Cambridge A Level Chemistry 9701 — 2014 Oct/Nov Paper 3 · Variant 6

This document consists of 10 printed pages and 2 blank pages. [Turn over IB14 11_9701_36/6RP © UCLES 2014 *9763634822* Session Laboratory For Examiner’s Use 1 2 3 Total 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 9701/36 Paper 3 Advanced Practical Skills 2 October/November 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

2 9701/36/O/N/14 © UCLES 2014 1 You are to determine the concentration of a solution of sodium thiosulfate, Na2S2O3. To do this you will fi rst produce a known amount of iodine by reacting iodate(V) ions, IO3 –, with an excess of iodide ions, I–. The equation for this reaction is below. IO3 – + 5I– + 6H+ → 3I2 + 3H2O The amount of iodine produced in this reaction can be found by titrating with thiosulfate ions. The equation for this reaction is below. I2 + 2S2O3 2– → 2I– + S4O6 2– FB 1 is aqueous sodium thiosulfate, Na2S2O3. FB 2 is aqueous potassium iodate(V) containing 3.60 g dm–3 KIO3. FB 3 is sulfuric acid, H2SO4. FB 4 is aqueous potassium iodide, KI. starch indicator (a) Method ● Fill a burette with FB 1. ● Pipette 25.0 cm3 of FB 2 into the conical flask. ● Use the measuring cylinder to add 25 cm3 of FB 3 into the conical flask. ● Use the measuring cylinder to add 10 cm3 of FB 4 into the conical flask. Brown iodine solution is produced. ● Add FB 1 from the burette until most of the iodine has been removed and the solution in the conical flask is yellow. ● Add 10 drops of starch indicator to the contents of the conical flask. The solution will turn blue-black. ● Continue adding FB 1, from the burette, until the blue-black colour just disappears. ● Carry out 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 FB 1 added in each accurate titration. [6] I II III IV V VI

3 9701/36/O/N/14 © UCLES 2014 [Turn over (b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value. 25.0 cm3 of FB 2 required … cm3 of FB 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 KIO3 present in 25.0 cm3 of FB 2. [Ar: O, 16.0; K, 39.1; I, 126.9] moles of KIO3 = … mol (ii) The equations for the production of iodine and its titration with thiosulfate are shown below. IO3 – + 5I– + 6H+ → 3I2 + 3H2O I2 + 2S2O3 2– → 2I– + S4O6 2– Use these equations to calculate the number of moles of thiosulfate present in the volume of FB 1 you calculated in (b). moles of S2O3 2– = … mol (iii) Calculate the concentration, in mol dm–3, of sodium thiosulfate in FB 1. concentration = … mol dm–3 [5] [Total: 12]

4 9701/36/O/N/14 © UCLES 2014 2 In Question 1 iodide ions were oxidised by iodate(V) ions. Iodide ions can also be oxidised by peroxodisulfate ions, S2O8 2–, and you are to investigate the rate of this reaction. 2I– + S2O8 2– → I2 + 2SO4 2– The rate of this reaction can be measured by adding thiosulfate ions, S2O3 2–, and starch solution to the mixture. As the iodine is produced, it reacts immediately with the thiosulfate ions and is reduced back to iodide according to the reaction below. I2 + 2S2O3 2– → 2I– + S4O6 2– After all the thiosulfate has reacted, the iodine turns the starch indicator blue-black. The rate of reaction may be determined by timing how long it takes for the mixture to turn blue-black. The rate of the reaction can be represented by rate = reaction time 1 . (a) Method FB 4 is aqueous potassium iodide, KI. FB 5 is 0.020 mol dm–3 potassium peroxodisulfate, K2S2O8. FB 6 is 0.010 mol dm–3 sodium thiosulfate, Na2S2O3. starch indicator Read through the method before you start any practical work and prepare a suitable table for your results in the space at the top of page 5. ● Empty and wash thoroughly the burette that you used in Question 1. Experiment 1 ● Use the measuring cylinder to add 20 cm3 of FB 4 into a 100 cm3 beaker. ● Use the 10 cm3 pipette to add 10.0 cm3 of FB 6 to the beaker. ● Add 10 drops of starch indicator to the beaker. ● Fill a burette with FB 5. ● Run 20.00 cm3 of FB 5 into a second 100 cm3 beaker. ● Add the contents of the first beaker to the second beaker and start timing immediately. ● Stir the mixture once and place the beaker on a white tile. ● Stop timing as soon as the solution goes blue-black. ● Record this reaction time to the nearest second. ● Wash out both beakers and shake dry. Experiment 2 ● Use the measuring cylinder to add 20 cm3 of FB 4 into a 100 cm3 beaker. ● Use the 10 cm3 pipette to add 10.0 cm3 of FB 6 to the beaker. ● Add 10 drops of starch indicator to the beaker. ● Fill the second burette with distilled water. ● Run 10.00 cm3 of FB 5 into the second 100 cm3 beaker. ● Run 10.00 cm3 of distilled water into this second beaker containing FB 5. ● Add the contents of the first beaker to the second beaker and start timing immediately. ● Stir the mixture once and place the beaker on a white tile. ● Stop timing as soon as the solution goes blue-black. ● Record this reaction time to the nearest second. ● Wash out both beakers and shake dry.

