition of the catalyst to each cylinder should be done as nearly simultaneously as possible – using two assistants will help. Start timing. Foam will rise up the cylinders. The lead dioxide will probably be fastest, followed by manganese dioxide and liver. Potato will be much slower and the iron oxide will barely produce any foam. This order could be affected by the surface areas of the powders. Time how long each foam takes to rise to the top (or other marked point) of the cylinder. The foam from the first three cylinders will probably overflow considerably. b Place a glowing spill in the foam; it will re-light confirming that the gas produced is oxygen.

Teaching tips Some students may believe that the catalysts – especially the oxides – are reactants because hydrogen peroxide is not noticeably decomposing at room temperature. The teacher could point out the venting cap on the peroxide bottle as an indication of continuous slow decomposition. Alternatively s/he could heat a little hydrogen peroxide in a conical flask with a bung and delivery tube, collect the gas over water in a test-tube and test it with a glowing spill to confirm that it is oxygen. This shows that no other reactant is needed to decompose hydrogen peroxide. NB: Simply heating 50 volume hydrogen peroxide in a test-tube will not succeed in demonstrating that oxygen is produced. The steam produced will tend to put out a glowing spill. Collecting the gas over water has the effect of condensing the steam. It is also possible to ‘cheat’ by dusting a beaker with a tiny, almost imperceptible, amount of manganese dioxide prior to the demonstration and pouring hydrogen peroxide into it. Bubbles of oxygen will be formed in the beaker.

Theory The reaction is : 2H2O2(aq) → 2H2O(l) + O2(g) This is catalysed by a variety of transition metal compounds and also by peroxidase enzymes found in many living things.

Extensions Repeat the experiment but heat the liver and the potato pieces for about five minutes in boiling water before use. There will be almost no catalytic effect, confirming that the catalyst in these cases is an enzyme that is denatured by heat. Investigate the effect of using lumpy or powdered manganese dioxide. The powdered oxide will be more effective because of its greater surface area. Try using other metal oxides or iron filings as catalysts. Animal blood may be used instead of liver if local regulations allow this. One teacher suggested measuring the height of the foam over suitable time intervals and plotting a graph.


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Controlled explosion of a hydrogenair mixture

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A large fizzy drink bottle, from which the base has been removed, is filled with hydrogen. The hydrogen is allowed to burn at a small jet in the stopper of the bottle. As the hydrogenair mixture changes in composition, an explosive mixture is reached, which then explodes with a load bang.

Lesson organisation This simple demonstration can be used for fun, such as on Open Days, or to provide an illustration of the effect of the composition of a mixture of a flammable gas and air on its explosive properties. The link can be made to domestic gas explosions. It can also be used to introduce the idea of the conversion of chemical energy into heat, light, sound and kinetic energy in the context of fuels. The time for carrying out the demonstration should be about 10 min.

Apparatus and chemicals Eye protection for the demonstrator Ear protectors for the demonstrator Safety screens

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‘Fizzy’ drink bottle, 1 dm3, no larger (see note 1) One-holed bung, to fit bottle Short length of glass tubing (about 5 cm), to fit in bung (see note 1) Short length (about 5 cm) of rubber tubing, to fit glass tube (see note 1) Hoffman or screw clip, for rubber tubing Gas delivery tube Large plastic or glass trough, for collecting gas over water Boss, clamp and stand A source of hydrogen gas (Extremely flammable): cylinder and regulator fitted with a length (about 50 cm) of rubber tubing (see note 2) Wooden splints (see note 3)

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Technical notes Hydrogen (Extremely flammable) Refer to CLEAPSS® Hazcard 48 1 Cut the bottom off the drinks bottle and replace the cap with a bung carrying a short length of glass tubing to which is attached a short length of rubber tubing and a clip – see the figure below. The glass tube must not protrude into the bottle beyond the bottom of the bung. In the past a large (500 - 750 g) tin can with a press-on lid (such as a catering size instant coffee tin) has often been used for this demonstration. Such tins are harder to find nowadays. If such a tin is used, then a small (about 4 mm in diameter) hole must be made in the lid, and a larger (about 1 cm diameter) hole in the base. Another alternative is a cardboard container fitted with a plastic lid (such as a Pringle® crisp container). In both cases the container must be filled with hydrogen by inserting the delivery tube into the hole in the base and allowing at least one minute to ensure that all the air has been flushed from the container. The hole in the lid should then be sealed with a small piece of plasticine or flexible adhesive material (such as BluTak). The container should then be placed on a tripod or clamped with some clearance below. Take care not to site it undeneath light fittings that could be damaged by the flying lid.

Hoffman clip rubber tubing glass tube 5mm bore clamp 1 litre plastic fizzy drink bottle cut off at the bottom

hydrogen in

Filling the chamber

Burning the hydrogen

Make sure the rubber tubing is an airtight fit, but it can be prised off easily before the gas is ignited. If the glass tube is too narrow, it may not support the flame as it ‘backfires’ down the tube into the bottle.

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2 It is recommended that this demonstration is not attempted using hydrogen from a chemical generator because of the difficulty of generating the gas fast enough to flush the air out of the apparatus reliably. 3 Some teachers may prefer to light the gas using a wax taper or candle attached to the end of a metre rule. It is useful to have a Bunsen burner lit some distance from the can, for lighting the splint, taper or candle. If the tin or bottle has not been completely flushed with hydrogen or some air has re-entered the tin (likely if there is a delay between filling the tin and igniting the gas), the explosion can occur immediately on igniting the gas. The demonstrator, and the class, should be prepared for this!

Procedure a Fill the trough with water and clamp the bottle in position with its bottom well below the water level. Place a safety screen between the bottle and the class, who should be at least 2 m away. b Fill the bottle with water by opening the clip and sucking up water from the trough until it is just below the glass tube in the bung. Tighten the clip sufficiently to prevent the water level dropping. c Fill the bottle with hydrogen, using the delivery tube, until bubbles start to escape from the bottom. Shut off the hydrogen supply and move it away d Raise the clamp holding the bottle so that the trough can be slid away. Re-tighten the clamp. Light a splint and, holding it in one hand, remove the rubber tubing and clip from the bottle with the other hand. Ignite the hydrogen escaping from the top of the tube and step back from the bottle. The gas should burn with a small flame, which may be yellow at first, then turn blue and be almost invisible in daylight. After about 20 - 30 seconds the flame will decrease in size and appear to go out. Shortly afterwards there will be a loud explosion. (If a tin or cardboard tube is used - see Technical Notes 1), the lid will fly into the air.)

Teaching notes The reaction occurring is the combustion of hydrogen to form water: 2H2(g) + O2(g) → 2H2O(g) ΔH = -484 kJ mol–1 The energy released appears as heat, light, sound and kinetic energy, similar to the situation in an internal combustion engine. Mixtures of air and flammable gases usually have quite narrow explosive limits but hydrogen-air mixtures are explosive over a much broader range (4 - 77 mol% hydrogen). Archival footage of the explosion of the hydrogen-filled airship ‘Hindenburg’ in 1937 (see Web Links) could add interest here. This explosion would have been much more serious but for the fact that because of hydrogen’s very low density, most of the flames swept rapidly upwards rather than spreading sideways and downwards. The source of the energy for this reaction could be discussed, with a suitable group, in terms of the breaking and making of bonds. In view of the current interest in fuel cells as a source of energy for vehicles, it could be pointed out that a hydrogen fuel cell extracts the energy from this reaction in the form of electricity, rather than explosively in the form of heat. This electrical energy can be used much more efficiently to propel a vehicle than the equivalent amount of heat energy.

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Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/energy-changes-and-fuels/ controlled-explosion-of-a-hydrogen-air-mixture,293,EX.html

Useful resource Links to other Practical Chemistry experiments: Experiment 67: Controlled explosion of a methane-air mixture Experiment 68: Exploding balloons Combustion of hydrogen in air (http://www.practicalchemistry.org/experiments/combustion-ofhydrogen-in-air,292,EX.html) Video clips of the Hindenburg disaster 1. In colour, with commentary: http://www.vidicom-tv.com/tohiburg.htm 2. Black and white footage, with commentary: http://www.metacafe.com/watch/311189/hindenburg_disaster/ 3. Good quality b& w images, silent, with more extended footage: http://video.google.com/videoplay?docid=-375557973653978433 (Last accessed December 2009)

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Controlled explosion of a methaneair mixture

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A large tin fitted with a press-on lid and a glass chimney is filled with methane. The gas is lit at the top of the chimney. After a while the flame burns down the chimney and, as the methane-air mixture in the tin changes in composition, an explosive mixture is reached and the lid of the tin is blown off with loud bang.

Lesson organisation This simple demonstration can be used for fun, such as at Open Days, or to provide an entertaining illustration of the effect of the composition of a mixture of an inflammable gas and air on its explosive properties. A link can be made to domestic gas explosions. A darkened room will heighten visibility of the explosion flame. The time for carrying out the demonstration should be about 5 min.

Apparatus and chemicals Eye protection for the demonstrator Safety screens

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A large tin with a press-on lid (see note 1) Glass tubing, 2 - 3 cm in diameter, 30 - 50 cm long Epoxy resin adhesive (e.g. Araldite) Length of rubber tubing Boss, clamp and stand Methane (Natural Gas) supply (Extremely flammable) Wooden splints

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Technical notes Methane (Natural Gas) (Extremely flammable) Refer to CLEAPSS® Hazcard 45A 1 A catering size (500 – 750 g) instant coffee tin works well. Make a small hole about 1 cm in diameter in the base of the can. Make a larger hole halfway up the side of the tin to take the glass tubing. Use epoxy adhesive to glue the glass tube in place – see the figure below.

flame

glass chimney epoxy resin adhesive

coffee tin

lid CH4

Procedure

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a Put the lid on the tin, place it on its side and use the glass chimney to clamp it in position so that the lid is facing away from the class – see the figure above. Place a safety screen between the tin and the class. b Insert the rubber tube attached to the gas source into the hole in the base of the tin and fill the tin with methane. Allow at least a minute to ensure that the methane has displaced all the air in the tin. c Turn off the gas supply and remove the tubing. Without delay use a lighted splint at arms length to ignite the gas emerging from the chimney. It will initially burn with a yellow luminous flame. This will change to a blue flame as more air is drawn into the tin. d After a short while the flame will start to descend the chimney. As it reaches the bottom, the gas mixture in the can will explode, blowing the lid off the can. The explosion is fairly gentle and the lid must not be too tightly in place or it will not be blown off.

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Teaching notes The reaction is the combustion of methane to form carbon dioxide and water: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l), ΔH = - 890 kJ mol–1 The similarity of the flame to the flames of a Bunsen burner with the air hole open and shut should be pointed out. The flame descends the chimney because the combustion reaction is using up gas faster than the gas is rising up the chimney. The energy released appears as heat, light, sound and kinetic energy (of the flying lid), similar to the situation in an internal combustion engine. Methane-air mixtures have quite narrow explosive limits (4 -17 mol %), whereas hydrogen-air mixtures are explosive over a much broader range (4 -77 mol %). The source of the energy produced by the reaction could be discussed in terms of the breaking and making of bonds. This demonstration will not work with other domestic gases, such as propane and butane, because they are denser than air. This difference also affects how these gases behave in the event of leaks. Methane (natural gas) would concentrate near the ceiling or move upstairs and could escape through open high windows. Propane and butane would sink, building up at floor level, and could migrate down stairs or into cellars. Many mining disasters, especially coal mining, have been caused by explosion of methaneair mixtures. Methane levels have to be constantly monitored. Canaries and the Davy safety lamp have been used in the past.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/energy-changes-and-fuels/ controlled-explosion-of-a-methane-air-mixture,295,EX.html

Useful resources Related experiment: Experiment 66: Controlled explosion of a hydrogen-air mixture News report of methane explosion in Ukranian coal mine in 1999. http://uk.youtube.com/watch?v=LTsfyv8tT8g Photo and some details of a volcanic methane explosion. http://volcanoes.usgs.gov/images/pglossary/methane.php (Last accessed December 2009)


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Exploding balloons In this demonstration experiment, mixtures of hydrogen and oxygen gases in party balloons are ignited. Varying the proportions of the two gases alters the vigour of the resulting explosive, exothermic reaction.

Lesson organisation This is a very noisy demonstration and should be done in a large room (e.g. a school hall) or outdoors so the audience can sit or stand well away from the exploding balloons. Be aware as well that the noise will affect others in the vicinity, including possibly people living near the school.

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Apparatus and chemicals Eye protection Ear protectors The teacher will require: Small balloons, at least 4 (see note 1) Metre rule Small candle, wax taper or wooden splint (see note 2) Cotton thread Boss, clamp and stand Matches Access to cylinders (with regulators and rubber delivery tubing) of hydrogen (Extremely flammable) and oxygen gas (Oxidising agent) (see note 3) Students will require: Eye protection

Technical notes Hydrogen (Extremely flammable) Refer to CLEAPSS® Hazcard 48 Oxygen (Oxidising agent) Refer to CLEAPSS® Hazcard 69 1 Small party balloons are ideal. Do not fill the balloons too far in advance of the lesson. 2 Attach the candle, wax taper or wooden splint to the end of the metre rule with adhesive tape or Blue Tak. 3 Gases generated chemically will not have sufficient pressure to inflate the balloons. Filling the balloons with the different mixtures of gases can be tricky and teachers may prefer to have some balloons which are already filled.

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Procedure HEALTH & SAFETY: Wear eye protection throughout. The teacher should also wear ear protection as indicated. Students should be instructed to protect their ears at relevant points in the demonstration.

