Cambridge A Level Chemistry 9701 — 2012 Oct/Nov Paper 3 · Variant 6

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 a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fl uid. DO NOT WRITE IN ANY BARCODES. Answer all questions. 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 15 and 16. 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 Advanced Practical Skills 2 October/November 2012 2 hours Candidates answer on the Question Paper. Additional Materials: As listed in the Confi dential Instructions UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS General Certifi cate of Education Advanced Subsidiary Level and Advanced Level This document consists of 13 printed pages and 3 blank pages. [Turn over IB12 11_9701_36/5RP © UCLES 2012 *8712516860* Session Laboratory For Examiner’s Use 1 2 3 Total

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3 9701/36/O/N/12 © UCLES 2012 [Turn over For Examiner’s Use I II III IV V VI 1 You are to determine the concentration of aqueous copper(II) sulfate by titration. The concentration of Cu2+ ions in a solution can be found by reaction with an excess of aqueous iodide ions to produce iodine. The amount of iodine formed can be found by titration with thiosulfate ions, S2O3 2–. FB 1 is aqueous copper(II) sulfate, CuSO4. FB 2 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. FB 3 is aqueous potassium iodide, KI. starch indicator Read through the instructions carefully before starting any practical work. (a) Method ● Fill the burette with FB 2. ● Pipette 25.0 cm3 of FB 1 into a conical flask. ● Use a measuring cylinder to add 10 cm3 of FB 3 into the conical flask. ● Titrate this mixture with FB 2 until the colour of the mixture changes from brown to yellow-brown. An off-white precipitate will also be present in the flask throughout the titration. ● Add approximately 1 cm3 of starch indicator. ● Continue the titration until the blue-black colour of the starch-iodine complex just disappears leaving the off-white precipitate. ● 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 FB 2 added in each accurate titration. [6]

4 9701/36/O/N/12 © UCLES 2012 For Examiner’s Use (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 FB 1 required … cm3 of FB 2 [1] (c) Calculations The equations for the formation of iodine and its reaction with thiosulfate ions are given below. 2Cu2+ + 4I– → 2CuI + I2 I2 + 2S2O3 2– → S4O6 2– + 2I– 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 thiosulfate ions, S2O3 2–, present in the volume of FB 2 in (b). moles of S2O3 2– = … mol (ii) Using the equations above, deduce the number of moles of Cu2+ ions present in each 25.0 cm3. moles of Cu2+ = … mol (iii) Calculate the concentration, in mol dm–3, of copper(II) sulfate in FB 1. concentration of CuSO4 = … mol dm–3 [3]

5 9701/36/O/N/12 © UCLES 2012 [Turn over For Examiner’s Use (d) Three students repeated the experiment but each obtained different values for the concentration of CuSO4. The students each suggested possible improvements. Student 1 suggested that a larger quantity of starch should be added. Student 2 suggested that a larger volume of potassium iodide, FB 3, should be added. Student 3 suggested that the contents of the conical fl ask should be fi ltered before titration. Comment on the effectiveness of each of these possible improvements. Explain your answers. Student 1 … … Student 2 … … Student 3 … … [2] [Total: 12]

6 9701/36/O/N/12 © UCLES 2012 For Examiner’s Use 2 You are to determine the enthalpy change for the reaction between aqueous copper(II) sulfate and zinc. The enthalpy change of reaction can be found by measuring the temperature change when powdered zinc is added to aqueous copper(II) sulfate. FB 4 is 1.10 mol dm–3 aqueous copper(II) sulfate, CuSO4. powdered zinc (a) Method ● Weigh a 100 cm3 beaker. ● In the beaker weigh out between 2.1 g and 2.3 g of powdered zinc. ● Record the weighings and the mass of zinc in the space below. mass of zinc used = … g ● Support the plastic cup in a 250 cm3 beaker. ● Use a measuring cylinder to transfer 50 cm3 of FB 4 into the plastic cup. ● Measure and record in the table below, the initial temperature of FB 4 in the cup. ● Start the stop watch. Measure and record the temperature of FB 4 in the cup after 1 minute, 2 minutes and 3 minutes. ● At time 3½ minutes, add the weighed zinc to FB 4 in the cup and stir the mixture. ● From time 4 minutes, continue to stir the mixture and measure the temperature of the contents of the cup to complete the table. Results time / min 0 1 2 3 4 5 6 7 8 9 10 11 12 temperature / °C [2] (b) (i) On the axes opposite, plot the temperature (y-axis) against time (x-axis). The temperature axis should allow you to include a point at least 5 °C greater than the maximum temperature recorded.

