Practical 1 Finding the formula of Epsom salts REQUIREMENTS Each student will need: Eye protection, Rack and one clean, dry, hard-glass test tube, Solid hydrated magnesium sulphate (Epsom salts) (approx. 2-3 g) Spatula, Bunsen burner, Test tube holder, Access to balance Time required 45 minutes Introduction Epsom salts are hydrated magnesium sulphate. Their formula is sometimes written as MgSO 4 .xH 2 O. In this experiment, you will be trying to find the value of x (i.e. the number of moles of water associated with one mole of MgSO 4 in Epsom salts). Principle When Epsom salts are heated, their water of crystallisation is driven off, leaving anhydrous magnesium sulphate, MgSO 4 . MgSO 4 .xH 2 O(s) MgSO 4 (s) + xH 2 O(g) By weighing the hydrated and anhydrous magnesium sulphate before and after heating, it is possible to find the mass of MgSO 4 and the mass of H 2 O in a sample of Epsom salts. From these masses, you can calculate the number of moles of water associated with one mole of MgSO 4 . You can then write the precise formula for Epsom salts. Procedure Weigh a clean, dry test tube. Put about 1 cm depth of Epsom salts in the test tube and reweigh. Record these weighings and those which follow in a clear table. Now, heat the test tube gently for a minute and then strongly for 5 minutes. Allow the tube to cool and then reweigh it. Heat the tube again for 2 minutes. Allow it to cool and reweigh. If the last two weighings differ by more than 0.05 g, repeat the heating and weighing again. This heating of a substance, like Epsom salts, until its weight does not change any more is called 'heating to constant weight'. 1) What do you understand by the terms: a hydrated, b anhydrous, c water of crystallisation? 2) Describe what happens as the Epsom salts are heated. 3) How could you show that water is given off when Epsom salts are heated? 4) Why must the Epsom salts be heated to constant weight? 5) From your results table, find by subtraction of the appropriate weighings: a. the mass of hydrated MgSO 4 you used, b. the mass of water in this hydrated salt, c. the mass of MgSO 4 in this hydrated salt. 6) Calculate the relative formula mass of: (a) water (H 2 O), (b) magnesium sulphate (MgSO 4 ). (H = 1, O = 16, Mg = 24, S = 32) 7) Rewrite the sentences below, filling in the missing data. From my results: ___ g of H 2 O combine with __ g of anhydrous MgSO 4 in Epsom salts Therefore ___ moles of H 2 O combine with ___ moles of anhydrous MgSO 4 in Epsom salts Therefore ____ moles of H 2 O combine with 1 mole of anhydrous MgS0 4 . Conclusion: 8) Write a precise formula for Epsom salts.
Skill: analysis/interpretation
Criteria for assessment 1) Calculating mass of Epsom salt 2) The water lost = the difference in mass =Initial mass of Epsom salt – final mass 3) moles of water =mass / RMM
=mass/ 18
Marks 1 1
2
4) Final moles of Epsom salt = mass of MgSO 4 /molar mass.
2
5) Ratio=moles of water /moles of MgSO 4
2
answers to Questions: 6) What do you understand by the terms: a hydrated, b anhydrous, c water of crystallization? 7)
How could you show that water is given off when Epsom salts are heated?
8) Why must the Epsom salts be heated to constant weight? 9) Suitable conclusion TOTAL
3 1 1 1 14
Structure, bonding and properties
REQUIREMENTS Each student. or pair of students, will need: Eye protection Polythene rod and fur for charging Burette 3 beakers (100 ml) Rack with 6 test tubes Pair of carbon electrodes·100mA ammeter· 6 V battery or power pack· leads and crocodile dips Iodine (harmful) Calcium chloride (solid) (irritant) Powdered graphite isopropyl alcohol (highly flammable) benzene (toxic) Distilled water Silver nitrate solution (about 0.1 mol dm-3) Introduction Most of the physical and chemical properties of a substance can be related to the type of bonding present in it. In this practical you will study properties such as volatility and conductivity and try to explain these properties in terms of the bonding in the substance involved . Substances that are ionically bonded contain positive and negative ions. Substances that are covalently bonded contain molecules. If these molecules contain different elements, they may be polar due to the unequal sharing of electrons between the different atoms. The degree of polarity in a molecule, determined by the shape of the molecule and the relative e1ectronegativities of the atoms in it, has a significant effect on the properties of the substance. experiment 2: The effect of a charged rod on thin streams of liquid Charge a polythene rod by rubbing it on a piece of fur. Hold the rod about 1 cm from a thin stream of water running from a burette. Note what happens. 1 What sort of particles does water contain? 2 Are these particles polar? 3 Explain the effect of the charged rod on the stream of water. 4 What do you think would happen with a rod of opposite charge? Explain your answer. Repeat the experiment using benzene, and then isopropyl alcohol, instead of water. 5 Relate the behaviour of streams of these liquids towards a charged rod to the nature of the particles they contain. 6 Look up the boiling points and relative molecular masses of water, isopropyl alcohol and benzene. Try to explain the relative magnitudes of their boiling points in terms of polarity. Experiment 3: Miscibility of liquids Test the miscibility of a water and isopropyl alcohol, b water and benzene, c benzene and isopropyl alcohol. 7 Try to explain the results of experiment 2 in terms of the polarities of the molecules in the different liquids. Experiment 4: The solubility of iodine in different liquids Keep the solutions from this experiment and from experiments 4 and 5 for use in experiment 7. Put a very small crystal of iodine in a test tube. Add 5 cm3 of distilled water, put a stopper in the tube and shake. Try to decide roughly how soluble iodine is in water. Repeat the experiment using isopropyl alcohol as the solvent instead of water, and finally using benzene as the solvent. 8 What sort of particles does iodine contain? 9 What forces hold these particles together? 10 Explain the relative solubility of iodine in the three solvents. 11 Why do you think iodine is a different colour in different solvents? Experiment 5: The solubility of graphite in liquids Repeat experiment 3, using powdered graphite instead of iodine. Answer questions 8 to 10 with reference to graphite instead of iodine. Experiment 6: The solubility of calcium chloride In liquids Repeat experiment 3, using calcium chloride instead of iodine. As solutions of calcium chloride are colourless, use the following procedure to decide how much has dissolved. After shaking the crystal with the solvent, decant the 3 liquid into a test-tube, leaving the excess crystal behind. Add 2 cm of silver nitrate solution to the decanted liquid
and shake. Judge the relative solubility of calcium chloride in the three solvents from the amount of silver chloride precipitated. Answer questions 8 to 10 with reference to calcium chloride instead of iodine. Experiment 7: The volatility of iodine, graphite and calcium chloride CARE! Iodine vapor is corrosive and toxic. Use only a very small crystal of iodine and work in a fume cupboard. Heat a crystal of each of the solids in turn in a hard-glass test tube and judge their relative volatilities. 12 Try to explain the relative volatility of these three solids in terms of the particles they contain and the forces between them. Experiment 8: Conductivity Collect each of the solutions produced in experiments 3, 4 and 5. Test the conductivity of each solution by the following procedure. Connect a pair of carbon electrodes in series with a 6 V battery and a100 mA ammeter. Dip the electrodes in the solution under test and note the meter reading in each case. Test the same depth of solution each time. (If the solid showed no sign of dissolving, do not bother to perform the test in that particular case.) You should also test the conductivity of each of the pure solvents. Note: keep the electrodes clean and make sure they are not contaminated with liquid from a previous rest. 13 Try to explain the relative conductivities of the different solutions.
