Edexcel A2 Chemistry Paper 6

Answer ALL the questions. Write your answers in the spaces provided. 1

The table shows a series of tests carried out on a soluble crystalline compound A, which contains one anion and one cation. For each test, complete the table by filling in the inference column. Test (a)

Observation

Observe the appearance of A.

Inference

Pale green solid. ...................................................................................... ..................

(b)

Measure the pH of a dilute aqueous solution of A using a pH meter.

The pH is 6.0.

The type of reaction that has occurred when A dissolved in water is ...................................................................................... ..................

(c)

Add a few drops of dilute sodium hydroxide solution to a solution of A.

A green precipitate forms.

Leave a sample of the green precipitate formed in (c) to stand in air.

(1)

The sodium hydroxide is acting as ...................................................................................... ..................

The formula of the green precipitate is ...................................................................................... ..................

(d)

(1)

The green precipitate turns brown on the surface.

(2)

The type of reaction that has occurred is ...................................................................................... ..................

The formula of the brown precipitate is ...................................................................................... ..................

(e)

(f )

Add excess sodium The green hydroxide solution to precipitate a sample of the green does not precipitate formed in (c). dissolve. Add barium chloride solution, BaCl2(aq), acidified with hydrochloric acid, to a solution of A.

A white precipitate forms.

(2)

...................................................................................... .................

(1) The white precipitate is ...................................................................................... ..................

(1)

(g) Identify compound A by name or formula. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(Total for Question 1 = 9 marks)

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2

Two organic compounds, X and Y, are colourless liquids. Both compounds contain four carbon atoms and one functional group. (a) A series of tests was carried out on compound X. (i) When a few drops of 2,4-dinitrophenylhydrazine solution were added to X, an orange precipitate was formed. What deduction can be made from the result of this test alone? (1)

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(ii) When X was warmed with Fehling’s solution, a red precipitate was formed. What further deduction can be made from the result of this test? (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(b) Give the two possible displayed formulae of X. (2)

(c) A series of tests was carried out on compound Y. (i) A dry sample of Y reacted with phosphorus(V) chloride, producing steamy fumes. What deduction can be made from the result of this test alone? (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(ii) No reaction was observed when Y was added to sodium carbonate solution, Na2CO3(aq). What further deduction can be made from the result of this test?

(1)

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(iii) A sample of Y rotated the plane of plane-polarized light. What deduction can be made about the structure of Y from the result of this test? (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(iv) Use your answers to parts (i), (ii) and (iii), and the fact that each molecule of Y contains four carbon atoms, to deduce the displayed formula of Y. (1)

(v) Describe what you would expect to see if a sample of compound Y was added to iodine, I2, in alkaline conditions.

(1)

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3

The compound 2-ethanoylaminobenzoic acid can be made by reacting 2-aminobenzoic acid with ethanoic anhydride. O

O

C OH

+ (CH3CO)2O

C

heat under reflux

OH

NH2 2-aminobenzoic acid

+ CH3COOH

NHCOCH3 ethanoic anhydride

2-ethanoylaminobenzoic acid

The steps of the experimental procedure are as follows: 1. Measure out 4.00 g of 2-aminobenzoic acid into a pear-shaped flask. Add ethanoic anhydride. 2. Add anti-bumping granules to the flask, fit a reflux condenser and bring the mixture slowly to the boil. Heat under reflux for 15 minutes. 3. Allow the reaction mixture to cool and add 5 cm3 of water. Bring the contents of the flask back to the boil and then remove from the heat. 4. Let the reaction mixture cool to room temperature. A pale brown crystalline solid will form. 5. Collect the solid by suction filtration. 6. Purify the solid by recrystallization using ethanoic acid as the solvent. 7. Determine the melting temperature of the dry solid. (a) (i) Calculate the minimum volume, in cm3, of ethanoic anhydride needed for 4.00 g of 2-aminobenzoic acid to react completely. [Molar masses / g mol–1: (CH3CO)2O = 102; C6H4(NH2)COOH = 137 Density (CH3CO)2O = 1.082 g cm−3.]

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(3)

(ii) A student obtained 2.97 g of 2-ethanoylaminobenzoic acid from 4.00 g of 2-aminobenzoic acid. Calculate the percentage yield obtained by this student. Give your answer to two significant figures. (3)

(b) (i) When this experiment is carried out, the actual volume of ethanoic anhydride used is greater than that calculated in (a). Suggest why this is so. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(ii) Anti-bumping granules are added in step 2. What would be observed if ‘bumping’ occurred? (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(iii) Ethanoic anhydride is corrosive to both the skin and the respiratory system. Suggest two precautions to minimise the risks when using ethanoic anhydride, other than wearing eye protection and a lab coat. (2) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(iv) Outline how you would carry out the recrystallization in step 6. (4) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(v) Suggest a reason why the recrystallization will slightly reduce the yield of 2-ethanoylaminobenzoic acid. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(vi) Draw a labelled diagram of the apparatus that could be used to find the melting temperature of the dry solid in step 7. (2)

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(vii) State two ways you would use the results from (vi) to check the identity and purity of the product. (2) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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4

The concentration of a solution of sodium dichromate(VI), Na2Cr2O7, can be found by titration with a solution containing Fe2+(aq) ions in acidic conditions. A 20.0 cm3 sample of a solution of sodium dichromate(VI), of unknown concentration, was titrated with a solution of Fe2+(aq) ions, of concentration 0.0500 mol dm −3. An indicator, diphenylamine, was used. This turned an intense violet colour at the end point. The titration was repeated several times and some of the results are shown in the table below. Titration number