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a Inflate one of the balloons with air (by mouth) and seal it by tying a knot in the neck. Clamp the knotted end of the balloon to hold it in position. Place the clamp stand on a desk well away from any combustible materials. Check that the ceiling space above the clamp stand is also clear. b Light the candle or wax taper and touch the balloon with the candle flame. The balloon will burst with a familiar pop, due solely to the rubber bursting. Extinguish the candle flame. c Inflate a second balloon with hydrogen from a cylinder and seal it. This balloon should float in air. Tether it to the clamp stand with a length of cotton thread. Ensure that the students are at least 3 m away. d Instruct the students to place their fingers in their ears. The teacher should wear ear protection. e Light the candle and, holding the metre rule at arm’s length, touch the candle flame on the balloon. The balloon will explode with a loud bang. Flames from the combustion of the hydrogen with the oxygen in the air can be seen. Extinguish the flame. f Inflate the third balloon with a little hydrogen (about one third of the full balloon size) and complete the inflation with air from the mouth. The correct ratio of hydrogen to air for complete combustion is 2:5 but it is preferable to have rather more hydrogen than this so that the balloon will float upwards. g Attach the filled balloon to the clamp stand with a cotton thread as before. Ensure that ears are protected and then ignite the balloon as before. The explosion will be louder this time because the fuel (hydrogen) and the oxygen in the air are thoroughly mixed. Again, extinguish the candle flame. h Finally, inflate the fourth balloon - first with some hydrogen and then fill it with oxygen from the cylinder. The ideal mixture is two volumes of hydrogen to one volume of oxygen but this may need adjusting in order to ensure that the balloon still floats. i Attach the balloon to the clamp stand with thread as before. Wear ear protectors and ensure that members of the audience place their fingers in their ears. Ignite the balloon at arm’s length. There will be a very loud explosion! Note: Exploding stoichiometric (exactly reacting) mixtures of hydrogen and oxygen on a large scale can shatter glass apparatus and laboratory windows (flying glass may cause injury) and permanently damage the hearing of people in the same room. It is recommended that this reaction is done ONLY on the small scale as described in experiment 70 and experiment 71.

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Teaching notes Hydrogen-air mixtures will explode if they contain between 4 to 75% hydrogen. The exothermic reaction: 2H2(g) + O2(g) → 2H2O(g) releases 484 kJ of energy from the amounts (reacting masses) shown in the equation. A good way of presenting the demonstration is to have the balloons tethered in a row some distance apart, prepared shortly before the demonstration. They can then be ignited one-byone in order of increasing vigour of reaction. The explosions increase in loudness as the percentage of oxygen in the mixture increases. The class could be asked to predict what would happen if a balloon filled only with oxygen is ‘ignited’. This could then be demonstrated, with suitable theatricality, and the class asked to explain the rather disappointing result. With no fuel (hydrogen) for the oxygen to combine with, the bang is due only to the bursting of the rubber of the balloon, as when filled only with air (or helium).

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/energy-changes-and-fuels/ exploding-balloons,47,EX.html

Useful resources Hydrogen air reaction: http://www.jce.divched.org/JCESoft/CCA/CCA4/MAINPT/DH_elt/H.HTM#Air

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The howling/screaming jelly baby

69

The following 3 pages reproduce the CLEAPSS® Supplementary Risk Assessment SRA01 – The howling/screaming jelly baby, with permission from CLEAPSS®

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The Howling / Screaming Jelly Baby: Reacting a ‘jelly baby’ with molten potassium chlorate

Supplementary Risk Assessment (to meet the COSHH and / or Management Regulations) (To supplement Model Risk Assessments in other CLEAPSS publications, such as Hazcards, the Laboratory Handbook, Recipe Cards and various guides and guidance leaflets. This will eventually be incorporated into one of those publications.)

Details of operation: 15 g of reagent-grade potassium chlorate(V) is weighed into a Pyrex boiling tube. The tube is clamped at a slight angle to the vertical. The apparatus is surrounded by safety screens and the potassium chlorate(V) is heated until it melts. The demonstrator, wearing a face shield and heat-resistant gloves, uses tongs to drop a jelly baby into the melt. (The above details are similar to those in various publications, including those of the Royal Society of st Chemistry, the 21 Century Science project and the Salters’ Chemistry Club Handbook.) Schools are advised not to deviate from the details described in this risk assessment. If any variation is required, members should contact CLEAPSS for a Special Risk Assessment. (a) Potassium chlorate(V).

Substance(s) possibly hazardous to health, etc:

(b) Jelly babies are a form of confectionery, with stated contents of sugar, glucose syrup, water, gelatine, citric acid, flavourings, colours. (a) OXIDISING and HARMFUL (R9: Explosive when mixed with combustible material; R20/22: Harmful by inhalation and if swallowed).

Classification under CHIP3 Regulations 2002:

(b) See CLEAPSS Hazcard 77.

Particular risks / precautions:

th Bretherick’s Handbook of Reactive Chemical Hazards (6 edition, 1999, Butterworth) states: “Potassium chlorate: Although most explosive incidents have involved mixtures of the chlorate with combustible materials, the exothermic decomposition of the chlorate to chloride and oxygen can accelerate to explosion if a sufficient quantity and powerful enough heating are involved…”.

Potassium chlorate with sugars: A stoichiometric mixture with sucrose ignites at 159 °C and has been evaluated as a rocket propellant. Dry powdered mixtures with glucose containing above 50% chlorate explode under a hammer blow. Pyrotechnic mixtures with lactose begin to react exothermically at about 200 °C, when the lactose melts and carbon is formed...”. A recirculatory fume cupboard caught fire with this experiment. It appears that the sparks were sucked onto the prefilter which is made of paper. The result is shown overleaf. Workplace Exposure Limits: SRA 01 11/08 Page 1 of 3

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© CLEAPSS® , The Gardiner Building, Brunel Science Park, Uxbridge UB8 3PQ Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk

Burnt prefilter!

Risk assessment Potassium chlorate(V) is notoriously unstable. The thermal decomposition of potassium chlorate(V) has been a common activity in school science in the past. Catalysed by manganese dioxide, it demonstrates catalysis in a clear and striking manner, although problems have arisen when the manganese dioxide has been impure or charcoal (very similar in appearance) has been used in its place. Nevertheless, the use of potassium chlorate(V) in schools is well established and guidance on safe use in some contexts is given on the relevant CLEAPSS Hazcard. This activity involves the use of a jelly baby which is not covered by the Hazcard or similar safety texts. Most of the texts used by education employers as model risk assessments warn of the risk of dangerous or unstable mixtures with sugars and recommend that such mixtures should not be made. A jelly baby contains sugars but it is a single lump rather than crystals or powder, thus the surface area exposed (and hence the rate of reaction) is less. In addition, the sugars will be diluted by the water present, further reducing the rate of reaction and making an explosion much less likely. The conclusion is that, notwithstanding the general advice that chlorate / sugar mixtures should not be made in schools, jelly babies present a safe but spectacular demonstration of the power of potassium chlorate as an oxidising agent and the energy stored in foodstuffs, provided that certain safety precautions are adhered to. •

The activity must be carried out only by teachers who should practice it in advance. They should NOT be tempted to increase the scale of the operation.

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Make sure that there are no fire alarms that use smoke sensors in the laboratory (or in the corridor if the door is opened to disperse the fumes). Laboratory fire alarms should use heat sensors as recommended by the DCSF in Science Accommodation in Secondary Schools, BB80.

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Teachers must take steps to prevent theft of the chemicals, in case pupils are tempted to repeat the activity outside school.

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In case of explosion, the apparatus should be surrounded by safety screens (or in part by a wall). A closed / sealed apparatus must NOT be used.

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This reaction should be carried out on the open bench with the windows open. Fume cupboards are NOT designed to cope with this amount of smoke in such a short time and smoke will leak out. Ejected molten liquids have been known to melt plastic or crack glass windows. The sparks have ignited the prefilter in recirculatory fume cupboards.

SRA 01 11/08 Page 2 of 3


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The reaction is very vigorous and molten potassium chlorate(V) and decomposition products are likely to shoot out of the boiling tube. The safety screens should be arranged to prevent the ejected particles from scattering around the room and especially from landing on combustible objects. There should be heat-resistant mats to protect the bench.

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Bench mats and safety screens may be spattered with potassium chlorate(V). After the reaction, this should be carefully washed away with plenty of water, to prevent the possibility of inadvertent combustion.

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Spectators must be several metres away from the demonstration and should wear eye protection.

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The demonstrator will, inevitably, be closer to the demonstration than the spectators and should make use of the additional protection provided by a face shield.

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The demonstrator’s hand is at some risk during the few seconds when the jelly baby is being dropped in to the melt, even if tongs are used. Heat-resistant gloves should be worn.

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As impurities can cause an explosion, care should be taken to ensure that the potassium chlorate(V) is pure (use reagent grade) and that the boiling tube is clean (and free, for example, from traces of carbon).

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Clamps used should not have rubber grips, as these may melt because of the heat of the reaction and so permit the boiling tube to move.

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Do not substitute sodium chlorate(V) for potassium chlorate(V). It does not work (it has the wrong melting point).

th This risk assessment was produced on 6 November 2008. You are advised to check for any update on the CLEAPSS web site.

Notes: COSHH stands for Control of Substances Hazardous to Health. The regulations require that an assessment of risk must be made before substances hazardous to health are handled. The substances covered are the reactants, the products and any intermediate or side products that are very toxic, toxic, harmful, corrosive or irritant. Just because a substance carries no hazard label does not mean that it is completely safe. The Management of Health and Safety at Work Regulations require a similar risk assessment for substances with other hazard classifications or activities involving hazardous procedures.

SRA 01 11/08 Page 3 of 3


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Hydrogen/oxygen explosion Exploding stoichiometric (exactly reacting) mixtures of hydrogen and oxygen on a large scale can shatter glass apparatus and laboratory windows (flying glass may cause injury) and permanently damage the hearing of people in the same room. It is recommended that this reaction is done ONLY on the small scale as described below and in experiment 71. The function of risk assessment is to adjust the procedure to avoid these events. Soap bubbles make a useful alternative means of holding the gases. Using the method below ensures that there is no flying glass and the explosion is loud enough for a laboratory. Two holes are drilled with a very fine drill bit through the rubber bung that fits the widenecked, 100 ml bottle. A wider hole is bored though the bung which is then fitted with glass tubing (use 6 mm medium-wall thickness borosilicate glass tubing). Two copper wires are fed through the bung and the nickel-foil electrodes are soldered onto the wire. The glass tubing is bent into shape using a non-luminous Bunsen burner flame to melt the glass and being careful not to heat the bung.

Procedure

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HEALTH & SAFETY: Wear eye protection a Pour the 0.2 M sodium sulfate solution into the 100 ml bottle so it reaches the very top. b Have a beaker handy to collect the overspill as the bung containing the electrodes is inserted into the neck of the bottle. The overflow rises up the delivery tube and empties into the beaker.

detergent solution in a crucible on wooden blocks or a laboratory jack nickel electrode soldered onto copper wire

c Connect the copper wires to 0.2M sodium sulfate solution the low-voltage supply (6-8 volts is a suitable setting) and pass current until no more solution is pushed out of the bottle into the beaker. All this can be prepared earlier and the current turned off. d Place the crucible filled with 50% liquid detergent solution on the laboratory jack or wooden blocks so that gases from the bottle will bubble through. Switch on the current to collect bubbles of gas. e Switch off the current, lift the bottle so that the tube is clear of the crucible and move the crucible closer to an ignited Bunsen burner. f Light a splint with the Bunsen burner flame and apply to the bubbles on the top of the crucible.

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Technical notes

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1 Wear eye protection. 2 The Bunsen burner required for this demonstration should be at least 1 m from the bottle. 3 The first gas sample may not explode with a loud crack as air might be present, so repeat the procedure. 4 The glass tubing should be flush with the bottom of the bung. 5 The electrolyte is 0.2 M sodium sulfate solution rather than sulfuric acid. More advanced students will appreciate that water molecules are being oxidised and reduced at the electrodes to form oxygen and hydrogen. 6 This method uses nickel electrodes in place of platinum which are expensive and difficult to solder. However, the nickel anode does oxidise during the process if the electrolysis cell is left on for long periods. The solution becomes green and nickel(II) hydroxide precipitates out. Disposal: All solutions can be poured down the foul-water drain.

Reference This experiment has been reproduced from CLEAPSS®, L195 Safer chemicals, safer reactions p.26 with permission from CLEAPSS®

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Exploding bubbles of hydrogen and oxygen

71

This demonstration involves electrolysing sulfuric acid and the explosive re-combination of the hydrogen and oxygen. It is an alternative to experiment 70: Hydrogen/oxygen explosion and can be used if you school doesn’t have soldering equipment. Please note this method is considered slightly more hazardous by CLEAPSS® than experiment 70. Exploding stoichiometric (exactly reacting) mixtures of hydrogen and oxygen on a large scale can shatter glass apparatus and laboratory windows (flying glass may cause injury) and permanently damage the hearing of people in the same room. It is recommended that this reaction is done ONLY on the small scale as described below and in experiment 70.

Lesson organisation This experiment works well as a class demonstration and involves an impressive explosion between hydrogen and oxygen. Students are usually impressed by such reactions. A dilute solution of sulfuric acid is electrolysed using lead electrodes. The hydrogen and oxygen evolved at the electrodes are mixed and used to blow soap bubbles. These bubbles can be exploded, giving a very loud ‘crack’. The demonstration should take around 10 minutes.

Apparatus and chemicals Eye protection Ear protection

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Low voltage DC power pack, capable of supplying a current of at least 4 A at 12 V Connecting leads and crocodile clips Ammeter (0–4 A) Side arm boiling tube Solid bung to fit boiling tube Short length of glass tube (6 mm diameter) Length of flexible rubber, or silicone plastic tubing Plastic beaker (250 cm3) or ice cream tub Bunsen burner Spatula with spoon-shaped end (see note 1) 1 m wooden rule A little washing up liquid 2 pieces of lead foil (20 X 1 cm ) (see note 1) (Toxic, Refer to CLEAPSS® Hazcard 56) Sulfuric acid, 2 mole.dm-3, (Corrosive, Refer to CLEAPSS® Hazcard 98A)

Technical notes Hydrogen (Extremely flammable, Refer to CLEAPSS® Hazcard 48) Oxygen (Oxidising, Refer to CLEAPSS® Hazcard 69) 1 If this type of spatula is unavailable use a teaspoon taped to a 1 m rule

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Before the demonstration a Set up the apparatus as shown in the diagram. The electrodes can be folded over the top of the tube and kept apart with a short length of plastic tubing. bung –

+ delivery tube

acid level lead foil hydrogen bubbles

oxygen bubbles

side arm test tube plastic tube

detergent in water beaker

b Add acid until it is just below the side-arm. c Carefully fit the bung but do not damage the lead foil. Use Vaseline to make a gas tight seal if necessary. d Half fill the beaker with water, add a few drops of washing up liquid, and stir the mixture gently. Do not place the plastic tube in the beaker at this stage. e Light a Bunsen burner, but keep it at least 1 m away from the electrolysis experiment.