Skill: observation/recording/reporting Experiment Title number 2 The effect of a charged rod on thin streams of liquid 3 Miscibility of liquids
4 5 6
7
8
The solubility of iodine in different liquids The solubility of graphite in liquids The solubility of calcium chloride In liquids
The volatility of iodine, graphite and calcium chloride
observation
marks
Jets of Water and isopropyl alcohol deflected Benzene no deflection Water and isopropyl alcohol are miscible Water and benzene immiscible isopropyl alcohol and benzene immiscible however a layer of emulsion formed distinctive layer still seen
3 3
liquid isopropyl alcohol soluble
benzene soluble
3
insoluble
insoluble
3
soluble
soluble
insoluble
3
iodine
substance graphite
calcium chloride
high
low
low
Water Slightly soluble insoluble
iodine Conductivity
3
substance Calcium chloride
solvent
water benzene
Does not conduct Does not conduct
conducts Does not conduct
2 2
isopropyl alcohol
Does not conduct
conducts
2
TOTAL
24
YEAR 12 PRACTICAL 1 Aim: To determine the molarity of a solution of sodium hydroxide by titration against hydrochloric acid using methyl orange indicator Procedure 1.
Rinse a burette with 0.500 moldm-3 hydrochloric acid.
2.
Using a funnel, fill the burette with the acid until it is just below the zero mark. Check there are no air bubbles in the tip of the burette. If there are, open the tap until the air bubbles disappear and re-fill the burette. Then remove the funnel.
3.
Use a pipette to place 25.0 cm3 of the sodium hydroxide solution into a clean conical flask. When emptying the pipette into the conical flask, allow it to empty under gravity, and then touch the surface of the liquid with the pipette for approximately one second.
4.
Add two drops of methyl orange indicator into the conical flask.
5.
Record the initial volume in the following table: Titration 1 Titration 2 Titration 3 (rough)
Final volume (cm3) Initial volume (cm3) Titre volume (cm3) Concordant?
Titration 4
Titration 5
-
6.
Titrate the acid into the alkali, 1 cm3 at a time until the mixture changes from yellow to pink. Record the final volume in the table and work out the volume delivered (the titre volume).
7.
Repeat steps 3 – 6, but now add the acid rapidly until the volume added is close to the previous titration, and then dropwise until the mixture changes colour. Record the final volume in the table.
8.
Repeat this process until two concordant titre volumes (ie within 0.1 cm3 of each other) have been achieved. If you do not have three concordant values after five attempts, stop anyway.
Analysis 1. 2. 3. 4. 5.
Work out the number of moles of hydrochloric acid used in the titration. Give the equation for the reaction. Hence deduce the number of moles of sodium hydroxide which must have been present in the conical flask. Hence deduce the concentration of the sodium hydroxide solution. The standard errors for the apparatus used in this experiment were balance ± 0.05g 250 cm3 volumetric flask ± 0.5 cm3 25 cm3 pipette ± 0.1 cm3 burette ± 0.15 cm3 Estimate the percentage error in using each piece of apparatus, and the overall apparatus error. Evaluation: 1.
Calculate the a. absolute error b. relative error in the molarity of the sodium hydroxide solution that was determined, given that the accepted value of its molarity is 0.563 mol dm-3 Compare your calculated molarity with the figure supplied. Can you account for any discrepancy?
2.
Comment on your titration technique. Are your titration results reliable?
3.
Suggest improvements to increase the accuracy of the experiment.
TO DETERMINE THE MOLARITY OF A SOLUTION OF SODIUM HYDROXIDE YEAR 12 PRACTICAL 1 Typical titre readings: Rough
1
2
3
4
Initial reading
1.15
1.20
1.00
1.10
2.35
Final reading
30.15
28.70
28.00
27.70
29.05
Titre (cm3)
29.00
27.50
27.00
26.60
26.70
Titres 3 and 4 are concordant. 1)
Average titre = (26.60 + 26.70)/2 = 26.65
Overall equation. NaOH +
HCl
NaCl + H 2 O
Information •
Concentration of HCl = 1.0 mol dm-3
•
Volume of HCl = 26.65 cm3
•
Volume of NaOH = 25 cm3
Calculation where c= concentration and v= volume in dm3.