1 (trial)

2

3

Burette reading (final) / cm3

21.45

41.35

21.95

Burette reading (initial) / cm3

1.20

21.45

Volume of Fe2+(aq) used / cm3

4

21.95 20.00

19.80

Titre used to calculate mean () (a) Explain why a trial titration (titration 1) is carried out. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(b) (i) Complete the table and indicate with a tick () those titres most suitable for calculating a mean titre. Use the titres you have chosen to calculate the mean titre. (4)

Mean titre =

*P41217A01116*

cm3

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(ii) Use the equation below, and your mean titre, to calculate the concentration of the sodium dichromate(VI) solution, in mol dm–3. (3) Cr2O72−(aq) + 6Fe2+(aq) + 14H+(aq)  2Cr3+(aq) + 6Fe3+(aq) + 7H2O(l) orange

green

(c) Assuming the accuracy of the burette is ± 0.05 cm3 each time the burette is read, calculate the % error of the titre in titration 3. (1)

(d) Suggest one reason why the indicator diphenylamine is needed, even though the solution in the titration flask changes colour from orange to green when no indicator is used. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(e) A student carrying out one titration left an air bubble in the tip of the burette before taking the initial reading. This bubble was no longer present when the student took the final reading. State and explain what effect, if any, this would have on the titre value. What effect would the use of this titre have on the calculated concentration of sodium dichromate(VI)? (3) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(Total for Question 4 = 13 marks) TOTAL FOR PAPER = 50 MARKS

*P41217A01316*

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Write your name here Surname

Other names

Centre Number

Candidate Number

Edexcel GCE

Chemistry Advanced Unit 6B: Chemistry Laboratory Skills II Alternative Wednesday 15 May 2013 – Morning Time: 1 hour 15 minutes Candidates may use a calculator.

Paper Reference

6CH08/01 Total Marks

Instructions    

Use black ink or ball-point pen. Fill in the boxes at the top of this page with your name, centre number and candidate number. Answer all questions. Answer the questions in the spaces provided – there may be more space than you need .

Information   



The total mark for this paper is 50. The marks for each question are shown in brackets – use this as a guide as to how much time to spend on each question. You will be assessed on your ability to organise and present information, ideas, descriptions and arguments clearly and logically, including your use of grammar, punctuation and spelling. A Periodic Table is printed on the back cover of this paper.

Advice    

P41653A ©2013 Pearson Education Ltd.

10/5/6/7/7/

Read each question carefully before you start to answer it. Keep an eye on the time. Try to answer every question. Check your answers if you have time at the end.

*P41653A0116*

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(a) Compound Z is a crystalline solid that contains a nickel cation and one type of anion. Complete the table below. Test (i)

Observation

Add dilute sulfuric acid to compound Z

Inference

Bubbles of a colourless gas are released. The gas turns limewater milky

Name of gas released is ........................................................................

Formula of anion in Z is and ........................................................................

a .................................................................... coloured solution is formed (ii)

Add concentrated hydrochloric acid to the solution containing [Ni(H2O)6]2+ ions

(iii) Add a few drops of dilute aqueous ammonia to the solution containing [Ni(H2O)6]2+ ions (iv) Add excess dilute aqueous ammonia to the solution containing [Ni(H2O)6]2+ ions until no further change is observed

2

(2) (1)

Yellow-brown solution forms

Formula of the complex ion formed is [Ni(H2O)6]2+(aq) Formula of the complex ion formed is ........................................................................

(1) Green precipitate forms

Formula of the precipitate formed is ........................................................................

(1) Formula of the complex ion formed is [Ni(NH3)6]2+

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........................................................................

(1)

*P41653A0216*

(b) A 10.0 cm3 sample of a solution containing [Ni(H2O)6]2+ ions was titrated with a solution of concentration 0.010 mol dm –3 with respect to the ligand EDTA4− ions. The equation for the reaction is [Ni(H2O)6]2+ + EDTA4−  [Ni(EDTA)]2− + 6H2O (i) The mean titre of the solution containing EDTA4− ions was 24.20 cm3. Use this information, and the equation above, to calculate the concentration in mol dm−3 of the solution containing [Ni(H2O)6]2+ ions.

(2)

(ii) Assuming the total error in the measurement of the mean titre is ±0.10 cm3, calculate the percentage error in this titre. (1)

(iii) A similar solution, containing the same concentration of [Ni(H2O)6]2+ ions, also contained a small amount of an impurity, copper(II) sulfate. Suggest what effect this impurity would have on the titre. Justify your answer. (2) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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2

A colourless liquid, compound X, was extracted from raspberries. X has the molecular formula C10H12O2 and contains a benzene ring. (a) What would you expect to see if a sample of compound X was burned in air? (1)

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(b) A series of tests was carried out on compound X. In each test, state what you can deduce about the structure of compound X from the results described.

(i) X forms a white precipitate with aqueous bromine solution. (1)

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(ii) X forms an orange precipitate with 2,4-dinitrophenylhydrazine. (1)

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(iii) Fehling’s (or Benedict’s) solution remains blue when warmed with compound X. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(c) The high resolution proton nmr spectrum of compound X is shown below below.. This spectrum shows that there are six different proton environments in the molecule of X. The relative number of hydrogen hydrogen atoms in each environment is indicated on the spectrum. Use this spectrum, the data below and your answers answers to (a) and (b) to help answer the questions that follow.