The demonstration Eye protection and ear protection must be worn throughout this demonstration because of the hydrogen produced (Extremely flammable) f Connect the electrodes to the 12 V power pack, incorporating an ammeter in the circuit. g Put the delivery tube under the water h Switch on the power pack and adjust to give a current of about 1 - 2 A. Bubbles of hydrogen are formed at the negative electrode (cathode) and oxygen at the positive electrode (anode). It is worth pointing out that the volume of hydrogen formed is twice that of the oxygen produced. i Observe these changes for a minute or so, to allow air in the delivery tube to be displaced by the mixture of hydrogen and oxygen. j Scoop up some of the bubbles in the spatula and hold them in the Bunsen flame. They explode with an impressively loud sharp ‘crack’. If the bubbles do not explode, wait a little longer for the gas mixture to displace air from the tubing. (Under no circumstances try to ignite gas at the end of the tubing.)

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Teaching notes Some students may need the following explanation: Water comprises hydrogen and oxygen and electrical energy splits water into these elements. The formula of water is H2O so Avogadro’s hypothesis predicts the volume of hydrogen formed is double that of oxygen: 2H2O(l) → 2H2(g) + O2(g) The explosion is caused by the rapid release of energy when the gases re-combine to form water. More able students can be given the following explanation. Water, although covalent, ionises very slightly forming H+ ions and OH- ions: H2O(l) → H+(aq) + OH-(aq) However, there are insufficient of these ions to allow a reasonable current to flow, so an electrolyte – sulfuric acid – is added to increase the conductivity of the liquid. At the cathode hydrogen ions gain electrons to form hydrogen atoms which then combine to form hydrogen molecules as a gas. 2H+ (aq) + 2e- → H2(g) Water molecules at the anode yield electrons forming oxygen gas and hydrogen ions. 2H2O(l) → O2(g) + 4H+(aq) + 4eIt can be seen that for every four electrons that flow round the circuit, 2 molecules of hydrogen and one of oxygen are liberated. It should be emphasised that, while electrical energy is being used to decompose the water into its component gases (an endothermic change), the explosion of these two gases represents the evolution of energy (an exothermic reaction). The second reaction is the reverse of the first. The 2:1 volume ratio can be demonstrated using a Hoffman voltameter. It is worth discussing with students the feasibility of producing hydrogen on a large scale from water and using this gas as non-polluting fuel. Much research is being done to find a way of splitting water catalytically. The effect of an explosive reaction depends more upon the rate of reaction than the amount of reactants. The reaction between a stoichiometric (exactly reacting) mix of hydrogen and oxygen is so fast that the shock wave from 1 cm3 of this mixture can permanently damage hearing, but 1000cm3 of pure hydrogen gas in a balloon takes a second to burn and the shock wave is slight. Damage is caused by large values of Joules/second not Joules alone. This apparatus is, in fact a lead-acid accumulator (micro car battery). It gets charged up during the electrolysis, as can be demonstrated by removing the power supply after the H2/O2 experiment, and attaching a 2V bulb, which will shine for many seconds. The chemistry is not too difficult for gifted and talented students to research.



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72

An oscillating reaction This is one of the simplest oscillating reactions to demonstrate. Bromate ions oxidise malonic acid to carbon dioxide. The reaction is catalysed by manganese(II) ions. On mixing the reactants and catalyst, the reaction mixture oscillates in colour between red-brown (bromine, an intermediate) and colourless with a time period of about 20 seconds.


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Eye protection Magnetic stirrer and follower (optional). 1 dm3 beaker. The chemical quantities given are for one demonstration. 575 cm3 of 2 mol dm–3 sulfuric(VI) acid (H2SO4) (Corrosive) 9 g of malonic acid (propanedioic acid, CH2(CO2H)2) (Harmful) 8 g of potassium bromate(V) (KBrO3) (Toxic and Oxidising) 1.8 g of Manganese(II) sulfate(VI)-7-water (MnSO4.7H2O) (Harmful and Dangerous for the Environment) 750 cm3 of deionised water.

Technical notes 575 cm3 of 2 mol dm–3 sulfuric(VI) acid (H2SO4) (Corrosive) Refer to CLEAPSS® Hazcard 98A 9 g of malonic acid (propanedioic acid, CH2(CO2H)2) (Harmful) Refer to CLEAPSS® Hazcard 36B 8 g of potassium bromate(V) (KBrO3) (Toxic and Oxidising) Refer to CLEAPSS® Hazcard 80 1.8 g of Manganese(II) sulfate(VI)-7-water (MnSO4.7H2O) (Harmful and Dangerous for the Environment) Refer to CLEAPSS® Hazcard 60 1 Potassium bromate(V) is toxic. It safe to use here because it is not being heated or ingested. 2 Demonstrators may also be interested in the oscillating reactions listed on CLEAPSS® Recipe Card 48. 3 The reaction will not work if tap water (Coventry) is used instead of deionised water. 4 Chloride ions, via the addition of a pinch of potassium chloride or dilute hydrochloric acid will immediately stop the oscillations. Clean apparatus is therefore essential.

Procedure

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Health & Safety: Wear eye protection.

Before the demonstration Place 250 cm3 deionised water in a 1 dm3 beaker. Add, with stirring, 575 cm3 of 2 mol dm-3 sulfuric acid. This will give 825 cm3 of 1.4 mol dm-3 sulfuric acid solution. Weigh out the malonic acid, potassium bromate and manganese sulfate in weighing boats. Care should be taken, since these chemicals are harmful or toxic.

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The demonstration Place the beaker of sulfuric acid on a magnetic stirrer and stir the solution fast enough for a vortex to form. A stirring rod can be used, but is tedious and tends to detract from the demonstration. Add the malonic acid and potassium bromate. When these have dissolved, add the manganese sulfate and observe what happens. A red colour should develop immediately. This will disappear after about one minute and thereafter the colour will oscillate from red to colourless with a time period of about 20 seconds for a complete oscillation. This will continue with a gradually increasing time period for over ten minutes – long enough for most audiences to lose interest!

Visual tips A white background is useful.

Teaching notes A member of the audience with a stopwatch could be asked to time the oscillation.

Theory This reaction is an example of a class of reactions called Belousov-Zhabotinsky (BZ) reactions. The overall reaction is usually given as: 3CH2(CO2H)2(aq) + 4BrO3–(aq) → 4Br–(aq) + 9CO2(g) + 6H2O(l) Oscillation is brought about by two autocatalytic steps. Bromine is an intermediate in the reaction scheme – the red colour that is observed. An analogy with predator-prey relationships might be one way to give students some idea of what is going on. For example a population of rabbits (analogous to the bromine) will increase rapidly (exponentially) if there is plenty of food (reactants). However, the plentiful supply of rabbits will stimulate a rapid increase in the fox population (another intermediate that reacts with the bromine) which will then deplete the rabbits. Lacking rabbits, the foxes will die, bringing us back to square one, ready for a rapid increase in rabbits and so on.

Extensions The reaction can be investigated using a colorimeter with a chart recorder or interfaced to a computer. See for example R. Edwards, Interfacing Chemistry Experiments, London: RSC, 1993.


Useful resources The theory of oscillating reactions is complex and not fully understood. Some of the more accessible articles are listed below. D. O. Cooke, Educ. Chem.,1975, 12, 144. I. R. Epstein, Chem. Eng. News., 1987, 65, 24. I. R. Epstein et al, Sci. Amer., 1983, 248, 112. M. D. Hawkins et al, Educ. Chem., 1977, 14, 53.


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73

The fractional distillation of crude oil This experiment simulates the industrial fractional distillation of crude oil in the laboratory.

Lesson organisation This is best done as a class experiment but can be used as a demonstration. The experiment time depends on the age and experience of the students. A year 7 (or equivalent) group needs about 50 mins and a year 12 (or equivalent) group about 30 mins.

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Apparatus and chemicals Eye protection Bunsen burner Heat resistant mat Side-arm hard-glass test-tube (see note 1) Bent delivery tube and rubber connection tubing Small sample tubes (20 mm x 5 mm) minimum size (small test-tubes can also be used), 4 Thermometer 0–360 °C with cork to fit side-arm test-tube Teat pipette Beaker (100 cm3) "Hard glass" (borosilicate) watch glass Mineral or ceramic fibre Wooden splints Crude oil substitute (Highly flammable and Harmful), about 2 cm3 (see notes 2 and 3)

Technical notes Crude oil substitute (Highly flammable and Harmful) Refer to CLEAPSS® Hazcard 45A and Recipe card 20 1 Side-arm boiling tubes produce more consistent results than boiling tubes fitted with bungs with two holes, one for a thermometer and one for a delivery tube. 2 Real crude oil contains more than 0.1% benzene, which is carcinogenic. Therefore its use is not permitted in schools. 3 It is important to try the experiment beforehand. It may be necessary to add an additional low boiling point fraction – eg cyclohexane – to obtain something below 70 °C. 4 This is quite a messy experiment. If it is done regularly, it is probably best to keep sets of apparatus – apart from the thermometer and watch glasses – dedicated to the experiment. This is because it is difficult to get clean, and it still works if oil residues are present.

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Procedure HEALTH & SAFETY: Wear eye protection a Place about a 2 cm3 depth of ceramic fibre in the bottom of the side-arm test-tube. Add about 2 cm3 of crude oil alternative to this, using the teat-pipette. b Set up the apparatus as shown in the diagram, with one addition – a beaker of cold water around the collecting tube. The bulb of the thermometer should be level with, or just below the side-arm. Heat the bottom of the side-arm test-tube gently, with the lowest Bunsen flame. Watch the thermometer. When the temperature reaches 100 °C, replace the collection tube with another empty one. The beaker of water is no longer necessary. Collect three further fractions, to give the fractions as follows: 1 Room temperature to 100 °C 2 100–150 °C 3 150–200 °C 4 200–250 °C

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thermometer bung clamp side arm test tube

delivery tube

crude oil alternative ceramic fibre

Bunsen burner

collecting tube

c A black residue remains in the side-arm test-tube. Test the four fractions for viscosity (how easily do they pour?), colour, smell and flammablility.

To test the smell, gently waft the smell towards you with your hand.

To test for flammablility, pour onto a hard glass watch glass and light the fraction with a burning splint. d Keep one set of fractions and see that they combine to form a mixture very like the original sample.

Teaching notes The fractions increase in viscosity with boiling temperature and should become more coloured as the temperature increases. With some artificial mixtures, the difference in colour can be difficult to observe. The descriptions of smells vary from student to student, but students can be encouraged to liken them to familiar smells – eg ‘like lubricating oil’. The samples become increasingly difficult to burn and burn with increasingly smokey flames. This experiment forms an important part of understanding how we obtain chemicals from oil.

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CLEAPSS® Recipe Card 20 ©CLEAPSS® 2007 20

Crude oil alternative

Comment:

Real crude oil and petrol contain benzene in concentrations greater than 0.1% and hence are not to be used (COSHH Regulations 2002). A synthetic mixture can be prepared using mainly aliphatic hydrocarbons to illustrate the principle of fractional distillation of crude oil treatment in industry. Further details can be found in section 13.2 of the CLEAPSS Laboratory Handbook.

Hazards

Petrolium spirit is HIGHLY FLAMMABLE and HARMFUL (see Hazcard 45A)

Control measures

Wear eye protection. Do not prepare the mixture near sources of ignition

Procedure to prepare 100 ml of synthetic crude oil ◆ Mix together 55 ml of liquid paraffin (medicinal), 20 ml of paraffin oil (kerosene), 11 ml of white spirit, 4 ml of petroleum ether (100-120°C), 4 ml of petroleum ether (80-100°C) and 6 ml of petroleum ether (60-80°C). ◆ Add a squeeze of oil paint (eg, Windsor and Newton’s Ivory Black) from a tube and stir well. ◆ After adding to a labelled bottle, shake the mixture well. Label the container HIGHLY FLAMMABLE and HARMFUL. ◆ Always shake the mixture well before use.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/chemicals-from-oil/the-fractionaldistillation-of-crude-oil,142,EX.html

Useful resource Students can be encouraged to find web links for this experiment themselves – there are many. Wikipedia Fractional Distillation is a good example http://en.wikipedia.org/wiki/Fractional_distillation

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Health & Safety checked, February 2008 Updated 23 Jul 2009

Cracking hydrocarbons

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In this experiment the vapour of liquid paraffin (a mixture of saturated hydrocarbons) is cracked by passing it over a heated catalyst. The mixture of gaseous short-chain hydrocarbons produced is collected and tested for unsaturation with bromine water and acidified potassium manganate(VII) solution.

Lesson organisation This experiment is intended as a class practical, but could also be done as a demonstration. The main risk to be considered in making the choice is the reliability of the students involved in handling very hot glassware and manipulating the apparatus for the safe collection of the flammable gas mixture over water. As a class practical, it is best if the students work in pairs, with one student controlling the Bunsen burner and the other collecting the tubes of gas. Students not familiar with using bromine water and potassium managanate(VII) solution to test for unsaturation need to be taught these tests first, using cyclohexane and cyclohexene. The class experiment should take about 45 minutes. A demonstration will take take about 15 - 20 minutes, including testing of the gases.

Apparatus and chemicals Eye protection Safety screens (for demonstration)

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Each working group requires: Test-tubes, 4 Bungs, to fit test-tubes, 4 Test-tube rack Boiling tube (see note 1) Bung, one-holed, to fit boiling tube Delivery tube fitted with a Bunsen valve (see note 2) Small glass trough or plastic basin, for gas collection over water Bunsen burner Heat resistant mat Stand and clamp Dropping pipette Wooden splint Medicinal paraffin (Liquid paraffin - NOT the fuel) (Low hazard), about 2 cm3 Porous pot or pumice stone fragments (see note 3) Bromine water, 0.02 mol dm-3 - diluted to a pale yellow-orange colour (Harmful at concentration used), about 2 cm3 (see note 4) Acidified potassium manganate(VII) solution, about 0.001 mol dm-3 (Low Hazard at concentration used), about 2 cm3 Mineral wool (preferably 'Superwool')

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Technical notes Medicinal paraffin (Liiquid paraffin) (Low hazard) Refer to CLEAPSS® Hazcard 45B Bromine water (Harmful at concentration used) Refer to CLEAPSS® Hazcard 15B and Recipe card 28 Potassium manganate(VII) (potassium permanganate) solution, about 0.02 mol dm–3 (Low hazard at concentration used) Refer to CLEAPSS® Hazcard 91 and Recipe Card 86 Dilute sulfuric acid, 0.1 mol dm-3 (Low hazard at concentration used) Refer to CLEAPSS® Hazcard 98A and Recipe Card 69 Mineral wool (preferably 'Superwool') Refer to CLEAPSS® Hazcard 86 1 The boiling tube should be a hard glass (borosilicate) 150 mm x 25 mm test-tube. 2 It is important to ensure that the bung and the boiling tube fit well. Bunsen valves (see diagram below) can be made by attaching a 3 cm long piece of clean, unused, soft rubber tubing to the delivery tube, and then attaching a short length (1 - 2 cm) of glass rod, as shown in the diagram below. The rubber tubing should be slit on one side along about 1 cm of its length in the direction of the tubing. The use of a Bunsen valve should stop 'suck-back' occurring. See CLEAPSS® Laboratory Handbook 13.2.1. rubber tubing glass rod plug end of delivery tube

slit cut with scalpel

3 Porous pot chips can be made by crushing broken crucibles into pea-sized fragments. 4 Dilute the bromine water until it is yellow in colour. About 0.005 to 0.01 mol dm-3 is adequate. Above 1% concentration (0.02 mol dm-3), bromine water is Toxic and Irritant.