Moles of HCl = c x v =
0.1× 26.65 1000
=2.665 x 10-3 moles Mole Ratio
NaOH : HCl
=
So moles of NaOH = 2.665 x 10-3 moles Conc. of NaOH
=
n V
2.665 x 10 –3 ×1000 = 25 = 0.107 mol dm-3
Skill: observation/recording/reporting
1
:
1
Skill:_ Analysis and Interpretation Criteria for assessment a) Uses only concordant titres. b) Average titre c) Overall equation. NaOH + HCl NaCl + H 2 O d) Calculation Moles of HCl = c x v where c= concentration and v= volume in dm3. 0.1× 26.65 1000 = =2.665 x 10-3 moles e) Mole Ratio NaOH : HCl = 1 : 1 f) So moles of NaOH = 2.665 x 10-3 moles n V
g) Conc. of NaOH = –3 2.665 x 10 ×1000 = 25 = 0.107 mol dm-3 TOTAL
Skill: Manipulation/Measurement
Marks 1 1 0 3
1 1
3
10
Criteria for assessment 1.Follows instructions 2.Use of burette (a) Using a funnel to transfer the hydrochloric acid to burette without spillage. (b) Rinsing the inside of the burette with a small amount of the acid to minimize concentration error. (c) Makes sure that eyes are level with bottom of meniscus when reading the burette (d) Ensures that the burette is free of air bubbles
Marks 1 1 1 1
3. Use of pipette (a) Uses the forefinger to monitor the flow of liquid in pipette so that bottom of meniscus is at the graduation mark. (b) Allows liquid to flow freely from pipette under gravity.
1
4. Reaction vessel (a) Swirls conical flask to ensure proper mixing of reagents. (b) Takes care in adding acid so as not to exceed the end point. (c) Repeats titration to get agreeable results
1
TOTAL
1
1 1 1
10
Experiment # 10: Analyzing iron tablets Apparatus/materials: Iron tablets (5) sold by chemists as ferrous sulphate tablets, 1.0 mol dm-3 sulphuric acid3(irritant) 3 -3 3 3 (200 cm ), 0.01 mol dm potassium manganate(VII), (75 cm ), conical flasks (250 cm ), Standard flask (250 cm ), Burette and stand, Pipette (25 cm3) and safety filler, Filter funnel, Filter paper, Wash bottle and distilled water . Introduction: Iron is essential to the human body. Its principal role is as a constituent of haemoglobin, the oxygen-carrying agent in the blood. Iron is also present in a number of enzymes and co-enzymes involved in redox processes in the body. Some groups in the population need substantial amounts of iron in their diet in order to produce extra haemoglobin. Such people include growing children, pregnant and menstruating women, and people who have for various reasons lost considerable amounts of blood. A satisfactory intake of iron can normally be ensured by eating a suitable diet, because certain foods - liver, kidney, egg yolk and spinach for example - are rich in iron. Nevertheless, it is sometimes necessary to supplement the iron taken in the natural diet with 'iron tablets'. Iron tablets bought at the chemist usually contain iron(II) sulphate (ferrous sulphate) a cheap, soluble form of iron. In this practical you will attempt to find the actual percentage of iron(II) sulphate in the tablets and then compare this result with the quantity stated on the bottle. Assuming all the iron in the tablets is in the form of Fe2+, it is possible to estimate the iron content by titration against potassium manganate(VII), KMnO 4 .
The equation for the reaction of Fe2+ with 𝑀𝑛𝑂4− in acid solution is: 5Fe2+(aq) + 𝑀𝑛𝑂4− (aq) + 8H+(aq) 5Fe3+(aq) + Mn2+(aq) + 4H 2 0(1) 1. How many moles of Fe react with 1 mole of 𝑀𝑛𝑂4− - ? P
Procedure
Making a solution of the tablets Weigh accurately 5 of the iron tablets, then dissolve them in about 100 cm3 of 1.0 mol dm-3 sulphuric acid in a conical flask. This will probably require heating, but do not heat more than necessary to dissolve the tablets.
2. Why should the tablets not be heated more than necessary? 3. Why are the tablets dissolved in sulphuric acid instead of water?
The outer coating of the tablets will probably not dissolve, so the solution will need filtering. 4. What do you think the outer coating might be? Filter the mixture into a beaker, making sure you do not lose any of the solution, then wash out the conical flask with water and pour the washings through the filter. Finally pour distilled water over the residue and collect these washings as well. Pour the filtrate into a 250 cm3 standard flask, washing out the beaker and adding the washings to the standard flask. Make up to the mark with distilled water. Titration with potassium manganate(VII) Using a safety filler, pipette 25 cm3 of the iron(II) solution from the beaker into a conical flask. Add about 25 cm3 of 1.0 mol dm-3 sulphuric acid and titrate with 0.01 mol dm-3 potassium manganate(VII) solution. Repeat until two consistent results are obtained. 5. 6. 7. 8. 9. 10. 11.
How many moles of 𝑀𝑛𝑂4− were needed to react with 25 cm3 of your Fe2+ solution? How many moles of Fe2+ are there in 25 cm3 of the solution? How many moles of Fe2+ are there in all the tablets?
How many moles of Fe2+ are there in one tablet? What mass of: a Fe, b FeS0 4 , c FeS0 4 .7H 2 0 is there in one tablet? What mass of iron(II) sulphate is stated by the makers to be present in each tablet? Compare your result with the mass stated by the makers, and comment.
[Note: Each iron tablet is stated by the maker as 12 milligrams i.e., 0.012 grams which is 67.5% of the recommended daily allowance (RDA)] {Students will work in 8 groups comprising 3 persons+1 group of 2 persons} Lab report Format: experiment number, date, topic, aim, introduction, Apparatus/materials, procedure, results (use suitable table(s)), Questions, discussion (comment on possible sources of error, precautions taken), conclusion (must be concise and related to the aim). Comment: lab report must be neat with headings underlined, NO PENCIL WRITING!, incorrect grammar will be penalized. Procedure must be written in the past tense. Seek consultation about any misunderstanding rather than submitting rubbish! Avoid rounding off errors. Carry an extra digit or two in intermediate steps in calculations. Leave rounding off for final answers. State final answers using appropriate number of significant figures
Skill: Analysis and Interpretation
Criteria for assessment 1. How many moles of Fe react with 1 mole of MnO− 4 ? ans:1 2. Why should the tablets not be heated more than necessary? To prevent oxidation of Fe2+ 3. Why are the tablets dissolved in sulphuric acid instead of water? To prevent oxidation of Fe2+ 4. What do you think the outer coating might be? 3 2+ 5. How many moles of MnO− 4 were needed to react with 25 cm of your Fe solution? − [# of moles of MnO− 4 used = volume of MnO4 used× concentration] P
Marks 1 1 1 0 3
1000
6. How many moles of Fe2+ are there in 25 cm3 of the solution? [# of moles of Fe2+ used
= 5 x # of moles of MnO 4 - (aq ]
