Cambridge A Level Chemistry 9701 — 2014 May/June Paper 3 · Variant 1

This document consists of 11 printed pages and 1 blank page. [Turn over IB14 06_9701_31/4RP © UCLES 2014  READ THESE INSTRUCTIONS FIRST Write your Centre number, candidate number and name on all the work you hand in. Give details of the practical session and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fl uid. DO NOT WRITE IN ANY BARCODES. Answer all questions. Electronic calculators may be used. You may lose marks if you do not show your working or if you do not use appropriate units. Use of a Data Booklet is unnecessary. Qualitative Analysis Notes are printed on pages 10 and 11. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. CHEMISTRY 9701/31 Paper 3 Advanced Practical Skills 1 May/June 2014 2 hours Candidates answer on the Question Paper. Additional Materials: As listed in the Confi dential Instructions Cambridge International Examinations Cambridge International Advanced Subsidiary and Advanced Level Session Laboratory For Examiner’s Use 1 2 3 Total

2 9701/31/M/J/14 © UCLES 2014 1 Hydrogen peroxide, H2O2, is unstable and decomposes to give water and oxygen. In addition to the usual units of concentration, mol dm–3, the concentration of a solution of hydrogen peroxide can also be given in ‘volume strength’ or ‘vol’. For example, in Question 3 you will use a solution of ‘20 vol’ H2O2. This term means that when 1 dm3 of this solution is completely decomposed it generates 20 dm3 of oxygen at room temperature and pressure. The aim of the following titration is to determine the volume strength of a solution of hydrogen peroxide. To do this you will titrate an acidifi ed solution of hydrogen peroxide with potassium manganate(VII) solution. FA 1 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4. FA 2 is aqueous hydrogen peroxide, H2O2. FA 3 is 1.0 mol dm–3 sulfuric acid, H2SO4. (a) Method ● Fill the burette with FA 1. ● Pipette 25.0 cm3 of FA 2 into the conical flask. ● Use the measuring cylinder to add 25 cm3 of FA 3 to the conical flask. ● Run FA 1 from the burette into the conical flask until the pink colour remains. ● Perform a rough titration and record your burette readings in the space below. The rough titre is … cm3. ● Carry out as many accurate titrations as you think necessary to obtain consistent results. ● Make certain any recorded results show the precision of your practical work. ● Record, in a suitable form below, all of your burette readings and the volume of FA 1 added in each accurate titration. [6] I II III IV V VI

3 9701/31/M/J/14 © UCLES 2014 [Turn over I II III IV V (b) From your accurate titration results obtain a suitable value to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 2 required … cm3 of FA 1. [1] (c) Calculations Show your working and appropriate signifi cant fi gures in the fi nal answer to each step of your calculations. (i) Calculate the number of moles of potassium manganate(VII) present in the volume of FA 1 calculated in (b). moles of KMnO4 = … mol (ii) Use the following equation and your answer to (i) to calculate the number of moles of hydrogen peroxide used in each titration. 2KMnO4 + 5H2O2 + 3H2SO4  K2SO4 + 2MnSO4 + 5O2 + 8H2O moles of H2O2 = … mol (iii) Calculate the concentration, in mol dm–3, of H2O2 in FA 2. concentration of H2O2 = … mol dm–3 (iv) Write an equation for the decomposition of hydrogen peroxide to produce oxygen and water. … (v) Calculate the concentration in ‘volume strength’ of H2O2 in FA 2. You must assume that the molar volume of oxygen is 24.0 dm3 mol–1 at room temperature and pressure. concentration of FA 2 = … ‘vol’ [5] [Total: 12]