3H

1H 2H 2H 2H

7

2H

6

5

4

3

2

1

0

chemical shift / ppm

Nuclear Magnetic Resonance

H―C―C=C alkenes arenes

H―C―C=O aldehyde ketone ester amide acid

Ar―O―H phenol

TMS H―C―O− alcohol ether ester ―O―H alcohol

Ar―H arene ring

8.0

6

H―C=C alkene

7.0

6.0

Chemical shifts for hydrogen relative to TMS (tetramethylsilane)

H―C―C alkane R 3 CH>R 2CH 2> RCH3

5.0

4.0

3.0

2.0

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1.0

0.0

δ /ppm for TMS

(i) Which hydrogen atoms in compound X are most likely to have caused the peaks at 6.5 ppm and 7.2 ppm? (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(ii) Compound X has a side chain containing four carbon atoms attached to the benzene ring. Show all the atoms on this side chain and label label each hydrogen hydrogen environment on the side chain with its splitting pattern.

(3) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(iii) Suggest the structural formula of X. (1)

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(d) Compound X can be extracted from from raspberries by steam distillation. Draw a labelled diagram of the apparatus you could use to carry out this steam distillation. (3)


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3

Glucose can be oxidized using acidified potassium manganate(VII). The kinetics of the reaction can be studied using the procedure outlined below. 1. Measured volumes of glucose solution, sulfuric acid and water were added to a conical flask. 2. A measured volume of potassium manganate(VII) solution was added to the flask. The mixture was gently swirled and a stopwatch started. 3. The time taken for the mixture in the flask to change colour was recorded and the initial rate of the reaction was then calculated. 4. The experiment was repeated using different volumes of the solutions. The results of the experiments are shown in the table below.

Experiment

Glucose / cm3

Sulfuric acid / cm3

Potassium manganate(VII) / cm3

Water / cm3

Initial rate / mol dm−3 s−1

A

20.0

20.0

10.0

0.0

1.0 × 10−5

B

20.0

20.0

5.0

5.0

5.0 × 10−6

C

10.0

20.0

10.0

10.0

9.8 × 10−6

D

10.0

10.0

10.0

20.0

4.9 × 10−6

(a) (i) Which piece of equipment should be used to measure out the volumes used in each experiment? Justify your choice. (2) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(ii) What colour change would you see in step 3? (2)

From . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . to ... ...................................................................................... ........ (iii) Explain why water was added to the flask in experiments B, C and D. (1)

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(iv) Suggest a technique that could be used to continuously monitor the change in concentration of potassium manganate(VII) during the reaction. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(v) State the order with respect to glucose, sulfuric acid and potassium manganate(VII) and hence write the rate equation for the reaction. (3)

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(b) Experiment A was repeated at different temperatures and the time taken for the potassium manganate(VII) to change colour was recorded. The results were processed to find values of 1/temperature and ln (rate constant) and these are shown in the table below.

Experiment

1 / temperature / K −1

ln (rate constant)

E

3.00 × 10−3

−1.60

F

3.10 × 10−3

−2.60

G

3.21 × 10−3

−3.75

H

3.35 × 10−3

−5.20

(i) Plot a graph of ln (rate constant) against 1 / temperature on the axes below. (3) 1 / temperature / K –1

ln (rate constant)

(ii) Calculate the gradient of the graph. (1)

*P41653A01116*

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(iii) Use your answer to (ii) and the relationship below to calculate the activation energy, E a, for this reaction. Include a sign and units in your answer. Gradient =

− E a R

R = 8.31 J K −1 mol−1 (2)


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4

The procedure outlined below can be used to extract caffeine from tea. 1. Add 25 g of tea, 10 g of calcium carbonate and 250 cm3 of water to a large beaker. 2. Gently boil the mixture for 15 minutes. 3. While the mixture is still warm, filter using suction filtration. 4. Transfer the filtrate to a separating funnel and separate the caffeine from the aqueous mixture using solvent extraction, with dichloromethane as the solvent. 5. Dry the extract. 6. Remove the solvent. [Density of dichloromethane = 1.32 g cm−3] (a) (i) Outline how to carry out the solvent extraction in step 4, to obtain a solution of caffeine dissolved in dichloromethane. (3)

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(ii) How would you dry the extract in step 5? Include the name of a suitable drying agent in your answer. (2) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(b) (i) The solvent dichloromethane is harmful and can enter the body through inhalation and skin absorption. Suggest a possible way to minimise each of these risks when using dichloromethane. (2) Inhalation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...................................................................................... ...................................................................................... ...................................................................................... ..................

Skin absorption

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(ii) Suggest a suitable way to remove the solvent in step 6. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(c) The extraction can also be carried out using liquid carbon dioxide. Suggest an advantage of using this rather than dichloromethane. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(d) A student carrying out this extraction obtained 85 mg of caffeine. Calculate the percentage by mass of caffeine obtained from the sample of tea used. (2)

(e) Caffeine obtained in this way is often a pale green solid, due to impurities. State the name of another technique you could use to further purify the caffeine. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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*P41653A01516*

15


Other names

Pearson Edexcel

Centre Number

Candidate Number

International Advanced Level

Chemistry Advanced Unit 6: Chemistry Laboratory Skills II Thursday 16 January 2014 – Morning Time: 1 hour 15 minutes

Paper Reference

WCH06/01

Candidates may use a calculator.