5 9701/36/O/N/14 © UCLES 2014 [Turn over Record your results in the space below. You should show the volume of FB 5, the volume of water and the reaction time for each experiment. [3] (b) Carry out three further experiments to investigate how the reaction time changes with different volumes of peroxodisulfate. Remember that the combined volume of peroxodisulfate solution, FB 5, and distilled water must always be 20.00 cm3. Do not use a volume of FB 5 that is less than 4.00 cm3. Record the volume of FB 5, the volume of water and the reaction time for each experiment. [4] (c) Use your results from (a) and (b) to complete the table below. You should show the volume of FB 5, the reaction time and the volume of FB 5 × reaction time. [2] I II III IV

6 9701/36/O/N/14 © UCLES 2014 (d) A student who had carried out these experiments concluded that the rate of reaction was directly proportional to the volume of FB 5. Using your values from (c), explain whether your results agree with this conclusion. … … … … [1] (e) Another student thought that the experiment could be made more accurate by giving longer reaction times. To do this he repeated Experiment 1 with the same volumes but using 0.100 mol dm–3 sodium thiosulfate instead of FB 6. He found that the reaction never turned blue-black. Explain why. … … … [1] (f) Describe how you could modify this experiment to investigate the effect of the concentration of iodide ions, FB 4, on the rate of the reaction. … … … … [2] [Total: 13]

7 9701/36/O/N/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. Half fi ll the 250 cm3 beaker with water and heat it until the water is approximately 60 °C. Leave to stand. This water will be used as a water bath. Turn off the Bunsen burner when the water is hot. (a) FB 7 and FB 8 are aqueous solutions. One of the elements present in the different ions in FB 7 and FB 8 is the same. Carry out the following tests on FB 7 and FB 8 and complete the table. test observations (i) To a 1 cm depth of FB 7 in a test-tube add a 1 cm depth of aqueous silver nitrate. (ii) To a 1 cm depth of FB 7 in a test-tube add aqueous sodium hydroxide. (iii) To a 1 cm depth of FB 7 in a test-tube add a 1 cm depth of aqueous sodium hydroxide and then a 1 cm depth of hydrogen peroxide. Leave to stand. (iv) Before starting this test, the Bunsen burner must be turned off. To a 2 cm depth of dilute sulfuric acid add a few drops of FB 8 then add a 1 cm depth of ethanol. Leave to stand in the water bath. [5]

8 9701/36/O/N/14 © UCLES 2014 (b) What element is present in both FB 7 and FB 8? … [1] (c) Solid FB 9 and aqueous solution FB 10 both contain the same cation. (i) Transfer approximately half of the FB 9 into a hard-glass test-tube and heat. observations … … (ii) Transfer the remaining FB 9 into a boiling tube and carefully add dilute sulfuric acid. observations … … (iii) To a 1 cm depth of FB 10 in a test-tube, add a piece of magnesium ribbon. Leave to stand. observations … … (iv) To a 1 cm depth of FB 10 in a test-tube, add an equal depth of concentrated hydrochloric acid (care: this is very corrosive). observations … … (v) Suggest which cation is present in both FB 9 and FB 10. … (vi) What change in the oxidation state of magnesium is occurring during reaction (c)(iii)? Oxidation state change from … to … [8] (d) In which part of the Periodic Table are the elements identifi ed as being present in FB 7, FB 8, FB 9 and FB 10? … [1] [Total: 15]

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10 9701/36/O/N/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 9701/36/O/N/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 9701/36/O/N/14 © UCLES 2014 BLANK PAGE 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.

® IGCSE is the registered trademark of Cambridge International Examinations. CAMBRIDGE INTERNATIONAL EXAMINATIONS Cambridge International Advanced Subsidiary and Advanced Level MARK SCHEME for the October/November 2014 series 9701 CHEMISTRY 9701/36 Paper 3 (Advanced Practical Skills 2), 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 October/November 2014 series for most Cambridge IGCSE®, Cambridge International A and AS Level components and some Cambridge O Level components.