1
7. How many moles of Fe2+ are there in all the tablets? 250 × # of moles of Fe2+ in 25 cm3 25
2
8. How many moles of Fe2+ are there in one tablet? answer to question7 5
9. What mass of: a) Fe, b) FeS0 4 , c) FeS0 4 .7H 2 0 is there in one tablet?
mass = molesx molar mass mass = molesx molar mass mass = molesx molar mass
10. What mass of iron(II) sulphate is stated by the makers to be present in each tablet? 0.033g 11. Compare your result with the mass stated by the makers, and comment. TOTAL
1 1 1
0 1 14
Skill: Manipulation/Measurement
Criteria for assessment
Marks 1
1.Follows instructions 2.Use of burette (a) Using a funnel to transfer the potassium manganate (VII) solution to burette without spillage. (b) Rinsing the inside of the burette with a small amount of the KmnO 4 (aq) to minimize concentration error. (c) Makes sure that eyes are level with bottom of meniscus when reading the burette (d) Ensures that the burette is free of air bubbles
1 1 1 1
3. Use of pipette (a) Uses the forefinger to monitor the flow of liquid in pipette so that bottom of meniscus is at the graduation mark. (b) Allows liquid to flow freely from pipette under gravity.
1 1
4. Reaction vessel (a) Swirls conical flask to ensure proper mixing of reagents. (b) Takes care in adding KmnO 4 (aq) so as not to exceed the end point. (c) Repeats titration to get agreeable results
TOTAL
1 1 1 10
YEAR 12 PRACTICAL 11 Aim: To find the molar enthalpy change for the neutralisation of hydrochloric acid by sodium hydroxide Method: 1. 2. 3. 4. 5. 6.
Place a clean, dry polystyrene cup inside a glass beaker. Pipette 25.0 cm3 of 1.0 moldm-3 hydrochloric acid into the cup. Start the stopclock and record the temperature, to 1 decimal place, of the solution every 30 seconds for 3 minutes. While you are waiting, pour some 1.0 moldm-3 sodium hydroxide solution into a beaker and record its temperature using another thermometer. Using another pipette, prepare 25.0 cm3 of the sodium hydroxide solution and after the timer has been running for 3 minutes run the sodium hydroxide solution into the cup containing the acid. Place a lid on the cup and continue to measure the temperature of the solution every 30 seconds until the stopclock has been running for a total of 10 minutes. Continue to stir gently using the thermometer.
Analysis: 1. 2.
Plot a graph of temperature against time. Extrapolate the part of the graph with a negative gradient to find out what the temperature would have been after 3 minutes. Use this as your final temperature. Take the mean of the temperature of the acid and the alkali just before they were mixed. This is your initial temperature. Calculate the temperature rise using your initial and final temperatures. Taking the density of the solution to be 1.0 gdm-3, the specific heat capacity to be 4.18 JK-1g-1 and assuming the total volume of liquid has not changed, calculate the heat change during the reaction. Write an equation for the reaction and work out the number of moles of NaCl produced. Work out the enthalpy change per mole of salt produced. Include a sign and units in your answer.
3. 4. 5.
6. 7. Evaluation: 1. 2.
3. 4.
The correct answer is –57.6 kJmol-1. Work out the difference between your answer and the correct answer, and then work out your percentage error. Work out the percentage apparatus error using the following information: Thermometer ±0.1 oC Pipette (2 readings) ±0.05 cm3 Comment on your answer to parts 1 and 2. Identify the main source error in the experiment and suggest how accuracy could be improved.
YEAR 12 P11 worked answer Time 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
Temperature (deg C) 23.1 23.1 23.1 23.1 23.1 23.1 29.2 29.1 29.1 28.9 28.8 28.7 28.6 28.6 28.5 28.4 28.3 28.3 28.2 28.2
variation of temperature with time during neutralisation 35
temperature (deg C)
30 25 20 15 10 5 0 0
2
4
6 time (mins)
8
10
12
Skill:_ Observation/recording/reporting Criteria for assessment
Marks
1. Overall organization of work (a) Appropriate subheadings (b) Subheadings in logical order (c) Subject matter under appropriate subheadings
2. Report of method adopted (a) Logical sequence in steps (b) Concise account (c) Grammar and spelling: correct One or two errors Many errors
(f)
1 1 1
1 1 2 (1) (0)
3. Graph: (a) Correct quantities plotted on axes [temperature (y-axis) against time] (b) Title of graph with axes labeled with quantity and unit (1 mark deducted for each omission) (c) Suitable scale for each axis (d) Fine circled points or sharp crosses, thin line (e) Accurate plotting of all readings (all points correct (3), one point incorrect (2), two points incorrect (1), three or more incorrect (0) Line of best fit [as shown worked example] TOTAL
1 2 2 1 3 1 17
Skill: Manipulation/Measurement Criteria for assessment 1.Follows instructions
Marks 1
3. Use of pipette (a) Uses the forefinger to monitor the flow of liquid in pipette so that bottom of meniscus is at the graduation mark. (b) Allows liquid to flow freely from pipette under gravity.