Total Marks









P42975RA ©2014 Pearson Education Ltd.

6/6/6/6/2/2/3/

*P42975RA0116*

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Answer ALL the questions. Write your answers in the spaces provided. 1 Substance A is a green solid which dissolves in water to form a green solution. Substance A is an ionic compound containing one cation and one anion. (a) Give the formulae of two cations which could be responsible for the green colour in the solid.

(2) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(b) A student added dilute sodium hydroxide, drop by drop, to an aqueous solution of A. Initially, a green precipitate was formed. The precipitate did not dissolve in excess sodium hydroxide solution. The precipitate darkened on standing to give a brown solid. (i) Write the formula of the cation in substance A. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(ii) Write a formula for the green precipitate. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(iii) Write a formula for the brown solid. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(iv) State the type of reaction involved when the green precipitate turns brown. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(c) The student added a few drops of acidified potassium manganate(VII) solution to another sample of a solution of A in a test tube. Describe the colour change that occurs. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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2

*P42975RA0216*

(d) The student acidified about 2 cm3 of a solution of A with dilute nitric acid in a test tube and then added a few drops of aqueous silver nitrate solution. A white precipitate was formed. (i) Give the formula of the anion present in A. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(ii) The test in (d)(i) is usually followed by the addition of ammonia solution to test the solubility of the precipitate. Explain why this is not a useful procedure in this case. (2) ...................................................................................... ...................................................................................... ...................................................................................... ...................

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2 Solution B is copper(II) sulfate dissolved in water. (a) When an excess of concentrated hydrochloric acid is added to solution B, the colour of B changes from blue to yellow. Give the formula of the complex ion responsible for the yellow colour. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(b) What would you observe as dilute ammonia solution is added, drop by drop, to another sample of solution B? (3) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(c) 20.0 cm3 of solution B was added to excess aqueous potassium iodide solution and the volume made up to 250 cm3. The following reaction occurred: 2Cu2+(aq) + 4I–(aq)



2CuI(s) + I2(aq)

25.0 cm3 samples of the resulting mixture were titrated with 0.120 mol dm –3 sodium thiosulfate solution. The mean titre was 17.85 cm3. (i) Complete the ionic equation for the reaction of thiosulfate ions with iodine. State symbols are not required. (1) 2S2O

2– 3

+ I2  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +

................................................

(ii) Calculate the concentration of solution B, in mol dm–3. (3)

(Total for Question 2 = 8 marks) 4

*P42975RA0416*

3

An ester is hydrolysed in the presence of an acid catalyst forming a carboxylic acid and an alcohol C. The alcohol contains four carbon atoms. In order to investigate the kinetics of this reaction, two solutions, X and Y, were made up.

Solution X: 100 cm3 of a 0.20 mol dm –3 solution of the ester Solution Y: 100 cm3 of a 0.20 mol dm–3 solution of hydrochloric acid Flasks containing the two solutions were placed in a water bath at 50 oC and when both solutions had reached the temperature of the water bath, the solutions were mixed and a clock started. As soon as the clock was started, a 10 cm3 sample was taken from the reaction mixture, transferred to a cooled conical flask and titrated with 0.050 mol dm–3 sodium hydroxide solution. Other samples were taken at two minute intervals and analysed in the same way.

Results: Time/min

0

2

4

6

8

10

12

14

16

Titre/cm3

20.0

23.4

26.2

28.5

30.5

32.1

33.4

34.5

35.5

V = (40 – titre)/cm3

20.0

16.6

13.8

11.5

9.5

7.9

6.6

5.5

4.5

V is directly proportional to the concentration of the ester remaining in the solution. (a) Why was each sample cooled before titration? (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(b) Two indicators are available for the titrations: phenolphthalein and methyl orange. Which one should be used? Give a reason for your answer. (1) Indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reason

...................................................................................... ...................................................................................... ..................................................................................

...................................................................................... ...................................................................................... ...................................................................................... ..................

(c) (i) Explain why the titre at time zero is 20.0 cm3 rather than 0.0 cm 3. No calculation is required. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(ii) Explain why the titre increases as the reaction proceeds. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

...................................................................................... ...................................................................................... ...................................................................................... ..................

6

*P42975RA0616*

(d) Plot a graph of V on the vertical axis against time on the horizontal axis. Use your graph to determine the order of the reaction by measuring two successive half-lives. (4)

First half-life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Second half-life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

*P42975RA0716*

7 Turn over

(e) The alcohol C is oxidized using potassium dichromate(VI) and dilute sulfuric acid. State the colour change observed. (1)

From . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . To .. ..................................................................................... (f ) The oxidation results in the formation of either a carboxylic acid or a ketone. (i) Suggest a chemical test that could be used to show that the purified product is a carboxylic acid. Give the observation that you would make when this test is carried out. (2)

Reagent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Observation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...................................................................................... ...................................................................................... ...................................................................................... ..................

(ii) Suggest a chemical test that could be used to show that the purified product is a ketone. Give the observation that you would make when this test is carried out. (2) Reagent

...................................................................................... ...................................................................................... ...............................................................................

Observation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...................................................................................... ...................................................................................... ...................................................................................... ..................

(g) Tests show that C is oxidized to a carboxylic acid. What type of alcohol is C? (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

8

*P42975RA0816*

(h) A simplified nmr spectrum for alcohol C is shown below:

Q

5

4

3

2

1

0

Chemical shift  / ppm (i) What can you conclude from the fact that there are four sets of peaks? (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

...................................................................................... ...................................................................................... ...................................................................................... ..................