4 9701/31/M/J/14 © UCLES 2014 2 An acidifi ed solution of hydrogen peroxide is able to oxidise iodide ions, I–(aq), to iodine, I2(aq). H2O2(aq) + 2H+(aq) + 2I–(aq)  I2(aq) + 2H2O(l) In this experiment, you will investigate how the rate of this reaction depends on the concentration of the hydrogen peroxide and on the concentration of the iodide ions. The rate of this reaction can be measured by adding thiosulfate ions, S2O3 2–, and starch indicator to the mixture. As the iodine is produced, it reacts immediately with the thiosulfate ions and is reduced back to iodide ions. I2(aq) + 2S2O3 2–(aq)  2I–(aq) + S4O6 2–(aq) When all the thiosulfate has reacted, the iodine then turns the starch indicator blue-black. The rate of reaction may be determined by timing how long it takes the reaction mixture to turn blue-black. FA 2 is aqueous hydrogen peroxide, H2O2. FA 3 is 1.0 mol dm–3 sulfuric acid, H2SO4. FA 4 is 0.50 mol dm–3 potassium iodide, KI. FA 5 is 0.025 mol dm–3 sodium thiosulfate, Na2S2O3. starch indicator Read through the instructions carefully and prepare a table for your results before starting any practical work. (a) Method Experiment 1 ● Use the measuring cylinder to transfer the following volumes into the same 100 cm3 beaker. ○ 10 cm3 of FA 3 ○ 20 cm3 of FA 4 ○ 10 cm3 of FA 5 ○ 10 cm3 of distilled water ● Add 10 drops of starch indicator to the beaker. ● Rinse the measuring cylinder with water and shake dry. ● Use the measuring cylinder to measure 10 cm3 of FA 2. ● Add the contents of the measuring cylinder to the beaker and start timing immediately. ● Stir the mixture once and place the beaker on a white tile. ● Stop timing as soon as the solution turns blue-black. ● Record this reaction time to the nearest second. ● Wash out the beaker and the measuring cylinder with water and shake dry. Experiment 2 ● Use the measuring cylinder to transfer the following volumes into the same 100 cm3 beaker. ○ 10 cm3 of FA 3 ○ 20 cm3 of FA 4 ○ 10 cm3 of FA 5 ● Add 10 drops of starch indicator to the beaker. ● Rinse the measuring cylinder with water and shake dry. ● Use the measuring cylinder to measure 20 cm3 of FA 2.

5 9701/31/M/J/14 © UCLES 2014 [Turn over I II III IV V VI ● Add the contents of the measuring cylinder to the beaker and start timing immediately. ● Stir the mixture once and place the beaker on a white tile. ● Stop timing as soon as the solution turns blue-black. ● Record this reaction time to the nearest second. ● Wash out the beaker and the measuring cylinder with water and shake dry. Experiment 3 ● Use the measuring cylinder to transfer the following volumes into the same 100 cm3 beaker. ○ 10 cm3 of FA 3 ○ 10 cm3 of FA 4 ○ 10 cm3 of FA 5 ○ 10 cm3 of distilled water ● Add 10 drops of starch indicator to the beaker. ● Rinse the measuring cylinder with water and shake dry. ● Use the measuring cylinder to measure 20 cm3 of FA 2. ● Add the contents of the measuring cylinder to the beaker and start timing immediately. ● Stir the mixture once and place the beaker on a white tile. ● Stop timing as soon as the solution turns blue-black. ● Record this reaction time to the nearest second. Record all your results in a single table. You should include the volume of hydrogen peroxide, the volume of potassium iodide, the volume of distilled water and the reaction time. You should also include the ‘rate of reaction’ which is given by the following expression. rate of reaction = reaction time 1 [6] (b) From your results, what can you conclude about how the ‘rate of reaction’ is affected by, (i) the concentration of hydrogen peroxide, … (ii) the concentration of potassium iodide? … [1]

6 9701/31/M/J/14 © UCLES 2014 (c) It is not possible to draw a reliable conclusion about the effect of changing the concentration of a reagent on the ‘rate of reaction’ from only two experiments. Suggest three experiments that could be carried out in addition to Experiments 2 and 3 to investigate the effect of changing the concentration of FA 4, potassium iodide. In each case give the volumes of each solution / liquid that would be used. Do not carry out these experiments. [2] (d) A student was unable to complete all three experiments on the same day but had to return to the investigation the following morning. The student noticed that the results were signifi cantly different. Give two possible reasons for these differences and explain how the student could have overcome these problems. … … … [2] (e) There are many sources of error in these experiments. In the following, assume that the only error involves measuring the reaction time. (i) If the error in each recorded time is 1 s, calculate the percentage error in the time recorded in Experiment 1. percentage error = … % (ii) Another student decided to repeat Experiment 1 but used 0.050 mol dm–3 sodium thiosulfate in place of FA 5. How would this change the percentage error in the recorded time? Explain your answer. … … … [2] [Total: 13]