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Procedure HEALTH & SAFETY: Wear safety goggles For a demonstration the class and teacher should be protected by safety screens in case of unexpected suck-back causing the the hot tube to shatter. a Place about a 2 cm3 depth of mineral wool in the bottom of the boiling tube and gently press it in place with a glass rod. Drop about 2 cm3 of liquid paraffin on to the wool, using a dropping pipette, Use enough paraffin to completely soak the mineral wool, but not so much that the paraffin runs along the side of the tube when it is placed horizontally. b Clamp the boiling tube near the mouth so that it is tilted slightly upwards, as shown in the diagram below. Place a heap of catalyst (pumice stone or porous pot fragments) in centre of the tube and fit the delivery tube. c Fill the trough about two-thirds full with water and position the apparatus so that the end of the delivery tube is well immersed in the water.

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porcelain chips

product gas

mineral wool soaked in liquid paraffin hard-glass boiling tube

Bunsen burner

Bunsen valve fits here if desired

d Fill four test-tubes with water and stand them inverted in the trough. Also place the the test-tube bungs, upside down, in the water, . e Strongly heat the catalyst in the middle of the tube for a few minutes, until the glass is up to a dull red heat. Avoid heating the tube too close to the rubber bung. f While keeping the catalyst hot, flick the flame from time to time to the end of the tube for a few seconds to vaporise some of the liquid paraffin. Try to produce a steady stream of bubbles from the delivery tube. Be careful not to heat the liquid paraffin too strongly or let the catalyst cool down. To avoid suck-back do not remove the flame from heating the tube while gas is being collected. If suck-back looks as if it is about to occur, lift the whole apparatus by lifting the clamp stand. g When a steady stream of gas bubbles is established, collect four tubes full of gas by holding them over the Bunsen valve. Take care not to lift the water-filled tubes out of the water when moving them, to avoid letting air into them. Seal the full tubes by pressing them down on the bungs, then place them in a rack. h When gas collection is complete, first remove the delivery tube from the water by tilting or lifting the clamp stand. Only then stop heating. i Test the tubes of gas as follows: (i) What does the gas look like? Carefully smell the contents of the first test-tube. Of what does the smell remind you? Does liquid paraffin have a smell?

(ii) Use a lighted splint to see if the gas is flammable. The first tube may contain mostly air. If it does not ignite, try the second tube. Once the gas is lit, invert the test-tube to allow the heavier-than-air gas to flow out and burn.

(iii) To the third tube of gas add 2 - 3 drops of bromine water, stopper and shake well.

(iv) To the fourth tube add 2 - 3 drops of acidified potassium manganate(VII) solution, stopper and shake well.

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Teaching notes The demand for petrol is greater than the gasoline fraction obtained by distilling crude oil. Cracking larger hydrocarbons produces smaller alkanes that can be converted into petrol. It also produces small alkenes, which are used make many other useful organic chemicals (petrochemicals), especially plastics. This experiment models the industrial cracking process. You need to be particularly vigilant and lift the apparatus out of the water if suck-back starts to occur and cannot be reversed by stronger heating . It is very important to stress that students should not stop heating the boiling tube at the end of the experiment until the delivery tube is out of the water and contains no water. The gas mixture which collects has a characteristic smell, burns with a yellow flame, and decolourises bromine water and acidified potassium manganate(VII) solution. This shows the presence of unsaturated molecules. Students will find it helpful to build molecular models to understand the reaction and be able to write an equation for the reaction.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/chemicals-from-oil/crackinghydrocarbons,139,EX.html

Useful resource There are many web links, but few deal with this topic at a level appropriate to 14 - 18 chemistry teaching and learning - most are either far too technical or too elementary. A discussion of the cracking of hydrocarbons for A-level chemistry students and teachers: http://www.chemguide.co.uk/organicprops/alkanes/cracking.html A good overview of industrial processes for hydrocarbon cracking: http://en.wikipedia.org/wiki/Fluid_catalytic_cracking To view a video clip of this experiment, go to: http://media.rsc.org/videoclips/demos/Crackingahydrocaron.mpg (Last accessed December 2009)

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Health & Safety checked, July 2008 Updated 12 Feb 2009

Making nylon – the ‘nylon rope trick’

75

A solution of decanedioyl dichloride in cyclohexane is floated on an aqueous solution of 1,6-diaminohexane. Nylon forms at the interface and can be pulled out as fast as it is produced forming a long thread – the ‘nylon rope’.

Apparatus and chemicals Eye protection One 25 cm3 beaker. A pair of tweezers. Retort stand with boss and clamp.

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The chemical quantities given are for one demonstration. 2.2 g of 1,6-diaminohexane (hexamethylene diamine, hexane-1,6-diamine, H2N(CH2)6(NH2) (Corrosive) 1.5 g of decanedioyl dichloride (sebacoyl chloride, ClOC(CH2)8COCl) (Corrosive) 50 cm3 of cyclohexane (Highly Flammable and Harmful) 50 cm3 of deionised water

Technical notes 2.2 g of hexane-1,6-diamine (hexamethylene diamine,1,6-diaminohexane, H2N(CH2)6(NH2) (Corrosive) Refer to CLEAPSS® Hazcard 3B and CLEAPSS® Recipe Card 45 1.5 g of decanedioyl dichloride (sebacoyl chloride, ClOC(CH2)8COCl) (Corrosive) Refer to CLEAPSS® Hazcard 41 and CLEAPSS® Recipe Card 45 50 cm3 of cyclohexane (Highly Flammable and Harmful) Refer to CLEAPSS® Hazcards 45B 1 Wear eye protection and disposable nitrile gloves when pulling out the thread. 2 The room should be well ventilated and there must be no sources of ignition. 3 Details for waste disposal can be found in the CLEAPSS® Handbook section 7.5 4 This demonstration has been described in many sources using chlorinated solvents for the acid chloride. These are no longer considered safe and will soon become unavailable. Cyclohexane is less dense than water whereas chlorinated solvents are denser. The layers are therefore inverted compared with the old method. 5 Cyclohexane is preferred to hexane as it is less harmful. 6 Hexanedioyl dichloride (adipoyl chloride) can be used as an alternative to decanedioyl dichloride, but it does not keep as well. 7 Decanedioyl dichloride reacts with moisture in the air to produce decanedioic acid which forms nylon much less readily than the acid chloride. Ensure that the bottle is restoppered carefully after opening and consider storing it in a desiccator. The dichloride is also available in 5 cm3 sealed ampoules. The cyclohexane solution will still make nylon for a couple of days after being made up even if left unstoppered. A solution kept in a stoppered bottle is still usable after two weeks. The solution can be stored over anhydrous sodium sulphate or calcium chloride to keep it dry. 8 Solid 1,6-diaminohexane can be difficult to get out of the bottle. The easiest way to manipulate it is to heat the bottle gently in warm water until it melts at 42 °C and dispense the liquid using a dropping pipette.

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Procedure

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Health & Safety: Wear eye protection and disposable nitrile gloves when pulling out the thread. Dispose of the mixture as follows: First shake the reaction to mix the two layers. A lump of nylon will be produced which can be removed with tweezers, rinsed well with water, and disposed as solid waste. Failure to do this may result in the polymerisation taking place in the sink, leading to a blockage. The remaining liquids can be mixed with detergent and washed down the sink. rotate with finger and thumb metal rod glass tube nylon ‘rope’ nylon ‘rope’ nail metal rod cyclohexane layer aqueous layer

cotton bobbin nylon ‘rope’

Before the demonstration a Make up a solution of 2.2 g of 1,6-diaminohexane (Corrosive) in 50 cm3 of deionised water. This solution is approximately 0.4 mol dm-3. b Make up a solution of 1.5 g of decanedioyl dichloride (Corrosive) in 50 cm3 of cyclohexane (Highly Flammable and Harmful). This solution is approximately 0.15 mol dm–3.

The demonstration Pour 5 cm3 of the aqueous diamine solution into a 25 cm3 beaker. Carefully pour 5 cm3 of the cyclohexane solution of the acid chloride on top of the first solution so that mixing is minimised. Do this by pouring the second solution down the wall of the beaker or pour it down a glass rod. The cyclohexane will float on top of the water without mixing. Place the beaker below a stand and clamp as shown (see figure). A greyish film of nylon will form at the interface. Pick up a little of this with a pair of tweezers and lift it slowly and gently from the beaker. It should draw up behind it a thread of nylon. Pull this over the rod of the clamp so that this acts as a pulley. Continue pulling the nylon thread at a rate of about half a metre per second. It should be possible to pull out several metres. Take care, the thread will be coated with unreacted monomer and may in fact be a narrow, hollow tube filled with monomer solution. Wearing disposable gloves is essential.

Visual tips The beaker is rather small so allow the audience as close as possible consistent with comfort and safety.

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Teaching notes Point out that this demonstration is different from the industrial method of making nylon which takes place at a higher temperature. Molten nylon is then forced through multi-holed ‘spinnerets’ to form the fibres.

Theory The reaction is a condensation polymerisation nH2N(CH2)6 NH2 + nClOC(CH2)8COCl → H2N (CH2)6NHCO(CH2)8 nCOCl + nHCl

[

]

The nylon formed is nylon 6 –10 so called because of the lengths of the carbon chains of the monomers. Nylon 6 – 6 can be made using hexanedioyl dichloride (adipoyl chloride). The diamine is present in excess to react with the hydrogen chloride that is eliminated. An alternative procedure is to use the stoichiometric quantity of diamine dissolved in excess sodium hydroxide solution.

Extensions There are many ways of conveniently winding the nylon thread – for example using a windlass improvised from a cotton bobbin or a short length of glass tube slid over the rod of a clamp (see Fig).



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PVA polymer slime A solution of polyvinyl alcohol (PVA) can be made into a slime by adding borax solution, which creates crosslinks between polymer chains. In this activity, some interesting properties of the slime are investigated. Students are guaranteed to enjoy the activities involved. This experiment is easy to set up – providing the chemicals are available, and should take no more than about 30 mins. It can be done by students in groups of two or three.


•

Eye protection Each working group requires: Beaker (100 cm3) Measuring cylinder (50 cm3) Disposable plastic cup Metal spatula Petri dish (or watch glass) Water-based felt-tipped pen Spirit-based felt-tipped pen Disposable plastic gloves Polyvinyl alcohol, (-[CH2CH(OH)]n-), 4% (or 8%) aqueous solution, 40 cm3 (see note 1) Borax, hydrated sodium tetraborate (Na2B4O7.10H2O), 4% (or 8%) aqueous solution, (Low hazard), 10 cm3 (see note 1) Food colour or fluorescein (Low hazard) (optional) Hydrochloric acid, about 0.5 mol dm-3, (Low hazard at this concentration), 20 cm3 (optional) (see note 2) Sodium hydroxide, about 0.5 mol dm-3 (Corrosive), 20 cm3 (optional) (see note 2)

Technical notes Borax, hydrated sodium tetraborate (Na2B4O7.10H2O) (Low hazard) Refer to CLEAPSS® Hazcard 14 Fluorescein (Low hazard) Refer CLEAPSS® Hazcard 32 and Recipe card 35 Hydrochloric acid (Low hazard at concentration used) Refer to CLEAPSS® Hazcard 47A and Recipe card 31 Sodium hydroxide (Corrosive) Refer to CLEAPSS® Hazcard 91 and Recipe card 65 Slime. Refer to CLEAPSS® Recipe card 59 1 Polyvinyl alcohol (PVA) can be high MW (about 120 000) or low MW (about 15 000). If high MW PVA is used, prepare a 4% solution by placing 960 cm3 of water into a tall 1 dm3 beaker. Measure out 40 g of high MW PVA and add this slowly to the beaker of water, with stirring. If low MW PVA is used, prepare an 8% solution by placing 920 cm3 of water into a tall 1 dm3 beaker. Measure out 80 g of low MW PVA and add this slowly to the beaker of water, with stirring. In each case, heat the mixture gently, stirring occasionally, until the solution clears. Avoid boiling the solution. After cooling, this solution can be poured into suitable smaller containers, which can then be sealed and stored indefinitely. If a 4% aqueous solution of PVA is used a 4% aqueous solution of borax will be required. If an 8% aqueous solution of PVA is used an 8% aqueous solution of borax will be required.

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2 The hydrochloric acid and aqueous sodium hydroxide are best supplied in small glass bottles fitted with teat pipettes.

spatula

borax solution

disposable cup polyvinyl alcohol solution

Procedure HEALTH & SAFETY: Wear eye protection, and protective gloves if handling the slime. a Place 40 cm3 of the polyvinyl alcohol solution in the plastic cup.

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b If supplied, add one drop of food colour or fluorescein dye to the solution. Stir well. c Measure out 10 cm3 of borax solution into the beaker and add this to the polyvinyl alcohol solution, stirring vigorously until gelling is complete. This gel is sometimes known as a ‘slime’. d Wearing disposable gloves, remove the slime from the cup and knead it thoroughly to mix the contents completely. Roll the slime around in your hand, gently squeezing the material to remove air bubbles at the same time. Alternatively, place the slime in a plastic bag and mix and squeeze the mixture from outside the bag.