(ii) Using your answers to (g) and (h)(i), and the fact that alcohol C contains four carbon atoms, draw the displayed formula of alcohol C. (1)

(iii) On the displayed formula you have drawn in (h)(ii), circle the atom or group of atoms responsible for the peak labelled Q. (1) (Total for Question 3 = 17 marks)

*P42975RA0916*

9 Turn over

4

This question concerns the preparation of an ester, 3-methylbutyl ethanoate. The ester can be produced by the reaction of 3-methylbutan-1-ol and ethanoic anhydride: C5H11OH + (CH3CO)2O  CH3COOC5H11 + CH 3CO2H Reagents 

3-methylbutan-1-ol [molar mass = 88.0 g mol–1; density = 0.81 g cm–3]



ethanoic anhydride

Required product 

3-methylbutyl ethanoate [molar mass = 130.0 g mol–1; boiling temperature = 142 °C]

Safety information 

3-methylbutan-1-ol is highly flammable



3-methylbutyl ethanoate is highly flammable



ethanoic anhydride is corrosive, causing skin blistering and peeling

The steps of the experimental procedure are as follows: Step 1 Place 10.0 cm3 of 3-methylbutan-1-ol in a flask and add a few anti-bumping granules. Step 2 Set up the apparatus for reflux. Pour 12.5 cm3 of ethanoic anhydride (a slight excess) down the condenser. Warm the mixture until the reaction starts and then reflux gently for five minutes. Allow the mixture to cool. Step 3 Transfer the cooled mixture to a separating funnel, leaving the anti-bumping granules in the flask. Add about 25 cm3 of water and shake the mixture. Allow the two layers to separate and discard the lower aqueous layer. The addition of water converts any unreacted ethanoic anhydride into ethanoic acid. Step 4 Add about 10 cm3 of aqueous sodium hydrogencarbonate to the separating funnel and shake carefully. When the vigorous effervescence has finished, insert the stopper and shake the funnel, frequently releasing the pressure. Repeat the washing with further quantities of aqueous sodium hydrogencarbonate until no more gas is produced. Discard the lower aqueous layer each time. Step 5 Transfer the ester to a conical flask and shake the flask for five minutes with a suitable drying agent. Step 6 Filter the dried ester directly into a flask. Set up the apparatus for simple distillation, adding a few anti-bumping granules to the flask. Distil off the ester.

10

*P42975RA01016*

(a) State two safety precautions, each related to a specific hazard of this experiment. You may assume that eye protection and laboratory coats are being worn and that the experiment was carried out in a fume cupboard. (2) Hazard 1

...................................................................................... ...................................................................................... .............................................................................

Precaution 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hazard 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precaution 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(b) Draw a labelled diagram of the apparatus needed for heating under reflux in Step 2. You do not need to show stands or clamps. (3)

*P42975RA01116*

11 Turn over

(c) Why are anti-bumping granules added to the flask in Step 1? (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

...................................................................................... ...................................................................................... ...................................................................................... ..................

(d) What is the purpose of adding aqueous sodium hydrogencarbonate in Step 4? (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

...................................................................................... ...................................................................................... ...................................................................................... ..................

(e) In the following list of substances, only one would be a suitable drying agent to use in Step 5. Identify this drying agent, giving a reason for your choice: (2) concentrated phosphoric(V) acid sodium hydroxide solid anhydrous sodium sulfate concentrated sulfuric acid

   

Drying agent Reason

...................................................................................... ...................................................................................... ..................................................................

...................................................................................... ...................................................................................... ..................................................................................

...................................................................................... ...................................................................................... ...................................................................................... ..................

(f) In Step 6, the ester is distilled off. Suggest a suitable temperature range over which to collect the ester. (1)

From . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . °C to . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . °C .. .. (g) (i) Calculate the maximum mass of 3-methylbutyl ethanoate that could be obtained in this experiment from 10 cm3 of 3-methylbutan-1-ol. Give your answer to three significant figures. (3)

12

*P42975RA01216*

(ii) A student carried out the synthesis and obtained 9.45 g of 3-methylbutyl ethanoate. Calculate the percentage yield. (2)


*P42975RA01316*

13

*P42981A0112*

*P42981A0212*

*P42981A0312*

*P42981A0412*

*P42981A0512*

*P42981A0612*

*P42981A0712*

*P42981A0812*

*P42981A0912*

*P42981A01012*

*P42981A01112*

*P42981A01212*


Other names

Pearson Edexcel

Centre Number

Candidate Number


Chemistry Advanced Unit 6: Chemistry Laboratory Skills II Tuesday Tu esday 27 January January 2015 – Afternoon Afternoon Time: 1 hour 15 minutes

Paper Reference

WCH06/01


Total T otal Marks


Use black ink ink or ball-point pen. Fill in the boxes at the top of this page with your name, centre number and candidate number. Answer all questions. Answer the questions in the spaces provided – there may be more space than you need .

Information  

The total mark for this paper paper is 50. 50. The marks for each question are shown in brackets – use use this this as as a guide as as to how much much time to to spend spend on each question.





You will be assessed on your your ability to organise organise and present information, information, ideas, descriptions and arguments clearly and logically, including your use of grammar, punctuation and spelling. A Periodic Table Table is printed on the back cover cover of this paper. paper.



6/6/6/6/2/

Read each question carefully before you start to answer it. Keep an eye eye on the time. Try to answer answer every question. question. Check your answers if you you have time at the the end.