Page 2 Mark Scheme Syllabus Paper Cambridge International AS/A Level – October/November 2014 9701 36 © Cambridge International Examinations 2014 Question Indicative material Mark Total 1 (a) I The following data is shown • two burette readings for the rough titration • titre for rough titration • initial and final burette readings for two (or more) accurate titrations (Minimum of 2 × 2 boxes) Correct headings and units for accurate titrations. Headings should match readings • Initial / start and (burette) reading / volume (not V or vol) • Final / end and (burette) reading / volume • Titre or volume / FB 1 and used / added (not difference, total or change) Unit: / cm3 or (cm3) or in cm3 or cm3 for each reading. 1 II All accurate burette readings are to nearest 0.05 cm3. The need to record to 0.05 cm3 applies to the burette readings and not to the recorded titres but it does apply to 0.00 cm3. 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 III Has two uncorrected, accurate titres within 0.1 cm3 Do not consider the ‘rough’ even if ticked. Do not award this mark if having performed two titres within 0.1 cm3 a further titration is performed which is more than 0.10 cm3 from the closer of the initial two titres, unless any further titrations, within 0.1 cm3 of any other titration have also been carried out. Do not award this mark if any accurate burette readings (apart from initial 0) are given to zero dp. 1 Round any burette readings to the nearest 0.05 cm3. Examiner selects the ‘best’ titres using the hierarchy: • two (or more) accurate identical titres (ignoring rough), then • two (or more) accurate titres within 0.05 cm3, then • two (or more) accurate titres within 0.10 cm3 etc These best titres should be used to calculate the mean corrected titre to the nearest 0.01 cm3. Award IV, V and VI if δ <0.2 cm3 Award IV and V if δ >0.2 but < 0.3 cm3 Award IV if δ >0.3 cm3 but <0.4 cm3. Spread penalty: if the two best (corrected) titres used by the examiner were >0.5 cm3 apart, cancel one Q mark. 1 1 1 [6]

Page 3 Mark Scheme Syllabus Paper Cambridge International AS/A Level – October/November 2014 9701 36 © Cambridge International Examinations 2014 (b) Calculation of mean Candidate must average two (or more) accurate titres with total spread of no more than 0.2 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. Example: 26.667 must be rounded to 26.67. 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. Do not award this mark if: any selected titre is not within 0.20 cm3 of any other selected titre; the rough titre was used to calculate the mean; the candidate carried out only 1 accurate titration; burette readings were incorrectly subtracted to obtain any of the accurate titre values; all burette readings (resulting in titre values used in calculation of mean) are integers. 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) (i) Mr KIO3 = 214 Moles dm−3 = 214 40 3.60 × = 4.205 / 4.206 / 4.21 / 4.21 × 10−4 1 1 (ii) Moles S2O3 2− = (i) × 6 = (2.52 × 10−3) 1 (iii) (b) (ii) from vol 1000 x Answers given to 3 or 4 sf 1 1 [5] [Total: 12] 2 (a) Round times to nearest second. Supervisor calculates time with 10 cm3 / time with 20 cm3 (to 1 dp) and awards 3 marks if within 1.9 to 2.1 awards 2 marks if within 1.8 to 2.2 (but not within 1.9 to 2.1) awards 1 mark if within 1.6 and 2.4 (but not within 1.8 to 2.2) 1 1 1 [3] (b) I 3 additional volumes chosen with intervals not less than 2 cm3. These must include 1 of < 10 cm3 and 1 of > 10 cm3 and have none < 4 cm3. 1

Page 4 Mark Scheme Syllabus Paper Cambridge International AS/A Level – October/November 2014 9701 36 © Cambridge International Examinations 2014 II In all 3 additional experiments water is added to make a total of 20 cm3. III Tables in (a) and (b) to show volume FB 5, volume water and reaction time. All volumes measured to 0.05 cm3. IV All times recorded to nearest second. 1 1 1 [4] (c) Completes table correctly. Correct headings and units including cm3 s. 1 1 [2] (d) Agree: product FB5 × reaction time is (approx) constant Or Disagree: product of FB5 × reaction time is not constant 1 [1] (e) sodium thiosulfate is in excess – all the iodine reacts with the thiosulfate so no iodine produced (to turn blue-black). 1 [1] (f) (Carry out a series of reactions) keeping volume S2O8 2 (FB5) constant (and timing to blue-black) Alter volume I– (FB4) but keep total volume (I– and water) constant / keep I– and water volumes constant but change concentration of I–. 1 1 [2] [Total: 13]

Page 5 Mark Scheme Syllabus Paper Cambridge International AS/A Level – October/November 2014 9701 36 © Cambridge International Examinations 2014 FB 7 is MnCl2, FB 8 is KMnO4, FB 9 is CuCO3, FB 10 is CuSO4 3 (a) (i) (ii) (iii) (iv) White ppt Off-white / buff / beige / light brown ppt and darkens on standing / insoluble in excess. Brown / black colour Effervescence / bubbling / fizzing and relights glowing splint (Colour change) purple / pink to colourless 1 1 1 1 1 [5] (b) Manganese 1 [1] (c) (i) (ii) (iii) (iv) (v) (vi) (Solid goes) black Fizz / effervescence / bubbling and blue solution. Limewater turns milky Any three from Solution goes paler Pink / black / brown solid formed Solution gets warmer Fizz Pop with lighted splint 3 correct answers scores 2 2 correct answers scores 1 Solution turns / goes yellow / green Copper / Cu2+ 0 to (+)2 1 1 1 2 1 1 1 [8] (e) Transition (elements) / d-block 1 [1] [Total: 15]