7 9701/31/M/J/14 © UCLES 2014 [Turn over 3 Qualitative Analysis At each stage of any test you are to record details of the following. ● colour changes seen ● the formation of any precipitate ● the solubility of such precipitates in an excess of the reagent added Where gases are released they should be identifi ed by a test, described in the appropriate place in your observations. You should indicate clearly at what stage in a test a change occurs. Marks are not given for chemical equations. No additional tests for ions present should be attempted. If any solution is warmed, a boiling tube MUST be used. Rinse and reuse test-tubes and boiling tubes where possible. Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given. (a) FA 6 is a solution of two different salts. It contains two different cations, one of which is listed in the Qualitative Analysis Notes on page 10. It contains two anions, both of which are listed in the Qualitative Analysis Notes on page 11. (i) Choose reagents that will allow you to identify one of the cations. Carry out suitable tests using these reagents and record your results in the space below. One of the cations in FA 6 is … . I II III IV V

8 9701/31/M/J/14 © UCLES 2014 (ii) Carry out the following tests to identify the two anions present in FA 6. test observations To a 1 cm depth of FA 6 in a test-tube add a 1 cm depth of aqueous silver nitrate, then add aqueous ammonia. To a 1 cm depth of FA 6 in a test-tube add a 1 cm depth of aqueous barium chloride (or aqueous barium nitrate), then add dilute nitric acid. The anions in FA 6 are … and … . [9] VI VII VIII IX

9 9701/31/M/J/14 © UCLES 2014 [Turn over (b) FA 7 is an acidifi ed solution of iron(II) sulfate, FeSO4(aq). Carry out the following tests and record your observations. test observations (i) To a 1 cm depth of FA 7 in a test-tube add aqueous sodium hydroxide and leave for a few minutes. (ii) To a 1 cm depth of FA 7 in a boiling tube add a 1 cm depth of dilute sulfuric acid followed by a 1 cm depth of ‘20 vol’ hydrogen peroxide. Stir the mixture, then (iii) pour a 1 cm depth of the mixture into a clean boiling tube and add a 3 cm depth of aqueous sodium hydroxide. (iv) What type of reaction takes place in (ii)? … … (v) Explain your observations in (iii). … … [6] [Total: 15] I II III IV V VI

10 9701/31/M/J/14 © UCLES 2014 Qualitative Analysis Notes Key: [ppt. = precipitate] 1 Reactions of aqueous cations ion reaction with NaOH(aq) NH3(aq) aluminium, Al 3+(aq) white ppt. soluble in excess white ppt. insoluble in excess ammonium, NH4 +(aq) no ppt. ammonia produced on heating – barium, Ba2+(aq) no ppt. (if reagents are pure) no ppt. calcium, Ca2+(aq) white ppt. with high [Ca2+(aq)] no ppt. chromium(III), Cr3+(aq) grey-green ppt. soluble in excess giving dark green solution grey-green ppt. insoluble in excess copper(II), Cu2+(aq) pale blue ppt. insoluble in excess blue ppt. soluble in excess giving dark blue solution iron(II), Fe2+(aq) green ppt. turning brown on contact with air insoluble in excess green ppt. turning brown on contact with air insoluble in excess iron(III), Fe3+(aq) red-brown ppt. insoluble in excess red-brown ppt. insoluble in excess magnesium, Mg2+(aq) white ppt. insoluble in excess white ppt. insoluble in excess manganese(II), Mn2+(aq) off-white ppt. rapidly turning brown on contact with air insoluble in excess off-white ppt. rapidly turning brown on contact with air insoluble in excess zinc, Zn2+(aq) white ppt. soluble in excess white ppt. soluble in excess

11 9701/31/M/J/14 © UCLES 2014 2 Reactions of anions ion reaction carbonate, CO3 2– CO2 liberated by dilute acids chloride, Cl –(aq) gives white ppt. with Ag+(aq) (soluble in NH3(aq)); bromide, Br –(aq) gives cream ppt. with Ag+(aq) (partially soluble in NH3(aq)); iodide, I –(aq) gives yellow ppt. with Ag+(aq) (insoluble in NH3(aq)); nitrate, NO3 –(aq) NH3 liberated on heating with OH–(aq) and Al foil nitrite, NO2 –(aq) NH3 liberated on heating with OH–(aq) and Al foil; NO liberated by dilute acids (colourless NO  (pale) brown NO2 in air) sulfate, SO4 2–(aq) gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acids) sulfi te, SO3 2–(aq) SO2 liberated with dilute acids; gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids) 3 Tests for gases gas test and test result ammonia, NH3 turns damp red litmus paper blue carbon dioxide, CO2 gives a white ppt. with limewater (ppt. dissolves with excess CO2) chlorine, Cl 2 bleaches damp litmus paper hydrogen, H2 “pops” with a lighted splint oxygen, O2 relights a glowing splint sulfur dioxide, SO2 turns acidifi ed aqueous potassium manganate(VII) from purple to colourless