*P45046A0116*

Turn over


A white solid, A, has one metal cation, and an anion containing two non-metallic elements. (a) A flame test is carried out on A. (i) Describe how you would carry out this flame test in the laboratory. (3)

....................................................................................................................................................................................................................................................................................

....................................................................................................................................................................................................................................................................................

....................................................................................................................................................................................................................................................................................

....................................................................................................................................................................................................................................................................................

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....................................................................................................................................................................................................................................................................................

(ii) A yellow flame is seen. Give the formula of the metal ion present. (1) ....................................................................................................................................................................................................................................................................................

(b) Solid A dissolves in water to form a colourless solution. This solution decolorises a dilute aqueous solution of iodine. Dilute hydrochloric acid is added to a fresh solution of A. A very pale yellow precipitate, B, forms slowly and an acidic gas, C, is given off. Gas C turns acidified sodium dichromate(VI) from orange to green. (i) Identify, by name or formula, the precipitate B and the gas C. (2)

Precipitate, B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gas, C

2

.................................................................................................................................................................................................................................................................

*P45046A0216*

(ii) What is the colour of a dilute aqueous solution of iodine? (1) ....................................................................................................................................................................................................................................................................................

(iii) Give the name of the anion in compound A. (1) ....................................................................................................................................................................................................................................................................................

(iv) Give the formula of compound A. (1) ....................................................................................................................................................................................................................................................................................


*P45046A0316*

3 Turn over

2

A white solid, D, is formed when ethanoyl chloride is added to a concentrated solution of ammonia. The molecular formula of D is C 2H5ON. When solid D is heated with excess aqueous sodium hydroxide solution, ammonia gas is given off and a solution, E , is formed. (a) Ammonia has a distinctive smell. Give two other tests, each of a different type, which could be used to show the presence of ammonia. Give the result of each test. (3)

Test 1

..................................................................................................................................................................................................................................................................

....................................................................................................................................................................................................................................................................................

Test 2

..................................................................................................................................................................................................................................................................

....................................................................................................................................................................................................................................................................................

(b) Excess dilute sulfuric acid is added to solution E and an organic liquid, F, is distilled from the mixture. (i) Draw a labelled diagram of the apparatus used for this distillation. (2)

4

*P45046A0416*

(ii) Addition of pure liquid F to aqueous sodium carbonate gives effervescence. Identify liquid F by name or formula. (1) ....................................................................................................................................................................................................................................................................................

(c) (i) Give the name and displayed formula of solid D. (2)

Name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Displayed formula

(ii) Write an equation for the formation of solid D from ethanoyl chloride and concentrated ammonia solution. State symbols are not required. (1)


*P45046A0516*

5 Turn over

3 This is an experiment to determine the oxidation number of vanadium in a purple solution, T, of a vanadium compound. Preparation of solution T

Solution T was formed when 25.00 cm 3 of a 0.100 mol dm –3 solution of sodium vanadate(V), NaVO 3, was reduced by heating with excess zinc and dilute sulfuric acid. When the reduction was complete, the yellow NaVO 3 solution had turned purple. Titration of solution T

The mixture was filtered through glass wool, directly into 50.00 cm 3 of 0.0200 mol dm–3 potassium manganate(VII), KMnO 4, solution. Further potassium manganate(VII) solution of the same concentration was added from a burette to this reaction mixture, which was kept at a temperature of about 80 oC. The end point is reached when all the vanadium ions had been oxidized back into vanadate(V) ions by the manganate(VII) ions. The end point occurred when a further 25.00 cm3 had been added. (a) (i) Draw a diagram of the apparatus for carrying out the titration, while keeping the titration mixture at about 80 oC. (2)

6

*P45046A0616*

(ii) What is removed from the reaction mixture by filtering through glass wool? (1) ....................................................................................................................................................................................................................................................................................

(iii) Suggest why the mixture is filtered directly into potassium manganate(VII) solution before carrying out the rest of the titration. (1) ....................................................................................................................................................................................................................................................................................

....................................................................................................................................................................................................................................................................................

....................................................................................................................................................................................................................................................................................

(iv) Explain why an indicator is not required for this titration. (1) ....................................................................................................................................................................................................................................................................................

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....................................................................................................................................................................................................................................................................................

(b) (i) Calculate the number of moles of vanadate(V) ions, VO3–, in 25.00 cm 3 of a 0.100 mol dm–3 solution of sodium vanadate(V), NaVO 3. (1)

(ii) Calculate the total volume of potassium manganate(VII) solution. Hence the total number of moles of potassium manganate(VII) used to oxidize the purple vanadium solution, T. (2)

*P45046A0716*

7 Turn over

(iii) Complete the half equation for the reduction of manganate(VII) ions to manganese(II) ions. (1)

MnO4– + . . . . . . . . . . . . . . . . . . . .H+ + . . . . . . . . . . . . . . . . . . . .e–

Mn2+ + . . . . . . . . . . . . . . . . . . . .H2O

(iv) By considering either the number of electrons transferred or by using the changes in oxidation numbers, calculate the oxidation number of vanadium in the purple solution, T.