7 9701/36/O/N/12 © UCLES 2012 [Turn over For Examiner’s Use (ii) Complete the graph to show how the temperature of the contents of the cup varies with time. ● Draw one straight line through the points between time 0 minutes and 3 minutes. ● Draw one straight line through the points after the maximum was reached. ● Extrapolate these two lines and draw a vertical line at time 3½ minutes. [4]

8 9701/36/O/N/12 © UCLES 2012 For Examiner’s Use (c) Calculation (i) Use your graph to determine the change in temperature at 3½ minutes. change in temperature = ………… °C (ii) Calculate the heat energy produced in the reaction. (You may assume that 4.3 J are required to raise the temperature of 1.0 cm3 of any solution by 1.0 °C.) heat energy produced = … J (iii) The reaction between zinc and aqueous copper(II) sulfate is a displacement reaction shown in the equation below. Zn(s) + Cu2+(aq) → Cu(s) + Zn2+(aq) From the mass of zinc added and the concentration of FB 4, show that the copper(II) sulfate was in excess in your reaction. [Ar: Zn, 65.4] (iv) Assuming that the copper(II) sulfate was in excess, use your answer to (ii) to calculate the enthalpy change of the reaction between Zn(s) and Cu2+(aq). Give you answer in kJ mol–1 and include the relevant sign. enthalpy change of reaction = … … kJ mol–1 sign value [6]

9 9701/36/O/N/12 © UCLES 2012 [Turn over For Examiner’s Use (d) One source of error in this experiment is due to the accuracy to which the thermometer can be read. What is the maximum error in a single temperature reading on a thermometer with graduations at 1 °C? maximum error = … °C Calculate the maximum percentage error when measuring a temperature rise of 12.0 °C. maximum percentage error = … % [2] [Total: 14]

10 9701/36/O/N/12 © UCLES 2012 For Examiner’s Use 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. You are provided with solutions FB 5, FB 6, FB 7 and FB 8. FB 5 and FB 6 each contain a compound of a transition element. Half fi ll a 250 cm3 beaker with water. Heat to approximately 80 °C, then stop heating and switch off the Bunsen burner. You will need this as a hot water bath on (b)(i). Continue work on (a) while the water heats. (a) (i) Carry out the following tests on FB 5. test observations To 1 cm depth of FB 5 in a test-tube, add aqueous ammonia. To 1 cm depth of FB 5 in a test-tube, add aqueous sodium hydroxide. To 1 cm depth of FB 5 in a test-tube, add aqueous barium chloride or aqueous barium nitrate then, add an excess of either hydrochloric acid or nitric acid.

11 9701/36/O/N/12 © UCLES 2012 [Turn over For Examiner’s Use (ii) From these tests, what conclusions, if any, can you reach about the identity of FB 5? … [4] (b) (i) Carry out the following tests on FB 6. test observations To 1 cm depth of FB 6 in a boiling tube, add 1 cm depth of FB 7 then, add 1 cm depth of ethanol. Place the boiling tube in the warm water bath and leave for a few minutes. To 1 cm depth of FB 6 in a test-tube, add 1 cm depth of FB 8. (ii) From these tests suggest identities for the following. The anion in FB 6 is … The cation in FB 7 is … The cation in FB 8 could be … or … (iii) Suggest a test to determine which of the two possible cations is present in FB 8. Do not carry out this test. … … [7] (c) Using your conclusions about the possible identities of FB 5 and FB 8, predict the result of mixing solutions of each. Do not carry out this test. Prediction … [1]

12 9701/36/O/N/12 © UCLES 2012 For Examiner’s Use (d) Suggest what happened to the ethanol when it was warmed with the mixture of FB 6 and FB 7. … [1] (e) You are to devise and carry out a test to confi rm the identity of the cation in FB 7. Record the test you use and the results of the test in the space below. [1] [Total: 14]

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15 9701/36/O/N/12 © UCLES 2012 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 lead(II), Pb2+(aq) white ppt. soluble in excess white 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 [Lead(II) ions can be distinguished from aluminium ions by the insolubility of lead(II) chloride.]