Tests e Test the properties of your slime in the following ways. 1 Pull the slime apart slowly. What happens? 2 Pull the slime apart sharply and quickly. What happens? 3 Roll the slime into a ball and drop it on to the bench. What happens? 4 Place a small bit of slime on the bench and hit it hard with your hand. What happens? 5 Write your name on a piece of paper with a water-based felt-tipped pen. Place the slime on top, press firmly, and then lift up the slime. What has happened to the writing and to the slime? Try the same again, this time using a spirit-based pen. Does this show the same effect?

Tests 6–8 below are optional.

6 Place a very small piece of slime in a Petri dish. Add the dilute hydrochloric acid dropwise, stirring well after each drop. When you notice a change record the number of drops added and your observations. 7 Now add dilute sodium hydroxide solution to the same sample used above in 6, stirring after each drop. When you notice a change record the number of drops added and your observations. 8 Can tests 6 and 7 be repeated time and time again to give the same results?

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Teaching notes Tell students to keep the slime away from clothes as it can produce permanent stains. The slime can be stored in an air-tight container, such as a plastic bag with a twist-tie. It is advisable to dip the slime in some water before storing, to keep it from drying out. Slime gets dirty from handling and may become mouldy after several days. When this happens you should throw it away. Do not put it down the sink because it clogs the drain. Slime-type materials are available under a variety of different brand names, and can be found in many toy stores. Slime is sometimes described as a reversible cross-linking gel. The cross-linking between the polymer chains of polyvinyl alcohol occurs by adding borax, Na2B4O7.10H2O (sodium tetraborate). PVA glue contains the polymer polyvinyl alcohol (also called polyethenol) and has the structure: H C

H2 C

H C

OH

H2 C

H C

OH

OH

Borax forms the borate ion when in solution. This ion has the structure: H

O

H

O

O

H

O

H

_

B

The borate ion can make weak bonds with the OH groups in the polymer chains so it can link the chains together as shown below. This is called cross-linking. H C

H2 C

H C

O

H2 C

O

O

H

H

H C

H

H

O

O

H

O

H

B H

O

H

H

O CH

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H

O H2 C

CH

O H2 C

CH

Slime is a non-Newtonian fluid that is dilatant – ie under stress, the material dilates or expands. Other well known stress-thickening materials are quicksand, wet sand on the beach, some printer’s inks, starch solutions and ‘Silly Putty’. Dilatant materials tend to have some unusual properties. Under low stress, such as slowly pulling on the material, it will flow and stretch. If careful, you can form a thin film.

●

Pull sharply (high stress) and the material breaks.

●

Pour the material from its container then tip the container upwards slightly, the gel self siphons.

●

Put a small amount of the material on a table top and hit it with your hand, there is no splashing or splattering.

●

Throw a small piece onto a hard surface; it will bounce slightly.

●

Adding acid to the slime breaks the crosslinking producing a liquid with lower viscosity. Adding alkali reverses the process and the slime should be regenerated. Various types of slime have been manufactured. In this investigation you use the polymer polyvinyl alcohol, which is reasonably cheap and is readily available from suppliers because it is widely used as a thickener, stabiliser and binder in cosmetics, paper cloth, films, cements and mortars. In ethanol solution polyvinyl alcohol solution dries to leave a thin plastic film that is useful in packaging materials, especially as it is biodegradable.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/polymers/pva-polymerslime,153,EX.html

Useful resource This website has a short video of preparing slime. http://matse1.mse.uiuc.edu/polymers/e.html This website gives a background on slime www.madehow.com/Volume-6/Slime.html This website contains more links www.msm.cam.ac.uk/SeeK/slime.htm (Websites accessed December 2009)


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Reactions of positive ions with sodium hydroxide (microscale version) This is a microscale version of the common test-tube practical reacting various positive ions with sodium hydroxide. The main advantages of the microscale version are the tiny quantities of chemicals consumed, and there are no test-tubes to wash up. Instead of test-tubes, students have a results sheet which looks like a large results table. This is laminated or put inside a plastic document wallet and can be re-used many times.

Lesson organisation This version is far quicker than the more traditional test-tube version and only takes a few minutes to do. The main management issue is likely to be students wandering around looking for the various reagents. This can be avoided if you supply enough bottles – ideally one bottle of each chemical for each bench of students. Make sure that there are plenty of bottles of sodium hydroxide, as this is used the most. Students need to make or be given a second copy of the results table to record their observations.


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Eye protection Each working group requires: Results table either laminated or in a plastic document wallet (see note 1). This can be found after this experiment. Access to: Red litmus paper Bottles of (see note 2): Sodium hydroxide, <0.5 mol dm-3 (Irritant at this concentration) Iron(II) sulfate, 0.2 mol dm-3, in 0.1 mol dm-3 sulfuric acid (Low hazard at this concentration) Iron(III) nitrate, 0.2 mol dm-3 (Low hazard at this concentration) Copper(II) sulfate, 0.2 mol dm-3 (Low hazard at this concentration) Aluminium nitrate, 0.2 mol dm-3 (Low hazard at this concentration) Calcium chloride, 0.2 mol dm-3 (Low hazard at this concentration) Magnesium chloride, 0.2 mol dm-3 (Low hazard) Ammonium chloride, 0.2 mol dm-3 (Low hazard at this concentration)

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Technical notes Sodium hydroxide (Irritant at this concentration) Refer to CLEAPSS® Hazcard 91 and Recipe card 65 Iron(II) sulfate (Low hazard at this concentration) Refer to CLEAPSS® Hazcard 55B and Recipe card 40 Iron(III) nitrate (Low hazard at this concentration) Refer to CLEAPSS® Hazcard 55C Copper(II) sulfate (Low hazard at this concentration) Refer to CLEAPSS® Hazcard 27C and Recipe card 19 Aluminium nitrate (Low hazard at concentration used) Refer to CLEAPSS® Hazcard 2B Calcium chloride (Low hazard at concentration used) Refer to CLEAPSS® Hazcard 19A Magnesium chloride (Low hazard) Refer to CLEAPSS® Hazcard 59B Ammonium chloride (Low hazard at concentration used) Refer to CLEAPSS® Hazcard 9A 1 Laminated copies of the results table last longer. Results tables in document wallets are very likely to get wet and dirty. If this practical is done regularly, it is worth laminating the tables. 2 Bottles of chemicals are required for each bench of students. If enough bottles are supplied, students do not need to wander round looking for the reagents. Dropper bottles are best. The concentrations are not crucial, but the sodium hydroxide should be below 0.5 mol dm-3 to minimise the hazard. Exactly which salt is used is also not critical, ie sulfates, chlorides or nitrates could be used, as available.

Procedure HEALTH & SAFETY: Wear eye protection a Take a copy of the results sheet. If it is not laminated, put it into a plastic pocket. Put two drops of sodium hydroxide solution onto each of the empty boxes and then two drops of the positive ion solution, and observe what happens.

•

b Hold a piece of damp red litmus paper over the ammonium chloride and sodium hydroxide box. c Add more sodium hydroxide, dropwise, to the aluminium nitrate and sodium hydroxide box. Observe what happens.

Teaching notes It is important that students do not add more than a couple of drops of each solution to the boxes. If this happens, the drops spread and mix, obscuring the results. If the red litmus is not held near the ammonium chloride and sodium hydroxide box soon after the solutions have mixed, it may be difficult to see litmus changing colour. The solids formed are: iron(II) hydroxide; iron(III) hydroxide; copper(II) hydroxide; aluminium hydroxide; calcium hydroxide; magnesium hydroxide. The gas made is ammonia.

Equations Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s) Fe3+(aq)+ 3OH-(aq) → Fe(OH)3(s) Cu2+(aq)+ 2OH-(aq) → Cu(OH)2(s) Al3+(aq) + 3OH-(aq) → Al(OH)3(s)

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followed by the aluminium hydroxide dissolving in the excess hydroxide to give a solution of sodium aluminate Al(OH)3(s) + 3OH-(aq) → (Al(OH)6)3-(aq) Ca2+(aq) + 2OH-(aq) → Ca(OH)2(s) Mg2+(aq) + 2OH-(aq) → Mg(OH)2(s) NH4+ + OH- → NH3 + H2O

Expected results Here are the expected results, if students use the worksheet you can copy from the next page. Some of the boxes on the worksheet are shaded to increase the visibility of the white precipitates. Positive ion solution

Positive ion solution and sodium hydroxide solution

Iron(II), Fe

Grey green solid is formed

2+

Orange solid is formed

Iron(III), Fe

3+

Copper(II), Cu

Blue solid is formed

Aluminium, Al3+

White solid is formed which dissolves in excess sodium hydroxide

2+

White solid is formed

Calcium, Ca

2+

Magnesium, Mg

White solid is formed

Ammonium, NH4+

A gas is evolved which turns damp red litmus paper blue

2+

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/separation-and-analysis/ reactions-of-positive-ions-with-sodium-hydroxide-microscale-version,164,EX.html

Useful resource Further information and ideas for microscale experiments can be found on Learnnet www.chemsoc.org/networks/learnnet/microscale.htm and Inspirational chemistry www.chemsoc.org/networks/learnnet/inspirational/home.htm (Websites accessed December 2009)

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Student worksheet Reactions of positive ions with sodium hydroxide

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If this sheet is not laminated, put it into a plastic pocket.

Microscale experiment Combine the drops of solutions on this grid.

Positive ion solution

Positive ion solution and sodium hydroxide solution

Iron (II), Fe2+

Iron (III), Fe3+

Copper (II), Cu2+

Aluminium, Al3+

Calcium, Ca2+

Magnesium, Mg2+

Ammonium, NH4+

This student worksheet accompanies the ‘Reactions of positive ions with sodium hydroxide‘ microscale experiment instructions on www.practicalchemistry.org

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Testing for negative ions This activity is in two parts – in the first, students make observations while carrying out the tests for various negative ions. In the second, they use their observations to help them identify the negative ions present in a number of unknown solutions. To make the second part of the exercise more challenging, tests for positive ions could be introduced and students could be asked to identify both the positive and negative ions present in a solution.

•

Apparatus and chemicals Eye protection The exact concentrations of the test solutions are not important. Use approximately 0.1–0.5 mol dm–3 for salt solutions and 0.5–1.0 mol dm–3 for acid solutions, except for nitric acid, which is corrosive at such concentrations (use 0.4 mol dm–3 instead). Test-tubes Dropping pipettes (these can be used just for the carbon dioxide testing or also for dispensing solutions; if the latter, far more pipettes will be required) Nitric acid 0.4 mol dm–3 (Irritant) Silver nitrate solution 0.1 mol dm–3 Barium chloride solution 0.1 mol dm–3 (Harmful) Hydrochloric acid Aluminium powder (Highly flammable) Sodium hydroxide solution less than 0.5 mol dm–3 (Irritant) Limewater Red litmus paper Ammonia solution 0.4 mol dm–3. For the initial observations Sodium or potassium chloride solution Sodium or potassium bromide solution Sodium or potassium iodide solution Sulfate solution, eg sodium sulfate Carbonate solution, eg potassium carbonate Nitrate solution, eg potassium nitrate. For testing unknowns The number of unknowns required depends on the time available. It is a good idea to use at least four solutions to ensure students are challenged. Label the solutions A, B, C etc and make sure you know which is which.

Technical notes

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Health and safety: Wear eye protection. Barium chloride solid is toxic; the 0.1 mol dm–3 solution is harmful. Wash your hands after use and warn students to do the same. Ammonia solution is an irritant when concentrated but not at the concentrations used by students in this activity. However, it can give off ammonia vapour, which can irritate the eyes and lungs. Keep the lid on the bottle when not in use. Nitric acid is an irritant. Silver nitrate solution can stain skin and clothes.

Tests for negative ions – expected observations Negative ion

Test

Observations

CO32– carbonate

Put a small amount of limewater into a test-tube (no more than 1 cm3). Put your sample in a separate testtube and add a few drops of hydrochloric acid. Using a pipette, collect the gas given off and bubble it through the limewater. (Note: you can also do this test on a solid sample.)

Bubbles of gas form. The gas turns the limewater milky, which shows that it is carbon dioxide.

Cl chloride

Add a few drops of dilute nitric acid followed by a few drops of silver nitrate solution. Let the mixture stand for a few minutes and then add some ammonia solution.

A white precipitate forms which discolours on standing. The precipitate is soluble in ammonia solution.

Br bromide


A cream precipitate forms which discolours a little on standing. The precipitate is slightly soluble in ammonia solution.

I– iodide


A yellow precipitate forms which does not discolour on standing. The precipitate is insoluble in ammonia solution.

SO42– sulfate

Add a few drops of barium chloride solution and then a A white precipitate forms. few drops of hydrochloric acid.

NO3– nitrate

Add a few drops of sodium hydroxide solution and a little aluminium powder. Warm the solution in a Bunsen flame and test any gas given off using red litmus paper.

–

–

A gas is given off which turns the litmus blue. This shows that the gas is ammonia.

Equations NaCl(aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s) Cl–(aq) + Ag+(aq) → AgCl(s) (and similarly for Br– and I–) Na2SO4(aq) + BaCl2(aq) → 2NaCl(aq) + BaSO4(s) SO42–(aq) + Ba2+(aq) → BaSO4(s) 2HCl(aq) + Na2CO3(aq) → 2NaCl(aq) + H2O(l) + CO2(g) CO32–(aq) + 2H+(aq) → CO2(g) + H2O(l) For completeness, the reaction with the nitrate ion is shown below. It is unlikely that students will be able to construct this for themselves and the student sheet does not ask them to do so. 8Al(s) + 3NO3–(aq) + 5OH–(aq) + 18H2O(l) → 8[Al(OH)4]–(aq) + 3NH3(g)

Reference This experiment has been reproduced from Inspirational Chemistry, Royal Society of Chemistry, London, p.177-179 and Index 7.3.1

Further problem solving ideas There are several suggestions in C. Wood, Creative Problem Solving in Chemistry, London: Royal Society of Chemistry, 1993.

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Student worksheet Testing for negative ions – making observations This activity is in two parts. In the first part you observe the reactions of various negative ions and in the second you use those observations to identify unknown solutions. Use the table Tests for negative ions to record your observations during each test. Use a clean test-tube each time or wash up thoroughly between tests using distilled or deionised water to avoid contamination. Use a small portion of the test solution each time (no more than 1 cm3). Write balanced symbol equations for the reaction that occurs in each of the tests (except the test for a nitrate).