You must show your working. (3)

(c) In acidic solution, the vanadate ions, VO3– are changed into VO2+. Write an ionic equation for this reaction. State symbols are not required. (1)

8

*P45046A0816*

(d) Some standard electrode potentials of tin and vanadium are given below. Sn2+(aq)| Sn(s)

–0.14 V

V2+(aq)| V(s)

–1.18 V

V3+(aq),V2+(aq)| Pt

–0.26 V

[VO 2+(aq) + 2H+(aq)], [V3+(aq) + H2O(l)]| Pt

+0.34 V

[VO 2+(aq) + 2H+(aq)], [VO2+(aq) + H2O(l)]| Pt

+1.00 V

Use these values to predict the lowest oxidation number of vanadium that can be produced from VO 2+ using tin as the reducing agent. Explain your reasoning. (2) ....................................................................................................................................................................................................................................................................................

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*P45046A0916*

9 Turn over

4

Cholesteryl benzoate was the first liquid crystal to be discovered. It can be prepared by the following procedure. Step 1

Dissolve 1.0 g of cholesterol in 3 cm 3 of pyridine in a conical flask.

Step 2

Add 0.40 cm3 of benzoyl chloride.

Step 3

Heat the mixture on a steam bath for about 10 minutes.

Step 4

Cool the mixture, and add 15 cm 3 of methanol.

Step 5

Collect the solid cholesteryl benzoate by suction filtration. Rinse the flask and the crude crystals with a little cold methanol.

Step 6

Recrystallize the cholesteryl benzoate using ethyl ethanoate as the solvent.

Some physical data for the chemicals involved are shown below. Molar mass / g mol –1 Cholesterol

386.7

Benzoyl chloride

140.6

Cholesteryl benzoate

490.8

Pyridine

79.1

Ethyl ethanoate

88.1

Density / g cm –3

Melting temperature / K

Boiling temperature / K 633

1.21

470 423 388 190

350

(a) Suggest the apparatus you would use to measure the volume of benzoyl chloride. (1) ....................................................................................................................................................................................................................................................................................

(b) The warning symbols on a bottle of benzoyl chloride are shown below. Write the meaning of each symbol in the space provided. (2)

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*P45046A01116*

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(c) 1 mol of cholesterol reacts with 1 mol of benzoyl chloride to form 1 mol of cholesteryl benzoate. (i) Determine which reactant is in excess by calculating how many moles of cholesterol and of benzoyl chloride are used in the preparation. (3)

(ii) Calculate the percentage yield when 0.65 g of cholesteryl benzoate is obtained. (2)

(d) Suggest how the mixture is cooled in Step 4. (1) ....................................................................................................................................................................................................................................................................................

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(e) Suggest why methanol is added to the cooled mixture in Step 4. (1) ....................................................................................................................................................................................................................................................................................

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*P45046A01216*

(f) Describe how to carry out the recrystallization to obtain pure dry crystals of cholesteryl benzoate in Step 6. (5) ....................................................................................................................................................................................................................................................................................

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(g) How would you show that the recrystallized cholesteryl benzoate crystals in Step 6 are purer than the crude crystals obtained in Step 5? (2) ....................................................................................................................................................................................................................................................................................

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*P45046A01316*

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Other names

Pearson Edexcel

Centre Number

Candidate Number


Chemistry Advanced Unit 6: Chemistry Laboratory Skills II Paper Reference

Friday 15 May 2015 – Morning Time: 1 hour 15 minutes

WCH06/01


Total Marks









6/6/6/5/


*P44884A0116*

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A white solid, A, contains one cation and one anion. When water is added slowly, the solid turns blue and then dissolves to form a blue solution, B. (a) When aqueous barium chloride is added to an acidified portion of solution B, a white precipitate forms. (i) Give the formula of the anion in B. (1)

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(ii) Name a suitable acid for acidifying solution B in this test. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(b) When aqueous ammonia is added to another portion of solution B, a blue precipitate forms. When more aqueous ammonia is added, this precipitate dissolves to form a deep blue solution, C. (i) Identify, by name or formula, the blue precipitate. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(ii) Give the formula of the ion responsible for the deep blue colour of solution C. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(c) Give the formula of the complex ion which gives the blue colour to solution B. Include the ligands in your answer. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(d) Give the formula of the white solid A. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(e) Why is solid A white and not coloured blue? Justify your answer. (2) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(Total for Question 1 = 8 marks) 2

*P44884A0216*

2

A salt, P, contains carbon, hydrogen, oxygen and one metallic element. (a) When a flame test is carried out on P, a yellow flame results. Give the formula of the metal ion in the salt. (1)

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(b) P dissolves in water to form a weakly alkaline solution, Q, with pH 8.1. The pH of Q can be measured by using a calibrated pH meter or an indicator. (i) Describe how to calibrate a pH meter. (2)

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(ii) Name a suitable indicator you could use and state the colour you would expect to observe. (2) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(iii) Which of the two methods will give the more accurate value for the pH of Q? Justify your answer. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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*P44884A0316*

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(c) Some of the solution Q is acidified with concentrated hydrochloric acid. An organic compound, R, forms in the solution. Methanol is added and the mixture warmed, forming a new organic compound S. This mixture is added to sodium carbonate solution in an evaporating basin. A fruity smell is detected. (i) Describe and explain what you would see as the mixture is added to the sodium carbonate solution. (2) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(ii) What type of compound is S? (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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*P44884A0416*

(d) The high resolution proton nuclear magnetic resonance (nmr) spectrum of S has only two peaks which are both singlets and have the same area. Deduce the structural formulae of S, R, and P. (3) S

R

P


*P44884A0516*

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3

Cupronickel is an alloy of copper and nickel. It is used to make ‘silver’ coins. A coin is analysed by the following method. Step 1

It is weighed on a balance which reads to two decimal places and found to have mass 4.00 g.