1 1
2. Use of thermometer (a) Positioning – immerses bulb completely in liquid (b) Reading – o Stirs liquid gently (just before taking reading especially) o Takes the reading on the thermometer when the lowest temperature is reached o Reads scale while bulb is immersed o Takes reading at eye level o Reads scale divisions correctly giving correct units
1 1 1 1 1 1
3Work area free from clutter o Disposes correctly of any waste/excess o Washes hands after use of any reagent TOTAL
1 1 11
Skill:_ Analysis and Interpretation Criteria for assessment a) extrapolate 0 – 3 mins to 3.5 mins gives 23.1 oC initial temp of NaOH = 23.3 oC so mean temp = 23.2 oC
Marks
2
b) extrapolate 4 – 10 mins to 3.5 mins gives 29.3 oC so temperature change = 6.1 oC
2
c) q = (25 + 25) x 4.18 x 6.1 = 1275 J = 1.275 kJ evolved d) moles of HCl (or NaOH) = 25/1000 x 1 = 0.025 moles
2 1
e) so ∆H = 1.275/0.025 = -51 kJmol-1
1
f) error = 57.6 – 51 = 6.6 o % error = 6.6/57.6 = 11.5 %
2
o apparatus error = 0.1/6.1 x 100 = 1.64 % (thermometer) = (0.05/25) x 100 = 0.20 % 0.20 % x2 = 0.40 % (2 pipettes) total 2.04 % error
2
o so error is greater than apparatus error
1
g) the main sources of error are - heat loss to the surroundings, particularly through the top of the cup - the heat absorbed by the polystyrene cup
2
h) -
the accuracy could be improved by: putting a lid on the cup better lagging of the polystyrene cup using a more accurate thermometer using more concentrated solutions to get a bigger temperature rise TOTAL
2 17
A SIMPLE KINETICS PLAN Hydrogen peroxide decomposes in the presence of metal oxide catalysts to form oxygen and water according to the following equation. 2H2O2 → 2H2O + O2 in the presence of the catalyst manganese (IV) oxide. Plan and design an experiment to investigate how the rate of decomposition of hydrogen peroxide is affected by temperature. (a)
Determine a suitable number of moles of hydrogen peroxide to be used in the experiment. (5)
(b)
Suggest a suitable volume and concentration of hydrogen peroxide for the experiment. (3)
(c)
Outline the procedure in full, giving details of any apparatus used. (10)
(d)
Indicate any hazards or safety precautions specific to the experiment. (2) (Total 20 marks)
KINETICS PLAN MS (a) n = pV/RT = (1 x 105 x 100 x 10-6)/(8.31 x 293) = 4.1 x 10-3
(3)
no moles of H2O2 used = 8.2 x 10-3
(1)
so vol of H2O2 used = n/C = 8.2 x 10-3/1 = 8.2 x 10-3 dm3 = 8.2 cm3
(1) 5
(b)
must use less moles than 8.2 x 10-3
(1)
but bigger volume eg 50 cm3 preferred
(1)
so suitable concentration = 0.1 moldm-3 (eg)
(1)
(not more than 82 cm3 of 0. 10M peroxide) 3 (c)
Collect oxygen into syringe
(1)
Known mass of catalyst into a conical flask thistle funnel and delivery tubing add peroxide solution (amount found above) start timing measure volume of gas collected in 2 minutes subtract volume of peroxide to find volume of oxygen formed repeat at different temperatures all other conditions identical
(1) (1) (1) (1) (1) (1) (1) (1) (1) max 10
(d)
hydrogen peroxide harmful or catalyst irritant
(1)
wear gloves
(1) 2
[20]
1
Experiment 13 Data Analysis Lab Aim : To determine the order of a reaction and the rate constant in a single species reaction For the experimental data Time (mins )
Concentration of reactant (M)
0
0.819
2
0.619
4
0.468
6
0.353
8
0.267
10
0.202
15
0.100
20
0.050 i)
Plot a graph of the data
4 mks
ii)
Determine the order of the reaction
2 mks
iii)
Calculate the rate constant , k
2 mks
iv)
Using k find the time for the concentration to drop to a quarter (¼ ) of the original concentration
2 mks
Data Analysis Lab Skill: Analysis/Interpretation Plot a graph of the data
4 mks
Determine the order of the reaction
2 mks
Calculate the rate constant , k
2 mks
Using k find the time for the concentration to drop to a quarter (¼ ) of the original concentration 2 mks Total = 10 marks
A practical study of the third period REQUIREMENTS Eye protection, Samples of elements from period 3. Because of the hazardous nature of sodium (highly flammable), phosphorus (toxic) and chlorine (toxic), it is best for the teacher to demonstrate these elements. It is important to avoid confusion between phosphorus (stored under water) and sodium (stored under oil).Ideally, phosphorus and sodium should not be in the laboratory at the same time. Simple apparatus to test electrical conductivity 1 rack + 4 test tubes Full-range indicator paper Samples of the following oxides: Na 2 O (or NaOH) (corrosive), MgO, AI 2 O 3 , SiO 2 , P 2 O 5 (corrosive) Access to SO 2 cylinder, Dilute hydrochloric acid (irritant) Dilute sodium hydroxide solution (corrosive) Samples of the following chlorides: NaCI, MgCI2, AICI3, PCI3, SCI2 (or S2CI2) AICI3, PCI3, SCI2 and S2CI2 are all corrosive Bunsen burner
Introduction The periodic table provides a very important unifying pattern to the study of chemistry. The intention of this practical is to study some of the trends in properties across the third period of the periodic table. A Patterns in the properties of elements The properties of the elements show a repeating pattern with increasing atomic number. The most obvious pattern is the steady change from metals on the left to non-metals on the right of each period. Procedure Examine samples of the elements sodium to argon in period 3. Test the electrical conductivity of those elements for which the results are not given in table 1. 1.
Copy and complete table 1. You will have to consult a data book, or Section 4.7 in Chemistry in Context, Fifth Edition, for boiling point values. Na
Mg
Al
Si
P
S
Cl
Ar
poor
poor
Atomic number Physical state and appearance Boiling point/°C Conductivity at room temperature
good
poor
Structure (giant metallic, giant molecular or simple molecular) Type of element (metal, non-metal or metalloid) Table 1 The elements of period 3
2. How do the boiling points of elements vary across the period from sodium to argon? How are the boiling points related to the structure of the elements? 3. Describe the changes in: a) the type of elements across period 3, b) the structure of elements across period 3.
B Patterns in the properties of oxides You should know quite a lot about the properties of these compounds already.