Health and safety

•

238

Wear eye protection. Take extra care when dealing with unknown solutions. At the concentrations used in this experiment: Barium chloride solution is harmful; wash your hands after use. Sodium hydroxide is an irritant. Ammonia solution can give off ammonia vapour, which can irritate the eyes and lungs. Keep the lid on the bottle when not in use. Nitric acid is an irritant. Silver nitrate can stain skin and clothes.

Tests for negative ions Negative ion

Test

CO32– carbonate

Put a small amount of limewater into a test-tube (no more than 1 cm3). Put your sample in a separate testtube and add a few drops of hydrochloric acid. Using a pipette, collect the gas given off and bubble it through the limewater. (Note: you can also do this test on a solid sample.)

Cl– chloride


Br– bromide


I– iodide


SO42– sulfate

Add a few drops of barium chloride solution and then a few drops of hydrochloric acid.

NO3– nitrate

Add a few drops of sodium hydroxide solution and a little aluminium powder. Warm the solution in a Bunsen flame and test any gas given off using red litmus paper.

Observations

239

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Student worksheet Testing for negative ions – Identifying unknowns Using the observations chart you made in Testing for negative ions – making observations, test the unknown solutions provided and identify the negative ions present. Make careful observations, including any negative results. You may need to try a number of tests before you get a positive result. Design a table to record your observations. You may wish to use the headings: Unknown sample; Test tried; Observations; and Conclusion.

•

Health and safety Wear eye protection. Take extra care when dealing with unknown solutions. At the concentration used in this experiment: Barium chloride solution is harmful; wash your hands after use. Sodium hydroxide is an irritant. Ammonia solution can give off ammonia vapour, which can irritate the eyes and lungs. Keep the lid on the bottle when not in use. Nitric acid is an irritant. Silver nitrate can stain skin and clothes.

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Flame tests (wooden splint method)

79

Teachers have traditionally used nichrome wire for carrying out flame tests. The main problems with this method are: the need to use concentrated hydrochloric acid (Corrosive, refer to CLEAPSS® Hazcard 47A). This presents considerable hazard that often deters teachers from using the procedure with students,

●

the problem of contamination of wires which are then difficult to clean,

●

the cost of regularly renewing wires.

●

Lesson organisation The method described in this experiment is intended for students to carry out and avoids the need for the use of concentrated hydrochloric acid. It also avoids the cost and contamination problems associated with the use of nichrome or platinum wires. A circus arrangement for the procedure would make classroom management much easier than if every group of students have to collect and test all the solutions at their own workstation. The time taken will depend on the number of tests to be carried out, but 30 minutes should be sufficient.

Apparatus and chemicals Eye protection Bunsen burners Heat resistant mat(s) Boiling tube racks Boiling tubes Wooden splints

•

Distilled water A selection from solutions of the following salts, each no more than 0.5 mol dm-3 Lithium chloride (Harmful) (see note 3) Sodium chloride (Low hazard) Potassium chloride (Low hazard) (see note 3) Rubidium chloride (Low hazard) Caesium chloride (Low hazard) Calcium chloride (Irritant) Strontium chloride (Irritant) Barium chloride (Harmful at the concentration used) Copper chloride (Harmful, Danger to the environment)

241

Technical notes

•

Wear eye protection Lithium chloride is Harmful. Refer to CLEAPSS® Hazcard 47B. Sodium chloride is Low hazard. Refer to CLEAPSS® Hazcard 47B. Potassium chloride is Low hazard. Refer to CLEAPSS® Hazcard 47B. Rubidium chloride is Low hazard. Refer to CLEAPSS® Hazcard 47B. Caesium chloride is Low hazard. Refer to CLEAPSS® Hazcard 47B. Calcium chloride is Irritant but Low Hazard at the concentration used. Refer to CLEAPSS® Hazcard 19A Strontium chloride is Irritant but Low Hazard at the concentration used. Refer to CLEAPSS® Hazcard 19A. Barium chloride is Harmful at the concentration used. Refer to CLEAPSS® Hazcard 10A Copper chloride is Harmful, Danger to the environment. Refer to CLEAPSS® Hazcard 27C. 1 Lead salts are best avoided. They carry an extra risk and the flame test result is not that impressive. 2 The chlorides of metals give the best results but other salts, such as sulfates, also work. Nitrates are best avoided. 3 Potassium iodide and lithium iodide can be used instead of the chlorides. As a general rule, chlorides are usually suggested, since they tend to be more volatile and more readily available. These two are in fact a little more volatile than the chloride, and potassium iodide is certainly likely to be available (refer to CLEAPSS® Hazcard 47B).

Procedure

•

HEALTH & SAFETY: Wear safety goggles. a Well before the lesson in which they are to be used, thoroughly soak a supply of wooden splints in distilled water. b Sets of boiling tubes should be up to half-filled with the solutions of the salts. c Each ’station’ around the laboratory should then consist of a boiling tube containing one of the above solutions, held in a test tube rack. Each should be labelled with the name and symbol of the metal ion present plus appropriate hazard warnings. There should also be as many pre-soaked splints as there are working groups. These should be immersed in the solution. d S tudents hold a soaked splint in a blue Bunsen flame to reveal the flame colour. It is important not to let the splint start to burn too vigorously. Bunsen burners could be clamped at an angle if desired: this helps avoid contamination caused by dripping onto the mouth of burner (but care is need in the direction of the flame). e A container (such as a beaker half filled with water) for the disposal of used splints will be needed at each workstation. f One station could be set up with distilled water as a control and another with a solution labelled as ‘unknown’ if wanted.

Reference This experiment was written on behalf of the RSC

Useful resource See also experiment 79: Flame colours – a demonstration for teaching notes and useful resources.

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Flame colours – a demonstration

80

This demonstration experiment can be used to show the flame colours given by alkali metal, alkaline earth metal, and other metal salts. This is a spectacular version of the ‘flame tests’ experiment that can be used with chemists and non-chemists alike. It can be extended as an introduction to atomic spectra for post-16 students.

Lesson organisation This experiment must be done as a demonstration. It takes about ten minutes if all is prepared in advance. Preparation includes making up the spray bottles and conducting a risk assessment. Your employer's risk assessment must be customised by determining where to spray the flame to guarantee the audience’s safety. Use a fume cupboard unless you are sure of an alternative space.

Apparatus and chemicals Eye protection Access to fume cupboard (unless a safe alternative space is available)

•

Trigger pump operated spray bottles (see note 1) Bunsen burner Heat resistant mat(s) Hand-held spectroscopes or diffraction gratings (optional) Samples of the following metal salts (no more than 1 g of each) (see note 2): Sodium chloride (Low hazard) Potassium chloride (Low hazard) (see note 3) Lithium chloride (Harmful) (see note 3) Copper sulfate (Harmful, Danger to the environment). Ethanol (Highly flammable), approx 10 cm3 for each metal salt. or IDA (industrial denatured alcohol) (Highly flammable, Harmful)

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Technical notes Sodium chloride is Low hazard. Refer to CLEAPSS® Hazcard 47B. Potassium chloride is Low hazard. Refer to CLEAPSS® Hazcard 47B. Lithium chloride is Harmful. Refer to CLEAPSS® Hazcard 47B Copper sulfate is Harmful, Danger to the environment. Refer to CLEAPSS® Hazcard 27C. Ethanol is Highly flammable. IDA (industrial denatured alcohol) is Highly flammable, Harmful. Refer to CLEAPSS® Hazcard 40A. 1 Spray bottles of the type used for products such as window cleaner should be used. These piston-operated spray bottles should be emptied, cleaned thoroughly and finally rinsed with distilled water. Ideally, one bottle is needed for each metal salt. Never use spray bottles with a rubber bulb - the flame may flash back into the container. 2 The chlorides of metals are the best but other salts also work. Make a saturated solution of each salt in about 10 cm3 ethanol. To do this, add the salt to the ethanol in small quantities, with stirring, until no more will dissolve – often only a few mg of salt will be needed. Place each solution in a spray bottle and label the bottle. The solutions can be retained for future use. They can be stored in the plastic bottles for several weeks at least without apparent deterioration of the bottles. 3 Potassium iodide and lithium iodide can be used instead. As a general rule, chlorides are usually suggested as they tend to be more volatile and more readily available. These two are in fact a little more volatile than the chloride, and potassium iodide is certainly likely to be available (refer to CLEAPSS® Hazcard 47B). Other metal salts (e.g. those of calcium and barium) can also be used provided an appropriate risk assessment is carried out. Barium chloride is toxic but gives a different colour (refer to CLEAPSS® Hazcard 10A), while calcium chloride (Irritant) and strontium chloride (Irritant) are different again (refer to CLEAPSS® Hazcard 19A). 4 Care should be taken not to allow excess ethanolic solution to accumulate on the heat resistant mats. There is a risk of this igniting with the proximity of the Bunsen burner flame.

Procedure HEALTH & SAFETY: Carry out the whole experiment in a fume cupboard or an area you have previously shown to be safe. Wear eye protection. Ensure that the spray can be safely directed away from yourself and the audience. a Darken the room if possible. b Light the Bunsen and adjust it to give a non-luminous, roaring flame (air hole open). c Conduct a preliminary spray in a safe direction away from the Bunsen flame. Adjust the nozzles of the spray bottles to give a fine mist. d Choose one spray bottle. Spray the solution into the flame in the direction you have rehearsed. Repeat with the other bottles. e A spectacular coloured flame or jet should be seen in each case. The colour of the flame depends on the metal in the salt used. f As an extension, students can view the flames through hand-held spectroscopes or diffraction gratings in order to see the line spectrum of the element. (Diffraction gratings work better. A better way to produce a steady source of light is to use discharge tubes from the Physics Department – with a suitable risk assessment.)

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Teaching notes The colours that should be seen are: sodium

yellow-orange (typical ‘street lamp’ yellow)

potassium

purple-pink, traditionally referred to as ‘lilac’ (often contaminated with small amounts of sodium)

lithium

crimson red

copper

green/blue

calcium

orange-red (probably the least spectacular)

barium

apple green

strontium

crimson

The electrons in the metal ions are excited to higher energy levels by the heat. When the electrons fall back to lower energy levels, they emit light of various specific wavelengths (the atomic emission spectrum). Certain bright lines in these spectra cause the characteristic flame colour. The colour can be used to identify the metal or its compounds (eg sodium vapour in a street lamp). The colours of fireworks are, of course, due to the presence of particular metal salts.

Reference This experiment has been reproduced from Practical Chemistry: http://www.practicalchemistry.org/experiments/intermediate/structure-and-bonding/flamecolours-a-demonstration,102,EX.html

Useful resource Flame colours gives a simple explanation of flame colours in terms of excited electrons: http://www.chemicalconnection.org.uk/chemistry/topics/view.php?topic=3&headingno=5&lang=en Flame tests gives another slightly different version, involving establishing some flame colours and then using them to identify unknowns: http://www.creative-chemistry.org.uk/activities/flametests.htm


245

Q.

Self test questionnaire Atomic structure and bonding - ‘dot-cross diagrams’ 1 W ork out the numbers of protons, neutrons and electrons in the following atoms; carbon, C; lithium, Li; uranium, U; manganese, Mn. 2 a) What is unique about the make-up of the sub-atomic particles in a hydrogen atom, H? b) W hat seems strange about the make-up of the sub-atomic particles in a chlorine atom, Cl? 3 What would be the numbers of protons, neutrons and electrons, in the following isotopes?

a) 126C, 136C, 146C, b) 11H, 12H, 13H

4 Neon, Ne, has isotopes

20 10

22 Ne and 10 Ne

20 22 They exist in the approximate ratio 9:1 10 Ne : 10 Ne. What will be the average relative atomic mass of neon?

5 Draw or write the electron arrangements for:

a) boron, B, b) fluorine, F, c) lithium, Li.

6 N eon atoms have 10 electrons (2,8) as do F- ions and Na+ ions. Why don’t fluorine and sodium become Ne when they react? 7 G ive the charge of the ions you would expect to be formed by a) calcium, b) oxygen, c) aluminium, d) nitrogen. 8 D raw diagrams to show the bonding between: a) sodium and chlorine, b) sodium and oxygen, c) calcium and fluorine 9 Write down the name and formula of each of the products formed in question 8. 10 Draw dot-cross diagrams to show the bonding in: a) Chlorine, Cl2, b) Methane, CH4, c) Nitrogen, N2 11 Which is the only metal element that does not have a giant structure at room temperature? 12 The table below gives some information about four substances. State which one is ionic, which metallic, which has a molecular structure and which has a covalent giant structure. Substance

246

Melting point/°C

Boiling point/°C

Electrical conduction As solid

As liquid

A

1083

2567

good

good

B

-182

-164

poor

poor

C

1723

2230

poor

good

D

993

1695

poor

poor

Formulae 1 Say how many atoms of which elements are combined together in: a) KF b) NaCl c) C2H6 d) C6H5NO2 2 Say how many atoms of each element are present in: a) (NH4)2SO4 b) Mg(NO3)2 c) Ca3(PO4)2 3 Name the following: a) LiBr b) Na2O c) MgO d) MgI2 e) FeS f ) Al2O3 4 Name the following: a) Li2CO3 b) Ca(OH)2 c) AlPO4 d) MgSO4 e) Cu(NO3)2 f ) NH4Cl

The charges on some ions Positive ions

Negative ions

1+

2+

3+

1-

2-

3-

Li Lithium

Mg Magnesium

Al Aluminium

Cl Chloride

O Oxide

PO43Phosphate

Na+ Sodium

Ca2+ Calcium

Br- Bromide

S2- Sulfide

K+ Potassium

Cu2+ Copper

I- Iodide

CO32Carbonate

NH4+ Ammonium

Zn2+ Zinc

NO3- Nitrate

SO42- Sulfate

+

2+

3+

-

2-

OH- Hydroxide

5 Use the table above to predict the formulae of the following compounds: a) lithium carbonate e) ammonium phosphate b) aluminium iodide f ) calcium oxide c) sodium nitrate g) copper chloride d) zinc sulfate h) aluminium phosphate 6 Use the Periodic Table to help you work out the electron arrangement of the following atoms and use it to predict the combining power of each. a) Nitrogen, N d) Sulfur, S b) Carbon, C e) Argon, Ar c) Chlorine, Cl f ) Silicon, Si 7 Draw dot-cross diagram for the following ions: a) the hydroxide ion OH b) the carbonate ion CO32-

247

Equations and balancing 1 Try the following a) Write a word equation for what happens when magnesium is burned in oxygen. b) Say what the following state symbols mean in an equation – (s), (l), (g), (aq). 2 Can you recognise a balanced equation? Which of the following equations is/are balanced? a) 2H2 + O2 → 2H2O b) CH4 + O2 → CO2 + H2O c) Na + H2O → 2NaOH + H2 d) H2 + Cl2 → 2HCl 3 Balance the two equations below. (Don’t change the formulae!) a) Li + F2 → LiF b) Fe + O2 → Fe2O3 4 Balance the following equations. a) Mg + HCl → MgCl2 + H2 b) Na + O2 → Na2O c) Ca(OH)2 + HNO3 → Ca(NO3)2 + H2O d) Ca + H2O → Ca(OH)2 + H2 5

Write the word equation and the balanced symbol equations for the following: a) iron reacting with sulfur b) magnesium reacting with nitrogen c) sodium reacting with bromine (The formulae you need to use are NaBr, Mg3N2, FeS).