Step 2

Water is added to the coin in a beaker. Concentrated nitric and sulfuric acids are added and the coin dissolves.

Step 3

When the coin is completely dissolved, the solution is neutralized.

Step 4

The neutral solution is transferred, with the washings, to a 100 cm3 volumetric flask, made up to the mark with water and mixed thoroughly.

Step 5

10 cm3 samples of the solution are taken and an excess of potassium iodide is added, producing iodine.

Step 6

The iodine is titrated with 0.200 mol dm–3 sodium thiosulfate solution.

(a) Why, in Step 2, is water added before, rather than after, the acids? (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(b) What is the colour of an aqueous iodine solution? (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(c) (i) To make the end point of the titration more obvious, an indicator is added just before the colour of the iodine disappears. Name this indicator. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

(ii) Suggest why the indicator is not added to the iodine solution earlier in the titration. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(iii) Give the colour change at the end point when the indicator is used in this titration. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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*P44884A0616*

(d) The results for the titrations are shown below. Titration number

1

2

3

4

Burette reading (final) / cm3

24.10

47.90

23.55

47.00

Burette reading (initial) / cm3

0.00

24.10

0.00

23.55

Titre / cm3

(i) Complete the table. (1) (ii) Which titres should be used to calculate the mean? Explain your choice. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(iii) Calculate the mean titre. (1)

*P44884A0716*

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(iv) Calculate the percentage by mass of copper in the coin. Use the equations below. 2Cu2+(aq)

+ 4I–(aq)

2S2O32–(aq) +



I2(aq) 

2CuI(s)

+

I2(aq)

– S4O2– 6 (aq) + 2I (aq)

(5)

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*P44884A0816*

(v) The uncertainty in each burette reading is ±0.05 cm3 and the uncertainty in each reading of the balance is ±0.005 g. Calculate the percentage uncertainty in the third titre value and in the mass measurement. Use your results to decide whether using a balance that weighs to three decimal places would significantly improve the accuracy of the result. (2)


*P44884A0916*

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4

Bromobenzene can be prepared from benzene by the following steps. Step 1

Reflux 20.0 cm3 of benzene with 6.0 cm3 of bromine and about 10g of iron filings, by heating on a water bath at 50 C.

Step 2

After the reaction has finished, remove the water bath and heat to boiling until no bromine vapour can be seen.

Step 3

Cool the mixture and add 25 cm3 of ethoxyethane (diethyl ether) to extract the bromobenzene.

Step 4

Wash the ethoxyethane layer with aqueous sodium hydroxide. Separate the ethoxyethane layer.

Step 5

Wash the ethoxyethane layer with water and repeat the separation.

Step 6

Dry the ethoxyethane layer with a suitable drying agent.

Step 7

Decant the dried solution.

Step 8

Distil the separated solution, collecting the fraction boiling around the boiling temperature of bromobenzene, 156 C.

(a) Calculate the number of moles of bromine, Br2, used in the experiment. [Density of bromine 3.1 g cm–3] (2)

(b) Bromine reacts with iron to form iron(III) bromide, which reacts with bromine to produce the attacking electrophile in Step 1. Write the chemical equations for these reactions. (2)

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*P44884A01016*

(c) Why is the ethoxyethane layer washed with sodium hydroxide solution in Step 4? (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(d) Draw a diagram of the apparatus used to separate the ethoxyethane layer from the aqueous layer in Step 5. Clearly label the ethoxyethane layer. [Densities: water 1.0 g cm–3, ethoxyethane 0.7 g cm–3] (2)

(e) The bromobenzene formed in this reaction can be nitrated to make 2,4-dinitrobromobenzene. Identify, by name or formula, the chemicals needed for this reaction. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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*P44884A01116*

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(f ) 2,4-dinitrobromobenzene reacts with hydrazine hydrate to make 2,4-dinitrophenylhydrazine crystals. The percentage yields for the reactions are: 75% for the formation of bromobenzene from benzene 70% for the formation of 2,4-dinitrobromobenzene from bromobenzene 70% for the formation of 2,4-dinitrophenylhydrazine from 2,4-dinitrobromobenzene Calculate the overall percentage yield of 2,4-dinitrophenylhydrazine from benzene, for this series of reactions. (1)

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*P44884A01216*

(g) The purity of the 2,4-dinitrophenylhydrazine crystals can be checked by carrying out a melting temperature determination using the Thiele tube apparatus shown below. thermometer stopper with wedge cut Thiele tube capillary tube with sample

heat here (i) The capillary tube must be sealed at one end. Describe how this is done. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(ii) When crystals are placed in the capillary tube they often stick in the top. Describe how to ensure the crystals reach the bottom of the capillary tube. (1) ...................................................................................... ...................................................................................... ...................................................................................... ..................

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(iii) Dibutyl phthalate is often used as the liquid in the Thiele tube.

Suggest two properties of dibutyl phthalate that make it a suitable liquid for this purpose. (2)

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*P44884A01316*

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(iv) The melting temperature for crystals of 2,4-dinitrophenylhydrazine is 201C. Suggest the temperature range over which you would expect the crystals to melt before and after purification by recrystallization. (2) Before recrystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . After recrystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (Total for Question 4 = 15 marks) TOTAL FOR PAPER = 50 MARKS

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*P44884A01416*

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Edexcel A2 Chemistry Paper 6

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