Cambridge A Level Chemistry 9701 — 2011 Oct/Nov Paper 3 · Variant 5

This document consists of 10 printed pages and 2 blank pages. DC (CW/DJ) 34877/5 © UCLES 2011 [Turn over UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS General Certificate of Education Advanced Subsidiary Level and Advanced Level * 5 6 3 6 5 1 9 9 6 1 * CHEMISTRY 9701/35 Advanced Practical Skills 1 October/November 2011 2 hours Candidates answer on the Question Paper. Additional Materials: As listed in the Confidential Instructions 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 fluid. 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 11 and 12. 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. Session Laboratory For Examiner’s Use 1 2 Total

2 9701/35/O/N/11 © UCLES 2011 For Examiner’s Use 1 You are required to investigate the effect of temperature on the rate of reaction of peroxydisulfate ions with iodide ions. Iodide ions are oxidised to iodine by peroxydisulfate ions. S2O8 2–(aq) + 2I–(aq) 2SO4 2–(aq) + I2(aq) FA 1 is aqueous potassium peroxydisulfate, K2S2O8. FA 2 is an aqueous solution containing a mixture of potassium iodide, KI, sodium thiosulfate, Na2S2O3, and starch. When FA 1 and FA 2 are mixed together the potassium peroxydisulfate reacts with the potassium iodide to make iodine. As soon as this iodine is formed, it reacts with the sodium thiosulfate and is turned back into iodide ions. Only when all the sodium thiosulfate has reacted does iodine remain in the solution. The solution then turns blue-black because of the presence of the starch indicator. The rate of reaction can be determined by the time it takes for a blue-black colour to first appear in the colourless mixture. (a) Method Read through the method and prepare a table on page 3 to record the initial and final temperatures and the reaction time for each experiment, before starting any practical work. • Half-fill a 250 cm3 beaker with water to act as a water bath. • Place it on a tripod and gauze and heat it with a Bunsen burner to about 65 °C then remove the Bunsen. While your water is being heated continue with the following steps of the method. • Fill the burette, labelled FA 1, with the aqueous potassium peroxydisulfate, FA 1. • Fill the burette, labelled FA 2, with the mixture of solutions, FA 2. • Measure 10.0 cm3 of FA 1 into a boiling tube. • Measure 10.0 cm3 of FA 2 into a second boiling tube. • Place both boiling tubes in the water bath. • Clamp one of the tubes and place a thermometer in this tube. • When the temperature of this solution has reached about 60 °C, pour the contents of the second tube into the clamped tube. Start timing immediately, note the temperature and stir the mixture. • Record this initial temperature. • Stop timing as soon as the blue-black colour appears. Record this reaction time to the nearest second and record the final temperature. • Repeat the experiment at decreasing temperatures as many times as necessary to generate data for plotting a graph. The experiment should not be performed at a temperature below about 30 °C. The temperature of the water bath may be adjusted by adding cold water or by reheating. (Boiling tubes may be rinsed and reused.)

3 9701/35/O/N/11 © UCLES 2011 [Turn over For Examiner’s Use [5] (b) The rate of reaction for each experiment can be represented by the following. ‘rate’ = 1000 reaction time in seconds Complete the following table for each of your experiments. The mean temperature is the average of the initial and final temperature for the experiment. mean temperature / °C ‘rate’ [6] I II III IV V I II III IV V VI

4 9701/35/O/N/11 © UCLES 2011 For Examiner’s Use (c) (i) Using your values in (b), plot a graph of ‘rate’ (y-axis) against mean temperature (x-axis). Choose suitable scales to allow you to extrapolate the graph to include the ‘rate’ at 20 °C. (ii) Use your graph to obtain a value for the ‘rate’ at 20 °C. ‘rate’ at 20 °C is … [4] I II III IV

5 9701/35/O/N/11 © UCLES 2011 [Turn over For Examiner’s Use (d) It has been suggested that an increase in temperature of 10 °C will double the rate of reaction. Use two pairs of temperatures from your graph to confirm or deny this statement. … … … …[2] (e) (i) Which of your experiments has the greatest percentage error in timing? … (ii) Calculate the percentage error in (i). You may assume that the error in measuring the time for a reaction is ±0.5 seconds. error in (i) = … % [2] (f) A student had difficulty in drawing a line of best fit. Identify a source of error in the experimental procedure. Do not include any errors involving the precision of apparatus. … …[1] (g) Suggest a modification that could be used to reduce this error. … …[1] (h) Using FA 1, FA 2 and distilled water, describe how you could investigate the effect of concentration of potassium peroxydisulfate on the rate of reaction. … … … … …[3] [Total: 24]