4. Copy out table 2, and complete the first three rows. Oxide formula
Na 2 O MgO Al203
SiO2
P 4 0 10 S0 2
C12O (g)
State at room temp. Appearance
yellow-red gas
Volatility Conductivity of molten oxide Solubility in water pH of solution in water Classification of oxide (acidic, basic or amphoteric) Structure of oxide (simple molecular, giant molecular, giant ionic) Table 2 Properties of the oxides in period 3
good
good
good
poor
poor
poor
high poor dissolves readily 2 acidic simple molecular
Test the solubility of each oxide in water. Add a very small measure of each solid to about 3 cm3 of distilled water and shake thoroughly. (In the case of sulphur dioxide, bubble the gas through 3 cm3 of water in a fume cupboard for a few seconds.) If Na 2 O is not available, use sodium hydroxide (NaOH) (CARE Corrosive. Wear protective gloves.) Some oxides will dissolve easily. Others will not dissolve at all. Test the pH of the solution obtained from each oxide using full-range indicator paper. Record your results in table 2.
5. Which of the oxides of period 3 elements: a) form acidic solutions with water, b) form alkaline solutions with water c) are insoluble in water? 6. Write equations for the reactions of the soluble oxides with water. If an oxide is insoluble in water, its reactions with acids and alkalis can be used to decide its acid-base character. If an oxide dissolves in acid, it 'must have reacted with it. The oxide can therefore be classified as basic. If the oxide dissolves in alkali, it can be described as acidic. If the insoluble oxide dissolves in both acids and alkalis, it is both basic and acidic. The adjective 'amphoteric' (from a Greek word meaning 'both') is used to describe these oxides. Test the solubility of the insoluble oxides in dilute hydrochloric acid and then in dilute sodium hydroxide solution. Remember to use small measures of solid. 7. Write equations for any reactions of the insoluble oxides: a) with hydrochloric acid, b) with sodium hydroxide solution. 8. Complete the last two lines in table 2. 9. Describe the following patterns in the properties of the oxides across period 3: a) their state, b) their character (acidic/amphoteric/basic), c) their structure. C Patterns In the properties of chlorides Examine samples of the chlorides of period 3 elements. 10. Copy out table 3 and complete the first two rows. (You will not be expected to test silicon tetrachloride yourself. The results for this are already inserted.) Element
Na
Mg
Al
Si
Formula of chloride
SiCl4
Appearance and state of chloride
colourless liquid
Volatility of chloride
high
Action of water on chloride
very vigorous reaction, HC1 fumes evolved
pH of solution of chloride in water
3
Structure of chloride
simple molecular
Working in a fume cupboard, warm separate small samples of the chlorides gently with a bunsen. Investigate the relative volatility of the chlorides and put your results in table 3. Investigate the effect of water on each chloride by adding a small quantity to about 3 cm3 of distilled water in a test tube. Test the pH of the mixture obtained using full-range indicator paper, then complete table 3. 11. Explain, with equations the action of sodium chloride, aluminium chloride and phosphorus chloride with water. 12. Describe the patterns across period 3 in: a) the formula of the chlorides, b) the state of the chlorides. c) the pH of the aqueous chloride solutions, d) the structure of the chlorides
P
S
Skill:_ Observation/recording/reporting Na
Mg
Al
Si
P
S
Cl
Ar
Atomic number
11
12
13
14
15
16
17
18
Physical state and appearance
Silvery solid 883
Silvery solid 1090
Silvery solid 2467
grey solid Red or Yellow white solid solid 2680 280 445
Yellow green gas -35
Colourless gas -186
Conductivity at room temperature good
good
good
fair
poor
poor
Structure (giant metallic, giant molecular or simple molecular)
giant metallic
giant giant giant simple simple simple simple metallic metallic metallic molecular molecular molecular molecular
Type of element (metal, non-metal or metalloid)
metal
metal
Boiling point/°C
Oxide formula
metal
poor
poor
metalloid non-metal nonmetal
Na 2 O MgO Al2O3
SiO2
P 4 O 10 SO 2
C12O
(s)
(g)
(g)
Colourless gas
yellow-red gas
high
high
poor
poor
insoluble soluble
soluble
dissolves readily
7
<7
2
2
acidic
acidic
acidic
(s)
(s)
(s)
(s)
State at room temp. Appearance
white
white
white
Colourle ss
Volatility
low
low
low
low
good
good
poor
good
Conductivity of molten oxide
non-metal nonmetal
Solubility in water
soluble
insoluble insoluble
pH of solution in water
13
7
7
basic Classification of oxide (acidic, basic or basic amphoteric) giant ionic giant Structure of oxide (simple molecular, ionic giant molecular, giant ionic)
Na
Element
high poor
amphoteri acidic c giant ionic
Mg
simple simple giant molecula molecular molecular r
Al
Si
simple molecular
P
S
PCl 3 PCl 5
SCl 6
high
Formula of chloride
NaCl
MgCl 2
AlCl 3 , Al 2 Cl 6
SiCl4
Appearance and state of chloride
White solid
White solid
liquid
colourless liquid
Volatility of chloride
low
low
high
high
high
Action of water on chloride
dissolves dissolve s
very vigorous reaction, HC1 fumes evolved
very vigorous reaction HC1
pH of solution of chloride in water
7
<7
3
<7
Structure of chloride
giant ionic giant ionic
simple molecular
simple molecular
simple simple molecular molecular
7
Criteria for assessment 1.Overall organization of work (a) Appropriate subheadings (b) Subheadings in logical order (c) Subject matter under appropriate subheadings 2. Tables (a) Neat tables (b) Headings for tables labeled (c) Answers in tables o All cells correct o One or two incorrect o 3 to 5 incorrect o 6 to 10 incorrect o 11 to 15 incorrect o 16 incorrect o >16 incorrect
3. Report of method adopted (a) Logical sequence in steps (b) Concise account (c) Grammar and spelling: correct One or two errors Many errors TOTAL
<7
Marks 1 1 1
1 1 10 (9) (8) (7) (6 to 2) (1) (0)
1 1 2 (1) (0) 19
Data Analysis lab To determine the relative molecular mass, Mr, of a volatile liquid Skills assessed Analysing (6 marks) and Interpretation (6 marks) Topic: Kinetic Theory Introduction Compound X is a volatile liquid. In order to determine the relative molecular mass, Mr, of X, a student carried out six experiments. In each experiment a measured mass of X was injected into an empty 100cm3 gas syringe maintained at a pressure of 100 kPa and a temperature of 373 K. In each of the six experiments the volume of gas produced was measured. The student’s results are shown in the table below. The apparatus used is shown in the diagram below.