6 Write the equations for the oxidation to their oxides of a) magnesium, b) lithium, c) aluminium, d) carbon, e) hydrogen 7 What is happening to sodium and chlorine in this reaction and why is it a redox reaction? 2Na + Cl2 → 2NaCl 8 a) Magnesium and sulfuric acid react to give magnesium sulfate and hydrogen. Write (or say to yourself ) the word equations for the reaction between each of the acids nitric, hydrochloric and ethanoic with magnesium. Then write balanced equations for the reactions. Pick out the formulae you will need: Mg(NO3)2, MgSO4, Mg(CH3COO)2, MgCl2. b) Do the same thing as above for the reaction between sodium hydroxide, NaOH, and the four acids. Pick out the formulae you will need: Na2SO4, NaCH3COO, NaNO3, NaCl. c) Write balanced symbol equations for the reactions between sodium carbonate and the four acids given in (b) and then magnesium carbonate and the four acids. The only new information you need is the formulae of sodium carbonate Na2CO3 and magnesium carbonate MgCO3. 9 CuSO4(aq) + Na2CO3(aq) → CuCO3(s) + Na2SO4(aq) The ions have ‘swapped partners’. This is called double decomposition. Describe what you would see happening in the above reaction. 10 Sodium and potassium salts and all nitrates are always soluble in water. Use this information to write word and symbol equations for the following double decomposition reactions which take place in solution. Include state symbols in your answers. a) lead nitrate and potassium iodide b) potassium carbonate and barium chloride c) sodium chloride and silver nitrate Pick out the formulae you will need: BaCl2, KI, Pb(NO3)2, K2CO3, NaCl, AgNO3, NaNO3, AgCl, PbI2, KCl, BaCO3, KNO3.

248

Chemical calculations – the mole 1 Use the Periodic Table to look up the relative atomic masses of the following atoms: a) sodium, Na c) lead, Pb b) oxygen, O d) chlorine, Cl 2 Imagine you had an atomic see-saw. A magnesium atom is placed on one side. What combinations of other atoms could be placed on the other side to balance it? For example two carbon atoms would work. There are lots of combinations. Give at least six. 3 Work out (using relative atomic masses in the Periodic Table) the relative molecular mass of: a) ammonia, NH3 b) calcium hydroxide, Ca(OH)2 c) oxygen molecule, O2 d) ethanol, C2H6O 4 Work out the reacting masses in grams for: a) 2H2 + O2 → 2H2O b) H2 + Cl2 → 2HCl 5 Find the mass of a mole of: a) Sn atoms b) NH3 molecules c) CO2 molecules d) NaCl 6 How many moles are there in a) 40 g of calcium, Ca? b) 980 g of sulfuric acid, H2SO4? c) 22 g of carbon dioxide, CO2? d) 3.2 g of oxygen molecules, O2? 7 For the following equations, write below each substance the quantity which will react in (i) grams and (ii) moles. a) CuCO3 → CuO + CO2 b) Mg + 2HCl → MgCl2 + H2 c) HCl + NaOH → NaCl + H2O 8 What is the formula of each of the following compounds? a) 196 g of sulfuric acid contains 4 g of hydrogen, 64 g of sulfur and 128 g of oxygen. b) 12.4 g sodium oxide contains 9.2 g of sodium and 3.2 g of oxygen. c) 17 g of ammonia contains 14 g of nitrogen and 3 g of hydrogen. d) 6.3 g of nitric acid contains 0.1 g of hydrogen, 1.4 g of nitrogen and 4.8 g of oxygen. 9 What is the concentration in mol dm-3 of a) 2 moles of H2SO4 in 1000 cm3 of solution? b) 2 moles of H2SO4 in 2000 cm3 of solution? c) 0.5 moles of H2SO4 in 100 cm3 of solution? 10 How would you make a a) 0.1 mol dm-3 solution of copper sulfate (relative molecular mass 160)? b) 1 mol dm-3 solution starting with 16 g of copper sulfate? 11 How many moles of solute are there in a) 10 cm3 of 1 mol dm-3 solution? b) 50 cm3 of 2 mol dm-3 solution? c) 20 cm3 of 0.01 mol dm-3 solution?

249

12 Describe how you would make the following: a) 1 dm3 of 2 mol dm-3 sodium chloride solution (NaCl). b) 2 dm3 of 0.01 mol dm-3 of silver nitrate solution (AgNO3). c) 250 cm3 of 0.5 mol dm-3 of sodium thiosulfate solution (Na2S2O3). d) 100 cm3 of 1 mol dm-3 iodine solution (I2). 13 a) 100 cm3 1 mol dm-3 sulfuric acid is just neutralised by 50 cm3 of 4 mol dm-3 of potassium hydroxide. In what proportion do they react?

_

b) 20 cm3 of 0.1 mol dm-3 nitric acid is just neutralised by 10 cm3 of 0.2 mol dm-3 sodium hydroxide. In what proportions do they react?

14 How many moles are there in the following volumes of oxygen (O2) at room temperature and pressure? a) 240 000 cm3 b) 120 cm3 c) 48 cm3 d) What difference (if any) would it make if the gas were neon?

250

A.

Answers Atomic structure and bonding – ‘dot-cross diagrams’ 1 C

p

n

e

6

6

6

Li

3

4

3

U

92

146

92

Mn

25

30

25

2 a) Hydrogen atoms have no neutron. b) Chlorine appears to have 18.5 neutrons 3 a)

p C

12 6

n

6

C

13 6

6

6

C

14 6

7

6

e

8

b)

p

n

e

H

1

0

1

H

1

1

1

H

1

2

1

6

1 1

6

2 1

6

3 2

4 20.2 5 a) 2,3; b) 2,7; c) 2,1 6 Only the number of electrons changes. The nucleus, which gives the atom its identity, does not vary. 7 a) Ca2+, b) O2-, c) Al3+, d) N38 a) Cl

Na

Na+

Cl-

b) Na

Na

O

Na+

Na+

O2-

c) Ca

Ca2+

F

F

F-

F-

9 a) NaCl, sodium chloride; b) Na2O, sodium oxide; c) CaF2, calcium fluoride.

251

10 a)

Cl

H

b) H

Cl

H

H

Cl-Cl

C

CH4

c) N

N N-N

11 Mercury, Hg, a liquid! 12 A metallic; B molecular; C ionic; D giant covalent

Formulae 1 a) One atom of potassium to one atom of fluorine b) One atom of sodium to one atom of chlorine c) Two atoms of carbon to six atoms of hydrogen d) Six atoms of carbon to five atoms of hydrogen to one atom of nitrogen to two atoms of oxygen. 2 a) Two atoms of nitrogen to eight atoms of hydrogen to one atom of sulfur to four atoms of oxygen b) One atom of magnesium to two atoms of nitrogen to six atoms of oxygen c) Three atoms of calcium to two atoms of phosphorus to eight atoms of oxygen 3 a) lithium bromide; b) sodium oxide; c) magnesium oxide; d) magnesium iodide; e) iron sulfide; f ) aluminium oxide. 4 a) lithium carbonate; b) calcium hydroxide; c) aluminium phosphate; d) magnesium sulfate; e) copper nitrate; f ) ammonium chloride. 5 a) Li2CO3; b) AII3; c) NaNO3; d) ZnSO4; e) (NH4)3PO4; f ) CaO; g) CuCl2; h) AlPO4. 6 a) 2,5; three; b) 2,4; four; c) 2,8,7; one; d) 2,8,6; two; e) 2,8,8; zero; f ) 2,8,4; four. 7 a)

-

O

H

b)

2O

O

C

O

252

Equations and balancing 1 a) magnesium + oxygen → magnesium oxide b) (s) is solid, (l) is liquid, (g) is gas and (aq) is aqueous which means dissolved in water. 2 a and d 3 a) 2Li + F2 → 2LiF b) 4Fe + 3O2 → 2Fe2O3 4 a) b) c) d)

Mg + 2HCl → MgCl2 + H2 4Na + O2 → 2Na2O Ca(OH)2 + 2HNO3 → Ca(NO3)2 + 2H2O Ca + 2H2O → Ca(OH)2 + H2

5 a) iron + sulfur → iron sulfide Fe + S → FeS b) magnesium + nitrogen → magnesium nitride 3Mg + N2 → Mg3N2 c) sodium + bromine → sodium bromide 2Na + Br2 → 2NaBr 6

a) b) c) d) e)

2Mg + O2 → 2MgO 4Li + O2 → 2Li2O 4Al + 3O2 → 2Al2O3 C + O2 → CO2 2H2 + O2 → 2H2O

7 Na → Na+, so it is oxidised (loss of an electron); Cl → Cl-, so it is reduced (gain of an electron). 8 a) Mg + 2HCl → MgCl2 + H2 Mg + 2HNO3 → Mg(NO3)2 + H2 Mg + H2SO4 → MgSO4 + H2 Mg + 2CH3COOH → Mg(CH3COO)2 + H2 b) NaOH + HCl → NaCl + H2O NaOH + HNO3 → NaNO3 + H2O 2NaOH + H2SO4 → Na2SO4 + 2H2O NaOH + CH3COOH → NaCH3COO + H2O c) Na2CO3 + 2HCl → 2NaCl + CO2 + H2O Na2CO3 + 2HNO3 → 2NaNO3 + CO2 + H2O Na2CO3 + H2SO4 → Na2SO4 + CO2 + H2O Na2CO3 + 2CH3COOH → 2 NaCH3COO + CO2 + H2O MgCO3 + 2HCl → MgCl2 + CO2 + H2O MgCO3 + 2HNO3 → Mg(NO3)2 + CO2 + H2O MgCO3 + H2SO4 → MgSO4 + CO2 + H2O MgCO3 + 2CH3COOH → Mg(CH3COO)2 + CO2 + H2O 9 A (blue) solution is added to a colourless solution and a (green) solid forms. 10 a) Pb(NO3)2(aq) + 2KI(aq) → 2KNO3(aq) + PbI2(s) b) K2CO3(aq) + BaCl2(aq) → 2KCl(aq) + BaCO3(s) c) NaCl(aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s)

253

Chemical calculations – the mole 1 a) 23; b) 16; c) 207; d) 35.5 2 Any combination of atoms whose Ars add up to 24, eg 24 hydrogens, 1 carbon plus 12 hydrogens etc. 3 a) 17; b) 74; c) 32; d) 46 4 a) 4 g H2 + 32 g O2 → 36 g H2O b) 2 g H2 + 71 g Cl2 → 73 g HCl 5 a) 119 g; b) 17 g; c) 44 g; d) 58.5 g 6 a) 1; b) 10; c) 0.5; d) 0.1 7 a) CuCO3 → CuO + 124 g 80 g 1 mole 1 mole

CO2 44 g 1 mole

H2 b) Mg + 2HCl → MgCl2 + 24 g 73 g 95 g 2g 1 mole 2 moles 1 mole 1 mole

c) HCl + NaOH → NaCl + H2O 36.5 g 40 g 58.5 g 18 g 1mole 1 mole 1 mole 1 mole

8 a) H2SO4; b) Na2O; c) NH3; d) HNO3 9 a) 2 mol dm-3; b) 1 mol dm-3; c) 5 mol dm-3; 10 a) Dissolve 16 g of copper sulfate in 1 dm3 of solution (or any other quantities in the same proportions). b) 16 g of copper sulfate in 100 cm3 of solution 11 a) 0.01; b) 0.1; c) 2 x 10-4 12 a) Dissolve 117 g of sodium chloride in 1 dm3 of solution b) Dissolve 3.4 g of silver nitrate in 2 dm3 of solution c) Dissolve 19.75 g of sodium thiosulfate in 250 cm3 of solution d) Dissolve 25.4 g of iodine in 100 cm3 of solution 13 a) 2 moles of potassium hydroxide to 1 mole of sulfuric acid b) 1 to 1 14 a) 10; b) 5 x 10-3; c) 2 x 10-3; d) None

254

RSC resources www.rsc.org/resources Visit this website to see how the RSC can support you in your chemistry teaching through teaching resources in a range of media - books, CD ROMs and online material. Listed on this page is a sample of the free online resources available:

Practical Chemistry www.practicalchemistry.org Good quality, appropriate chemistry experiments play a vital role in teaching and learning. They can be used to enhance learning and to clarify aspects of theory. Practical activities add to the fun of chemistry and allow students to apply their knowledge and understanding to what they experience. This website provides all teachers of chemistry with a wide range of experiments to illustrate concepts or processes, as starting-points for investigations and for enhancement activities such as club or open day events. It also enables the sharing of skills and experience of making experiments work in the classroom. There is lots of information for technicians too.

www.chemistryteachers.org www.chemistryteachers.org www.chemistryteachers.org is designed for chemistry and science teachers world-wide and allows you to quickly find resources from the Royal Society of Chemistry and elsewhere to use in your teaching. www.chemistryteachers.org works by linking curriculum or chemistry topics to resources via a 'tree' of chemistry. You can view this 'chemistry tree' by accessing the 'Search by Chemical Keyword' option. The resources available cover all age ranges, 5-11, 11-14, 14-16 and post-16, and are in a variety of formats (e.g. Word or pdf files, Excel spreadsheets, PowerPoint presentations, video clips, animations etc) or are links to external websites.

Inspirational Chemistry www.rsc.org/inspirational Resources for teaching chemistry in up-to-date contexts, set where possible in everyday situations, with teacher material and student worksheets.