6 9701/35/O/N/11 © UCLES 2011 For Examiner’s Use 2 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 identified 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 full name or correct formula of the reagents must be given. FA 3, FA 4 and FA 5 are aqueous solutions containing one cation and one anion. One of these solutions is a dilute acid and this is the only acid present. (a) (i) Select a single chemical reagent from those supplied which would allow you to identify the dilute acid. You may not use indicator paper. reagent … (ii) Use this reagent to test all three solutions and record your results in an appropriate form in the space below. (iii) From your observations in (ii), identify which solution is the dilute acid. FA … is the dilute acid. [5] I II III IV V

7 9701/35/O/N/11 © UCLES 2011 [Turn over For Examiner’s Use (b) The acid you have identified in (a)(iii) is dilute sulfuric acid. Complete the following table. test observations To 1 cm depth of FA 3 in a test-tube, add 1 cm depth of FA 4, then add excess hydrochloric acid. To 1 cm depth of FA 4 in a test-tube, add 1 cm depth of FA 5, then add excess hydrochloric acid. To 1 cm depth of FA 5 in a test-tube, add 1 cm depth of FA 3, then add excess hydrochloric acid. [4] (c) For the two unidentified solutions, complete the following table. test observations FA … FA … To 1 cm depth of unknown in a boiling tube, add NaOH(aq) warm the tube carefully [2] I II III IV

8 9701/35/O/N/11 © UCLES 2011 For Examiner’s Use (d) From your observations in (a), (b) and (c), identify the ions present in the two solutions tested in (c), giving the relevant evidence for each. If you have not been able to identify one or more of the ions, explain why the evidence obtained was insufficient. FA … cation … evidence … … … FA … anion … evidence … … … FA … cation … evidence … … … FA … anion … evidence … … … [4] (e) If one of the aqueous anions was a bromide, what would be the minimum evidence needed for its identification? … … …[1] [Total: 16] I II III IV

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11 9701/35/O/N/11 © UCLES 2011 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.]

12 9701/35/O/N/11 © UCLES 2011 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) sulfite, 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, Cl2 bleaches damp litmus paper hydrogen, H2 “pops” with a lighted splint oxygen, O2 relights a glowing splint sulfur dioxide, SO2 turns potassium dichromate(VI) (aq) from orange to green

UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS GCE Advanced Subsidiary Level and GCE Advanced Level MARK SCHEME for the October/November 2011 question paper for the guidance of teachers 9701 CHEMISTRY 9701/35 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 must be read in conjunction with the question papers and the report on the examination. • Cambridge will not enter into discussions or correspondence in connection with these mark schemes. Cambridge is publishing the mark schemes for the October/November 2011 question papers for most IGCSE, GCE Advanced Level and Advanced Subsidiary Level syllabuses and some Ordinary Level syllabuses.

Page 2 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – October/November 2011 9701 35 © University of Cambridge International Examinations 2011 Question Sections Indicative material Mark 1 (a) PDO Layout Recording I Pairs of thermometer readings and time unambiguously recorded. Minimum of three pairs. 1 II Correct headings and units. Units must have solidus: /s; brackets: (s); or describe in words: time in seconds or time in s, solidus/°C; or brackets (°C); or describe in words: temperature in °C. No repeats of unit in table to individual readings. 1 III Time recorded to 1 second and temperature to 0.5°C. (Must have at least one at 0.5°C.) 1 MMO Decisions IV Five (minimum) different experiments carried out. 1 V Initial temperatures span the range specified in the question. At least 1 at or below 40°C, at least 1 above 50°C and no two within 3°C (minimum 3 readings). If more than 5 readings can be within 3°C. 1 [5] (b) ACE Interpretation I Correct means and rates for highest 2 temperatures and lowest 2 temperatures. Use candidate’s times (not corrected). 1 PDO Display MMO Quality II 1000/time recorded 3–4sf. Calculate log rate for chosen temperatures. Calculate T3 T2 3 rate log 2 rate log − − to 3 significant figures (factor A). Calculate (T1 – T2) × factor A and add this to log rate 2. Compare this value with candidate’s log rate for T1 and calculate δ. 1 If δ > 0.05 but < or = 0.10 award III, If δ < or = 0.05 award III and IV. Repeat for (T3 – T4) × factor A and subtract from log rate T3. If δ > 0.05 but < or = 0.10 award V, If δ < or = 0.05 award V and VI. If 3 experiments, use slowest 2 for ‘standard’ and award Q marks for fastest (maximum 2) 1 1 1 1 [6]