Volume/cm3 Mass/g 19.0 0.04 33.5 0.07 38.0 0.08 46.0 0.09 62.0 0.13 76.5 0.16 Analysis Full marks can only be scored if you show all your working. 1. Plot a suitable graph to represent the data. 2. Draw a best fit straight line on the graph. 3. Use the graph to determine the volume of gas which would have been produced by 0.10 g of X. 4. State the ideal gas equation. 5. Use your answers to part (3) and part (4) to calculate the M r of X (R=8.31JK–1 mol–1). 6. Assume the maximum errors for the apparatus used in this experiment were: balance ± 0.01 g gas syringe ± 1cm3 Estimate the maximum percentage error in using each piece of apparatus, and hence calculate the overall apparatus error. Use a mass of 0.10 g and the volume from part (3) to calculate these errors. Evaluation Full marks can only be scored if you show all your working. 1. Consider the graph and comment on the accuracy of the results obtained by the student. Is your line of best fit good enough for you to use with confidence? Identify any anomalous results. 2. The correct value for the relative molecular mass of X is 72.0. Calculate the difference between the student’s value and this correct value. Express this difference as a percentage of the correct value. (If you could not complete the calculation in part (5) of the Analysis section, you should assume that the student’s value is 78.5. This is not the correct value.) Difference .................................................................................................................................. Percentage ................................................................................................................................. 3. Apart from loss of liquid during transfer to the gas syringe, identify one other source of error in this experiment. Suggest one improvement to minimise this other source of error. Source of error ........................................................................................................................... Improvement ............................................................................................................................... .................................................................................................................................................... 4. Would a loss of liquid during transfer to the gas syringe result in a lower value for the M r of liquid X? Explain your answer.
Skill assessed Analysing (6) Q3 volume for 0.10g is 47.0 - 48.0 cm3 1 mark Notes • Do not allow other answers Q4 PV = nRT 1 mark Q5 100 × 103 × 47.5 × 10-6 = 0.1 × 8.31 × 373 Mr Mr = 66.0 - 64.5 1 mark Notes • Consequential marking from answer to Q3 • Do not award this mark if candidate gets the correct answer by an incorrect method; don.t penalise again in awarding the nomenclature mark Q6 errors balance 0.01/0.1 × 100 = 10% 3 scoring points syringe 1/ 47-48 × 100 = 2.1% any 2 = 1 mark total error 12.1% Notes • Ignore precision of answers • Consequential marking for volume from Q3 and for overall error • Penalise doubled errors once • Lose mark if answers wrong because (x 100) missing from calculations; don’t penalise again in awarding the nomenclature mark • Which error being calculated is not stated; allow if the calculations are in the same order as in the question (balance, syringe). And do not penalise in nomenclature (a) The appreciation of precision quotes volume as integer or to 1 dp 2 scoring points (b) quotes Mr to 1 dp both = 1 mark Notes • If no answer to part 4 can.t score this mark (b) the correct use of nomenclature and terminology 3 scoring points second graph has sharp trace all 3 = 1 mark explains the calculation of the Mr clearly and logically explains the calculation of the errors clearly explains the calculation of the Mr clearly and logically explains the calculation of the errors clearly Notes • Incorrect units mean the nomenclature mark is lost • Don’t penalise missing units • Two blank sections mean the nomenclature mark is lost • Answer given in part 4 or 5 without working means the nomenclature mark is lost • Do not penalise for wrong calculation in Part 4 if explained clearly Total 6 marks Exercise 2 Skill assessed Evaluating(6) Q1. ignores result at 0.09g when plotting graph 2 scoring points line goes through the origin ( ± half a square ) both = 1 mark lines good/ can extrapolate with confidence/ technique good/ 1 mark results consistent or reliable Notes • Allow first point in written answer to Q1 or clearly from the graph; any contradictions loses mark • Must make a clear written comment for final point Q2. difference is 6.0 - 7.4 2 scoring points against 72.0 is a 8.3 to 10.3% error both = 1 mark Notes • Lose mark if no evidence of working in second part • Ignore precision of answers • Allow consequential answer from part 4 of Analysis • Difference must be clearly stated • Lose mark if the candidate answers a different question • Using 78.5 gives difference is 6.5, and a 9.0% error Q3. need more accurate mass one error and appropriate o use a 3 fig balance etc or a balance with more precision improvement = 1 mark o maintain constant temperature better o prevent heat loss from oven by better insulation/use a steam jacket o incomplete vaporisation o use a higher temperature or allow longer time (to equilibrate) or use longer needle o compound associates/ dissociates o use a higher/ lower temperature o syringe sticks o rotate syringe Notes • Do not allow more accurate balance