16 9701/36/O/N/12 © UCLES 2012 2 Reactions of anions ion reaction carbonate, CO3 2– CO2 liberated by dilute acids chromate(VI), CrO4 2–(aq) yellow solution turns orange with H+(aq); gives yellow ppt. with Ba2+(aq); gives bright yellow ppt. with Pb2+(aq) chloride, Cl –(aq) gives white ppt. with Ag+(aq) (soluble in NH3(aq)); gives white ppt. with Pb2+(aq) bromide, Br –(aq) gives cream ppt. with Ag+(aq) (partially soluble in NH3(aq)); gives white ppt. with Pb2+(aq) iodide, I –(aq) gives yellow ppt. with Ag+(aq) (insoluble in NH3(aq)); gives yellow ppt. with Pb2+(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) or with Pb2+(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 dichromate(VI) from orange to green 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. University of 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 October/November 2012 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 2012 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 – October/November 2012 9701 36 © Cambridge International Examinations 2012 Question Section Indicative material Mark Total 1 (a) PDO Layout PDO Recording PDO Recording MMO Quality I Records initial and final burette readings and titre for rough. Tabulates and records initial and final burette readings and volume of FB 2 run from burette for all accurate titrations. Not awarded if 50/50.0/50.00 shown more than once. II Appropriate headings and units: initial/final (burette) reading/volume, reading or volume at start/finish/beginning/end, volume used/volume added/FB 2 used/titre. Units are /cm3, (cm3) or volume in cm3. III All accurate burette readings recorded to 0.05 cm3 (this includes 0.00). Two (minimum) accurate titrations needed. Difference between candidate’s mean titre and Supervisor’s is calculated. Award IV, V and VI for δ Y 0.20 cm3 Award IV and V for 0.20 cm3 < δ Y 0.40 cm3 Award IV for 0.40 cm3 < δ Y 0.60 cm3 Spread penalty Titres (selected by Examiner) differ by [ 0.5 cm3 or only 1 accurate titration penalty -1. This mark is deducted from those awarded in IV to VI but no negative marks. 1 1 1 3 [6] (b) PDO Display Calculation of mean Candidate must average two (or more) accurate titres that are within 0.20 cm3 of another. 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 decimal places rounded to the nearest 0.01. Example: 26.667 must be rounded to 26.67. 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. 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]

Page 3 Mark Scheme Syllabus Paper GCE AS/A LEVEL – October/November 2012 9701 36 © Cambridge International Examinations 2012 (c) (i) (ii) (iii) ACE Interpretation 0.1 × answer to (b) calculated correctly (3 to 4 significant 1000 figures) Answer to (c)(i) (at least 2 significant figures) Answer to (c)(ii) × 1000 calculated correctly 25.0 (3 to 4 significant figures) 1 1 1 [3] (d) ACE Improvements Any two from Larger quantity of starch – no effect because starch is the indicator or wtte. Larger volume of KI – no effect because KI is already in excess or will have an effect because KI not (already) in excess. Filter before titration – effective because easier to see end-point/ colour change or not effective because iodine/chemicals stay on the filter paper. 1 1 [2] [Total: 12] 2 (a) MMO Collection PDO Recording Mass of zinc used between 2.1 and 2.3 g. Subtraction must be correct from unambiguous weighings. Table completed and all temperatures recorded to 0.0 or 0.5°C. Must include at least one ending in 0.0 and one ending in 0.5. 1 1 [2] (b) (i) (ii) PDO Layout PDO Layout MMO Collection PDO Layout Axes labelled temperature or T/°C or (°C) or temperature in °C (y-axis) and time (x-axis) or t/ minutes etc. Linear scales chosen so that graph occupies at least half the available length for both axes. This includes the 5 °C extension. Plotting accurate (within ½ small square and in correct square). Must plot all readings taken – minimum 10. 2 straight lines drawn – one before 3 minutes and one after maximum temperature. 3 appropriate lines drawn – including extrapolations (not falling). 1 1 1 1 [4]