12 9701/31/M/J/14 BLANK PAGE © UCLES 2014 Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included the publisher will be pleased to make amends at the earliest possible opportunity. Cambridge International Examinations is part of the Cambridge Assessment Group. Cambridge Assessment is the brand name of University of Cambridge Local Examinations Syndicate (UCLES), which is itself a department of the University of Cambridge.

CAMBRIDGE INTERNATIONAL EXAMINATIONS GCE Advanced Subsidiary Level and GCE Advanced Level MARK SCHEME for the May/June 2014 series 9701 CHEMISTRY 9701/31 Paper 3 (Advanced Practical Skills 1), maximum raw mark 40 This mark scheme is published as an aid to teachers and candidates, to indicate the requirements of the examination. It shows the basis on which Examiners were instructed to award marks. It does not indicate the details of the discussions that took place at an Examiners’ meeting before marking began, which would have considered the acceptability of alternative answers. Mark schemes should be read in conjunction with the question paper and the Principal Examiner Report for Teachers. Cambridge will not enter into discussions about these mark schemes. Cambridge is publishing the mark schemes for the May/June 2014 series for most IGCSE, GCE Advanced Level and Advanced Subsidiary Level components and some Ordinary Level components.

Page 2 Mark Scheme Syllabus Paper GCE AS/A LEVEL – May/June 2014 9701 31 © Cambridge International Examinations 2014 Question Sections Indicative material Mark Total 1 (a) PDO Layout I Initial and final readings and titre value given for rough titre and initial and final readings for two (or more) accurate titrations (minimum of 2 × 2 box) 1 PDO Recording II Appropriate headings and units for all accurate data. and volume FA 1 added recorded for each accurate titre. Headings should match readings. • initial/start (burette) reading/volume • final/end (burette) reading/volume • titre or volume/FA 1 used/added (not “difference”) unit: /cm3 or (cm3) or in cm3 or cm3 for each entry III All accurate burette readings recorded to 0.05 cm3. The need to record to 0.05 applies only to the burette readings and not to the recorded titres. Do not award this mark if: • 50(.00) is used as an initial burette reading • more than one final burette reading is 50.(00) • any burette reading is greater than 50.(00). 1 1 MMO Decisions IV Has two uncorrected, accurate titres within 0.1 cm3 Do not include a reading labelled ‘rough’. Do not award this mark if, having performed two titres within 0.1 cm3, a further titration is performed that is more than 0.1 cm3 from the closer of the two initial titres unless further titrations within 0.1 cm3 of any other have also been carried out. Do not award the mark if any ‘accurate’ burette readings (apart from initial 0) are given to zero dp. 1 Examiner rounds burette readings to the nearest 0.05 cm3, checks subtractions and then selects the ‘best’ titres using the hierarchy: two (or more) identical, then two (or more) within 0.05 cm3, then two (or more) within 0.1 cm3, etc. Examiner compares candidate mean titre with Supervisor mean titre. (a) MMO Quality Award V and VI for difference from Supervisor, δ ≤ 0.20 cm3 Award V only for 0.20 < δ ≤ 0.40 cm3 Spread penalty: if the two ‘best’ titres are ≥ 0.50 cm3 apart cancel one of the Q marks. 2 [6]