more accurate thermometer collect more gas repeat experiment Q4 volume lower than expected (as some liquid lost) 1 mark calculated Mr would be larger 1 mark Notes • Mark points independently • Accept less gas produced Total 6 marks
Observing the reactions of cations Testing with sodium hydroxide solution Add a few drops of sodium hydroxide solution to 3 cm3 of a solution containing Al3+(aq). What happens? Write an equation for the reaction. Does the precipitate remain or dissolve when excess NaOH(aq) is added? Testing with aqueous ammonia Repeat test1 using aqueous ammonia Testing with sodium carbonate solution Repeat test1 using aqueous sodium carbonate solution Repeat the experiment with solutions containing the following cations in place of Al3+: Ba2+, Ca2+, Cu2+, Fe2+, Fe3+, K+, Mg2+, Mn2+, Na+, NH4+, Pb2+, Zn2+. Record your results in a table similar to table 2. Cation Few drops of NaOH (aq) NaOH(aq) added Few drops of Excess tested added NH 3(aq) added NH 3(aq) added Al3+:
White ppt Al3+(aq) + 3OH- (aq) Al(OH) 3(s)
white ppt forms
White precipitate dissolves forming a clear solution Al(OH) 3(s) + OHAl(OH)- 4(aq) (aq)
Al(OH) 3(s) + OH- (aq) Al(OH)- 4(aq)
In excess, insoluble
Few drops of Na 2 CO 3 (aq) added White ppt 2Al3+(aq) + 3CO 3 2- (aq) Al 2 (CO 3 ) 3(s)
Ba2+,
White ppt Ba2+ (aq) + 2OH- (aq) Ba(OH) 2(s)
soluble
no ppt
Ca2+
White ppt Ca2+ (aq) + 2OH- (aq) Ca(OH) 2(s)
insoluble
milky suspension forms
In excess, insoluble
White ppt Ca2+ (aq) + CO 3 2- (aq) CaCO 3(s)
Blue ppt Cu2+ (aq) + 2OH- (aq) Cu(OH) 2(s)
In excess, insoluble
blue ppt forms
In excess, dissolves and a dark blue solution forms
Blue gelatinous ppt Cu2+ (aq) + CO 3 2- (aq) CuCO 3(s)
Cu2+
Ca2+ (aq) + 2OH- (aq) Ca(OH) 2(s) 2+ (
Cu aq) + 2OH- (aq) Cu(OH) 2(s)
Fe2+
green ppt Fe2+ (aq) + 2OH- (aq) Fe(OH) 2(s)
insoluble
Fe3+
Red brown ppt Fe3+(aq) + 3OH- (aq) Fe(OH) 3(s)
insoluble
green ppt forms
Fe2+ (aq) + 2OH- (aq) Fe(OH) 2(s)
A little, brown ppt forms
Fe3+(aq) + 3OH-
-
Cu(OH) 2(s) + 4NH 3 [Cu(NH 3 ) 4 ] (OH) 2
White ppt Ba2+ (aq) + CO 3 2- (aq) BaCO 3(s)
In excess, insoluble
Green gelatinous ppt Fe2+ (aq) + CO 3 2- (aq) FeCO 3(s)
In excess, insoluble
Brown ppt 2Fe3+(aq) + 3CO 3 2- (aq)
(aq)
Fe(OH) 3(s) Mg2+
White ppt Mg2+ (aq) + 2OH- (aq) Mg(OH) 2(s)
NH4+ no ppt
insoluble
milky suspension forms
Mg2+ (aq) + 2OH- (aq) Mg(OH) 2(s) -
no ppt
Fe 2 (CO 3 ) 3(s) In excess, insoluble
-
White ppt Mg2+ (aq) + CO 3 2- (aq) MgCO 3(s) No ppt
Pb2+
white ppt forms 2+ (
Pb
Zn2+
-
aq) + 2OH (aq) Pb(OH) 2(s)
White ppt Zn2+ (aq) + 2OH- (aq) Zn(OH) 2(s)
In excess, dissolves giving colourless solution
white ppt forms
In excess, insoluble
White ppt Pb2+ (aq) + CO 3 2- (aq) PbCO 3(s)
soluble
white ppt forms
In excess, dissolves giving colourless solution
White ppt Zn2+ (aq) + CO 3 2- (aq) ZnCO 3(s)
dissolves giving colourless solution
2+ (
Zn aq) + 2OH- (aq) Zn(OH) 2(s)
Zn(OH) 2(s) + 4NH 3 [Zn(NH 3 ) 4 ] (OH) 2 1 point for each correct observation 1 point for each correct equation Total 78 points divide no. of points obtained by 78 then multiply by 10
Skill:_ Observation/recording/reporting Subtract 1 mark for each omission. Anion Test Carbonate Add (dil)HCl(aq). Pass gas through lime pH>10 water, CO 3 2 –
universal indicator Hydrogen carbonate HCO 3 – pH 8-9 Chloride Cl– Bromide Br–
–
I Nitrate
2–
gas evolved turning lime water milky
Acidify with (dil)HNO 3 (aq). Add AgNO 3 (aq). Add dil NH 3 to ppt
White ppt AgCl forms. Ppt dissolves leaving colourless solution Ag+ (aq) + Cl- (aq) AgCl (s) AgCl (aq) + 2NH 3(aq) [Ag(NH 3 ) 2 ]+ (aq) + Cl- (aq) Cream ppt AgBr forms Ppt dissolves leaving colourless solution
– 𝐻𝐶𝑂3(𝑎𝑞)
+
H (aq) + Ca(OH) 2(s) + CO 2
Ag+ (aq) + Br- (aq) AgBr (s) AgBr (aq) + 2NH 3(aq) [Ag(NH 3 ) 2 ]+ (aq) + Br- (aq) Acidify with (dil)HNO 3 (aq) Add Yellow ppt AgI forms Ppt insoluble AgNO 3 (aq) Add (conc)NH 3 to ppt Ag+ (aq) + I- (aq) AgI (s)
Sulphite SO 3 2–
Add Barium nitrate Ba(NO 3 ) 2 (aq)/chloride BaCl 2 (aq) Add HCl(aq) Add Barium nitrate Ba(NO 3 ) 2 (aq)/chloride BaCl 2 (aq) Add HCl(aq)
White ppt. Insoluble
Or add (dil)HCl(aq)
SO 2 evolved turning potassium dichromate(VI) solution/paper from orange green
Heat
2+
Ba
(aq)
+ SO 4
2-
(aq)
2
2
2 2 Even without equation 2
BaSO 4
(s)
White ppt. Dissolves 2+
2
CO 2 (g) + H 2 O (l) (g) CaCO 3 (s) + H 2 O (l)
Add NaOH(aq) Add Devarda’s alloy NH 3 evolved, litmus paper red blue NO 3 – (powdered Zn, Al) Heat & hold moist red litmus at mouth of test tube
Sulphate SO 4 2–
marks 2
2H+ (aq) + 𝐶𝑂3(𝑎𝑞) CO 2 (g) + H 2 O (l) Ca(OH) 2(s) + CO 2 (g) CaCO 3 (s) + H 2 O (l)
Add (dil)HCl(aq). Pass gas through lime water,
Acidify with (dil)HNO 3 (aq) Add AgNO 3 (aq) Add (conc)NH 3 to ppt
Iodide
observation gas evolved turning lime water milky
2-
Ba (aq) + SO 3 (aq) BaSO 3 (s) BaSO 3 (s) + 2H+ (aq) Ba2+ (aq) + H 2 SO 3(aq)
TOTAL
2 Even without 2nd equation 2 18