Page 4 Mark Scheme Syllabus Paper GCE AS/A LEVEL – October/November 2012 9701 36 © Cambridge International Examinations 2012 (c) (i) (ii) (iii) (iv) ACE Interpretation PDO Display ACE Interpretation ACE Conclusion ∆T calculated. Examiner to check from graph and calculate to nearest 0.5°C. Candidate’s answer must be correct to nearest 0.5°C, decimal places not needed for 0.0. Allow ∆T at 3½ min, even if not max, provided some indication on graph. Correctly calculates 50 × 4.3 × candidate’s ∆T (from (i) unless value from graph was not using maximum vertical). Must be max. Moles Zn = candidate’s mass of zinc 65.4 (working must be shown and answer correct to significant figures shown) Moles Cu2+ = 50 x 1.1 = 0.055 1000 (working must be shown and answer correct to significant figures shown) (c)(ii) (correctly calculated, ecf possible) 1000 × moles of Zn (from (c)(iii)) Sign negative and answer to 2–4 significant figures (stand alone). 1 1 1 1 1 1 [6] (d) ACE Interpretation Error in one temperature reading = 0.5 (°C) Maximum % error = 1.0 × 100 = 8.3(3) % (ecf 2 × error) 12.0 Expression or correct answer. 1 1 [2] [Total: 14]

Page 5 Mark Scheme Syllabus Paper GCE AS/A LEVEL – October/November 2012 9701 36 © Cambridge International Examinations 2012 FB 5 is NiSO4(aq); FB 6 is K2CrO4(aq); FB 7 is H2SO4(aq); FB 8 is BaCl2(aq) 3 (a) (i) (ii) MMO Collection ACE Conclusions Solution/ FB 5/liquid goes from green to blue. Green precipitate and insoluble in excess. Unqualified white precipitate, insoluble in acid. FB 5 is a sulfate/nickel sulfate/NiSO4/SO4 2–. (allow conclusion even if green ppt in (i)) 1 1 1 1 [4] (b) (i) (ii) (iii) MMO Collection ACE Conclusions MMO Decisions Solution/FB 6/liquid goes from yellow to orange. (Solution) goes green/blue (allow grey-green or blue- green). Yellow precipitate. Anion in FB 6 is CrO4 2–/ chromate Cation in FB 7 is H+/ hydrogen Cation in FB 8 is Ba2+/ barium or Pb2+/ lead. Add named (aqueous) chloride – white ppt Pb2+ (not Ba2+) or Add (aqueous) NaOH – white ppt Pb2+ (not Ba2+) or Add (aqueous) ammonia - white ppt Pb2+ (not Ba2+) or Add (aqueous) KI – yellow ppt Pb2+ (not Ba2+) or Add (aqueous) named chromate (only allow if FB 6 not identified as chromate) – yellow ppt both Ba2+ and Pb2+ with indication that Ba2+ is paler. Allow ecf from candidate’s FB 8. 1 1 1 1 1 1 1 [7] (c) ACE Conclusions Prediction must follow identities of FB 5 and FB 8. (If these correct then should be white ppt.) If candidate’s ions in (b)(ii) and (a)(ii) would give two different results, both must be specified. 1 [1] (d) ACE Conclusions (Ethanol/it was) oxidised/ an aldehyde was formed/ a carboxylic acid was formed (not redox). 1 [1] (e) MMO Decisions Uses a (named) carbonate or a (named) reactive metal to produce effervescence/ positive test for the gas. Named indicator with correct colour. NaOH with temperature increase. (no ecf possible) 1 [1] [Total: 14]