Page 3 Mark Scheme Syllabus Paper GCE AS/A LEVEL – May/June 2014 9701 31 © Cambridge International Examinations 2014 Question Sections Indicative material Mark Total (b) ACE Interpretation Candidate must average two (or more) titres that are within 0.20 cm3. Working must be shown or ticks must be put next to the two (or more) accurate readings selected. The mean should normally be quoted to 2 dp rounded to the nearest 0.01. Two special cases where the mean may not be to 2 dp: allow mean to 3 dp only for 0.025 or 0.075 e.g. 26.325; allow mean to 1 dp if all accurate burette readings were given to 1 dp and the mean is exactly correct. e.g. 26.0 and 26.2 = 26.1 is correct but 26.0 and 26.1 = 26.1 is incorrect. Note: the candidate’s mean will sometimes be marked as correct even if it is different from the mean calculated by the Examiner for the purpose of assessing accuracy. 1 [1] (c) ACE Interpretation I Correctly evaluates 1000 0.0200 (b) × in (i) 1 II Correctly evaluates 25 5/2 × (i) in (iii) 1 PDO Display III Correct balanced equation in (iv) 1 ACE Interpretation IV Correctly evaluates ans (iii) × ½ × 24.0 in (v) (Allow ecf from incorrect equation) 1 PDO Display V All answers given to 3 or 4 sf (minimum of 3 answers attempted) 1 [5] Qn 1 Total [12]

Page 4 Mark Scheme Syllabus Paper GCE AS/A LEVEL – May/June 2014 9701 31 © Cambridge International Examinations 2014 Question Sections Indicative material Mark Total 2 (a) PDO Recording I Table to include • volume of hydrogen peroxide/FA 2, • volume of potassium iodide/FA 4, • volume of distilled water, • reaction time. volume/V in cm3//cm3/ (cm3), time/t in seconds//s/(s). (Minimum 2 expts recorded) 1 II All times recorded to the nearest second. (Minimum 2 expts) 1 ACE Interpretation III Correctly calculates all three rates (allow to 2 or 3 sf) Compare times for Expts 1 and 3 with those of the Supervisor. 1 MMO Quality Award IV, V and VI for both times within 3 s Award IV and V for one within 3 s and one within 6 s Award IV only for either within 6 s (If only 2 expts carried out IV is available – from either expt performed) 3 [6] (b) ACE Conclusion Rate increases with increasing concentration of hydrogen peroxide and potassium iodide (ora). Allow ecf from candidate’s results. 1 [1] (c) MMO Decisions Selects different volumes of FA 4 (less than 20 cm3, not 10 cm3 and not closer than 2 cm3 to suggested volumes or to 20 cm3 or to 10 cm3) 1 Volumes of distilled water selected so that vol of water + vol of FA 4 = 20 cm3 and FA 2 = 20 cm3 If FA 3 and FA 5 are shown then the volumes must be constant. 1 [2] (d) ACE Improvements Reason: change of temperature Use water bath to maintain constant temperature 1 Reason: decomposition of hydrogen peroxide Store H2O2(aq) in the fridge, make up fresh H2O2(aq), check conc. of H2O2(aq), keep H2O2(aq) in dark/dim light. 1 [2] (e) (i) ACE Interpretation Expression 100 1 Expt from time 1 × or correct value. 1 (ii) (Higher conc. of thiosulfate means) greater reaction time (allow reaction will be slower) and so a smaller percentage error. 1 [2] Qn 2 Total [13]

Page 5 Mark Scheme Syllabus Paper GCE AS/A LEVEL – May/June 2014 9701 31 © Cambridge International Examinations 2014 Question Sections Indicative material Mark Total FA 6 is ZnSO4(aq) and NaBr(aq); FA 7 is FeSO4(aq) 3 (a) (i) MMO Decisions I Selects NaOH(aq) and NH3(aq), and uses each in excess 1 PDO Layout II Unambiguous layout of all 4 observations (excess must be stated). 1 MMO Collection III White ppt with NaOH and soluble in excess. 1 IV White ppt with NH3 soluble in excess. 1 ACE Conclusion V Zn2+ 1 (ii) MMO Collection VI Cream ppt with AgNO3 and partially sol / insol in NH3 1 VII White ppt with BaCl 2/Ba(NO3)2 and insol in nitric acid. 1 ACE Conclusion VIII Br – 1 IX SO4 2– 1 [9] (b) (i) MMO Collection I Green ppt turning brown (in contact with air) 1 (ii) II No reaction/no change or yellow or green solution 1 (iii) III Red-brown/brown/green-brown ppt and effervescence 1 IV Gas relights a glowing splint 1 (iv) ACE Conclusion V Redox 1 (v) VI Decomposition of hydrogen peroxide to give oxygen or ppt is Fe(OH)3 / oxidation of Fe2+ to Fe3+ 1 [6] Qn 3 Total [15]