Chemistry demonstrations to enhance teaching and learning www.chemsoc.org/networks/learnnet/cldemo.htm This site gives descriptions and instructions for carrying out a variety of chemical demonstrations that will be useful in schools and colleges.

Alchemy? www.rsc.org/alchemy Alchemy? is a website built around 15 video clips, each lasting approximately five minutes. The clips cover the following industrial processes: aluminium, ammonia, chemicals from salt, copper refining, gases from the air, iron and steel, nitric acid, Nylon, oil refining, polythene, sodium, sulfuric acid, combinatorial chemistry, computational chemistry and making medicines.

255

Assessment for Learning www.rsc.org/aflchem Assessment for Learning is an effective way of actively involving students in their learning. The materials on the website allow you to use Assessment for Learning principles to structure sessions in which students investigate various chemical ideas.

ChemIT www.chemit.co.uk This site makes available a range of ICT materials to support the teaching of chemistry across the 11-19 age range. Many of these materials have been produced by chemistry teachers for use with their own students, others have been developed with the support of the RSC.

Joint Earth Science Education Initiative www.rsc.org/jesei Resources for teaching Earth Science, including notes for teachers and student worksheets. There is also material to help improve your own understanding of Earth Science and some stimulus material for students to help you put the ‘wow’ factor into your lessons.

Periodic Table of Data www.rsc.org/ptdata This interactive Periodic Table is designed to allow students and teachers to select from a wide range of data from the Periodic Table. Elements can be selected by Group, Period, individually, or a random selection. The data can then be exported to other packages such as word processors or spreadsheets. There are also tools to visualise trends in the properties of elements as well as a graph drawing facility. Another section of this resource covers the history of the Periodic Table and games for students, one of which puts them in the position of Mendeleev in 1869 and invites them to discover patterns in the elements.

Chemistry World – Interactive Periodic Table www.rsc.org/chemistryworld/podcast/element.asp Chemistry World takes a whirlwind tour of the periodic table: in a five minute podcast, a leading scientist or author tells the story behind a different element.

Anecdotes for chemistry teachers www.chemsoc.org/networks/learnnet/anecdotes.htm This page gives a collection of anecdotes, stories and unusual facts that will be of use to chemistry teachers to enliven their lessons.

Games for school students www.rsc.org/games The RSC provides a variety of interactive games for school students to help to enhance their learning. Visit this website to see the games available.

256

Index acid-alkali titration . . . . . . . . . . . . . . . . . . . . . . 57 alginate worms . . . . . . . . . . . . . . . . . . . . . . . . . 37 alkali metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 alkali metals - heating . . . . . . . . . . . . . . . . . . 148 alkali metals and chlorine . . . . . . . . . . . . . . . 148 allotropes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 aluminium and iodine . . . . . . . . . . . . . . . . . . 102 ammonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 ammonia fountain . . . . . . . . . . . . . . . . . . . . . . . 53 analysis . . . . . . . . . . . . . . . . . . . . . . . 232, 236, 241 balloons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 blue bottle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 bromine - handling . . . . . . . . . . . . . . . . . . . . . . 15 bromine water -preparing . . . . . . . . . . . . . . . 15 burning elements in chlorine . . . . . . . . . . . . 81 burning elements in oxygen . . . . . . . . . . . . . 77 burning money . . . . . . . . . . . . . . . . . . . . . . . . . . 91 calcium carbonate . . . . . . . . . . . . . . . . . . . . . . . 74 catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 change in mass when magnesium burns . . . . . . . . . . . . . . . . . . . . 109 charcoal - metals . . . . . . . . . . . . . . . . . . . . . . . 119 chemiluminescence . . . . . . . . . . . . . . . . . . . . 181 chlorine - burning elements in . . . . . . . . . . . 81 chocolate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 coins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 cold light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 colourful electrolysis . . . . . . . . . . . . . . . . . . . . 161 combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 combustion of iron wool . . . . . . . . . . . . . . . 107 competition for oxygen . . . . . . . . . . . . . . . . . 113 concentration and reaction rate . . . . . . . . 186 copper into 'silver' and 'gold' . . . . . . . . . . . 142 copper oxide and zinc . . . . . . . . . . . . . . . . . . 123 copper oxide formula . . . . . . . . . . . . . . . . . . . 133 copper oxide reduction . . . . . . . . . . . . . . . . . 131 cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 cracking hydrocarbons . . . . . . . . . . . . . . . . . 221 cross linking . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 cross linking polymers - worms . . . . . . . . . . 37 crude oil distillation . . . . . . . . . . . . . . . . . . . . . 218 cylinders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 diffusion in gases - ammonia and hydrogen chloride . . . . . . . . . . . . . . . . 13 diffusion in liquids . . . . . . . . . . . . . . . . . . . . . . . 18 diffusion of bromine in gases . . . . . . . . . . . . 16 displacement reactions . . . . . . . . . . . . . . . . . 116

disposable nappies . . . . . . . . . . . . . . . . . . . . . . 30 distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 distillation - steam . . . . . . . . . . . . . . . . . . . . . . . 41 dry ice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 effervescent Universal indicator rainbow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 electrolysis of potassium iodide solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 electrolysis of salt . . . . . . . . . . . . . . . . . . . . . . . 161 electrolysis of solutions . . . . . . . . . . . . . . . . . 168 electrolysis of zinc chloride . . . . . . . . . . . . . 164 elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 endothermic reactions . . . . . . . . . . . . . . . . . 178 endothermic solid-solid reactions . . . . . . . 176 exothermic reaction - spontaneous . . . . . 184 exothermic reactions . . . . . . . . . . . . . . . . . . . 178 exploding balloons . . . . . . . . . . . . . . . . . . . . . 204 exploding bubbles . . . . . . . . . . . . . . . . 211, 213 explosion - hydrogen-air . . . . . . . . . . . . . . . . 197 explosion - methane-air . . . . . . . . . . . . . . . . 201 extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 extraction of iron . . . . . . . . . . . . . . . . . . . . . . . 121 fat-pan fire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .88 filter paper neutralisation circles . . . . . . . . . 69 fire writing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 flame tests - wooden splint method . . . . 241 flame tests spray bottles . . . . . . . . . . . . . . . . 243 formula of copper oxide . . . . . . . . . . . . . . . . 133 fountain - ammonia . . . . . . . . . . . . . . . . . . . . . 53 fractional distillation of crude oil . . . . . . . . 218 gas cylinders - alternative . . . . . . . . . . . . . . . . 47 gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 gases - collecting . . . . . . . . . . . . . . . . . . . . . . . . 45 gases - testing . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Group 7 elements . . . . . . . . . . . . . . . . . . . . . . 152 hair gel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 halogen reactions aqueous solutions . . . . . . . . . . . . . . . . . . . . 152 halogen reactions with iron . . . . . . . . . . . . . 156 hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . 221 hydrochloric acid . . . . . . . . . . . . . . . . . . . . . . . . 57 hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 33 hydrogen and oxygen bubbles explosion . . . . . . . . . . . . . . . . . . . . . . . . 211, 213 hydrogen chloride . . . . . . . . . . . . . . . . . . . . . . . 13 hydrogen peroxide - catalysts . . . . . . . . . . . 195 hydrogen-air explosion . . . . . . . . . . . . . . . . . 197

257

indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 indigestion tablets . . . . . . . . . . . . . . . . . . . . . . . 61 iodine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 iodine and aluminium . . . . . . . . . . . . . . . . . . 102 iodine and zinc . . . . . . . . . . . . . . . . . . . . . . . . . 104 iodine clock reaction . . . . . . . . . . . . . . . . . . . 189 iron and halogen reactions . . . . . . . . . . . . . 156 iron and sulfur . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 iron on a match head . . . . . . . . . . . . . . . . . . . 121 iron wool combustion . . . . . . . . . . . . . . . . . . 107 jelly baby . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 limestone cycle . . . . . . . . . . . . . . . . . . . . . . . . . . 74 limonene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 lithium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 magnesium and steam . . . . . . . . . . . . . . . . . 136 magnesium burning change in mass . . . . . . . . . . . . . . . . . . . . . . . 109 match head - iron . . . . . . . . . . . . . . . . . . . . . . 121 metal extraction . . . . . . . . . . . . . . . . . . . . . . . . 119 metals and salts displacement reactions . . . . . . . . . . . . . . . 116 metals with charcoal . . . . . . . . . . . . . . . . . . . . 119 methane rocket . . . . . . . . . . . . . . . . . . . . . . . . . 93 methane-air explosion . . . . . . . . . . . . . . . . . . 201 moles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 money to burn . . . . . . . . . . . . . . . . . . . . . . . . . . 91 nappies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 negative ion tests . . . . . . . . . . . . . . . . . . . . . . . 236 neutralisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 neutralisation circles . . . . . . . . . . . . . . . . . . . . . 69 nitrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 nylon rope trick . . . . . . . . . . . . . . . . . . . . . . . . . 225 orange oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 oscillating reaction . . . . . . . . . . . . . . . . . . . . . 216 oxygen - burning elements in . . . . . . . . . . . .77 oxygen competition . . . . . . . . . . . . . . . . . . . . 113 plant water storage crystals . . . . . . . . . . . . . . 33 polymer - nylon . . . . . . . . . . . . . . . . . . . . . . . . 225 polymer - slime . . . . . . . . . . . . . . . . . . . . . . . . . 228 positive ion tests . . . . . . . . . . . . . . . . . . . . . . . . 232 potasium iodide . . . . . . . . . . . . . . . . . . . . . . . . 173 potassium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 products of combustion . . . . . . . . . . . . . . 83, 86 PVA polymer slime . . . . . . . . . . . . . . . . . . . . . . 228 Pyrex tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

258

rainbow fizz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 rainbow tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 rate of reaction - concentration . . . . . . . . . 186

rates of reaction - catalysts . . . . . . . . . . . . . . 195 rates of reaction - iodine clock . . . . . . . . . . 189 rates of reaction - rhubarb . . . . . . . . . . . . . . 192 reaction tube - making one . . . . . . . . . . . . . . 51 reduction of copper oxide . . . . . . . . . . . . . . 131 reversible reactions . . . . . . . . . . . . . . . . . . . . . 140 rhubarb - rates of reaction . . . . . . . . . . . . . . 192 rocket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 sand - silicon extraction . . . . . . . . . . . . . . . . . 128 screaming jelly baby . . . . . . . . . . . . . . . . . . . . 207 silanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 silicon from sand . . . . . . . . . . . . . . . . . . . . . . . 128 slime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 sodium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 sodium hydroxide . . . . . . . . . . . . . . . . . . . . . . . 57 spontaneous exothermic reaction . . . . . . 184 spray bottles - flame tests . . . . . . . . . . . . . . . 243 steam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 steam distillation . . . . . . . . . . . . . . . . . . . . . . . . 41 structure and bonding . . . . . . . . . . . . . . . . . . . 27 sulfur allotropes . . . . . . . . . . . . . . . . . . . . . . . . . 20 sulfur and iron . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 sulfur and zinc . . . . . . . . . . . . . . . . . . . . . . . . . . 100 testing for negative ions . . . . . . . . . . . . . . . . 236 testing for positive ions . . . . . . . . . . . . . . . . . 232 thermal decomposition . . . . . . . . . . . . . . . 74, 95 thermit reaction . . . . . . . . . . . . . . . . . . . . . . . . 125 thermometric titration . . . . . . . . . . . . . . . . . . . 64 titration - thermometric . . . . . . . . . . . . . . . . . 64 titration acid-alkali . . . . . . . . . . . . . . . . . . . . . . . 57 Universal indicator rainbow . . . . . . . . . . . . . . 66 whoosh bottle . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 wooden splint flame tests . . . . . . . . . . . . . . 241 worms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 zinc and copper oxide . . . . . . . . . . . . . . . . . . zinc and iodine . . . . . . . . . . . . . . . . . . . . . . . . . zinc and sulfur . . . . . . . . . . . . . . . . . . . . . . . . . . zinc chloride electrolysis . . . . . . . . . . . . . . . .

123 104 100 164

Notes

259

Notes

260


05/02/2010 12:03:54

48

V

51

52

57

(227)

137

56

(226)

88

133

59

59

63.5

65

55

(223)

87

178

40

181

41

93

23

24

184

42

96

25

26

44

190

186

101

43

(98)

27

192

45

103

28

(261)

72

(262)

73

(263)

74

(262)

75

(265)

76

(266)

77

29

47 197

108

59

58

144

30

(271)

78

(272)

79

(145)

104

150

105

152

106

157

107

159

108

162

109

165

110

167

111

169

80

201

48

112

32

119

31

115

175

71

70

82

207

50

173

81

204

49

91

90

Th Pa

231

232

#Actinide series

92

93

237

61

94

(244)

62

95

(243)

63

96

(247)

64

97

(247)

65

98

(251)

66

99

(252)

67

100

(257)

68

101

(258)

69

102

(259)

103

(260)

U Np Pu Am Cm Bk Cf Es Fm Md No Lr

238

60

Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

141

140

*Lanthanide series

75

73

15

P

14

Si

80

35

34

17

84

18

40

10

Ne

20

2

He

Cl Ar 79

16

S

35.5

9

F

19

7

33

122

128

I

85

(210)

53

86

(222)

54

Xe

atomic (proton) number

atomic symbol

relative atomic mass

Key

84

(209)

52

36

131

www.rsc.org/education

83

209

51

127

Pt Au Hg Tl Pb Bi Po At Rn

195

46

106

Fr Ra Ac Rf Db Sg Bh Hs Mt Ds Rg

89

32

8

31

7

O

16

6

N

14

5

4

0

Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

70

13

Al

28

6

C

12

4

Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te

91

22

56

Cs Ba La Hf Ta W Re Os Ir

139

39

Y

38

37

Rb Sr

89

21

88

20

Ca Sc Ti

85

19

K

55

40

39

45

12

11

Transition Metals

24

23

Na Mg

27

4

3 5

B

Li Be

11

3

9

2

The Periodic Table of Elements

7

1

H

1

1

ISBN 978-1-84973-112-6

9 781849 731126

Royal Society of Chemistry Education Registered Charity Number: 207890


Thomas Graham House Science Park, Milton Road Cambridge CB4 0WF UK

Tel: +44 (0)1223 432337 Fax: +44 (0)1223 432359 Email: [email protected] www.rsc.org

22/02/2010 11:02:09

Rsc Cfns Manual

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