Page 3 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – October/November 2011 9701 35 © University of Cambridge International Examinations 2011 (c) PDO Layout (i) I All points plotted to use at least 5 large squares on vertical axis and 4 on horizontal axis including position of 20°C. 1 II x-axis to allow extrapolation to 20°C. 1 III An appropriate line of best fit is drawn. 1 ACE Interpretation (ii) IV Correct value to 0.5°C or 1 dp of 1000/t from graph (ignore units). 1 [4] (d) PDO Display Uses temperature values 10 apart from graph and quotes rates/chooses rates that are doubled and quotes temperatures. 1 ACE Conclusions Relevant comment on data made. This can come from experimental results. 1 [2] (e) ACE Interpretation (i) Fastest reaction/first reaction. 1 (ii) Expression for % error ecf from (i). 1 [2] (f) ACE Interpretation Temperature change is not the same for each run of the mixture/FA 2 not at the same temperature as FA 1 before mixing/difficulty of gauging same level of colour/cannot start clock and pour solutions at same time/reusing boiling tubes could affect concentration. Not: human error/heat loss or gain/human reaction time. 1 [1] (g) ACE Improvements Use of thermostatically controlled water bath/data logger for colour intensity/colorimeter/get help from another student. Improvement must correspond to error specified. Not: automatic timer. 1 [1] (h) ACE Improvements Same volume FA 2. 1 Change volume of FA 1 and keep total volume constant by adding water for several volumes of FA 1. 1 All experiments carried out at the same temperature. 1 [3] [Total: 24]

Page 4 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – October/November 2011 9701 35 © University of Cambridge International Examinations 2011 FA 3 is BaCl2 or Ba(NO3)2; FA 4 is H2SO4; FA 5 is NH4Cl + Na2SO3 2 (a) MMO Decisions (i) I Selects named reagent involving CrO4 2– or CO3 2– (solution) or magnesium. 1 PDO Layout (ii) II Tabulates evidence of three tests carried out with no repeat headings (irrespective of reagents). 1 MMO Collection III FA 3 yellow ppt or white ppt or no change. 1 IV FA 4 (yellow solution turns) orange or effervescence or effervescence. 1 V FA 5 yellow solution/no reaction/no reaction. Do not allow NaOH for I but allow observations to include T rise for FA 4. If acid as reagent can score only II. Acidified potassium dichromate is 1 reagent. Do not credit as reagent but credit all observations FA 3 yellow ppt, FA 4 no change, FA 5 green. 1 [5] (b) MMO Collection I FA 3 + FA 4 white ppt. 1 II FA 4 + FA 5 no reaction or slow effervescence. 1 III FA 5 + FA 3 white ppt. 1 IV ppt insoluble in HCl in (i) and soluble in HCl in (iii). 1 [4] (c) MMO Collection Mark vertical columns FA 3 no ppt/ignore faint/slight white ppt. 2 FA 5 no ppt and gas/ammonia turning red litmus blue on warming. FA 4 no visible reaction. Cross out any identified as acid in (a) (iii). [2]

Page 5 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – October/November 2011 9701 35 © University of Cambridge International Examinations 2011 (d) ACE Conclusions All conclusions must follow observations. For each unknown. One mark for ion and one mark for satisfactory evidence. FA 3 must be Ba2+ or Ca2+ to gain credit. FA 3 if CrO4 2– in (a) (i), Ba2+ (1). 1 Evidence: yellow ppt or white ppt with FA 4/H2SO4 and no ppt/(faint) white ppt with NaOH (1) (Must have 2 pieces of evidence.) 1 If CrO4 2– not used in (a) (i) Ba2+ and/or Ca2+ (1). Evidence: faint white/no ppt NaOH and white ppt with FA 4/sulfuric acid/no NH3 when heated with NaOH (1). (Must have 2 pieces of evidence.). FA 5 NH4 + (1). 1 Evidence: formation NH3 in (c) (1). 1 SO3 2– (1). Evidence: formation SO2 in (a) or (b) (1). [4] (e) MMO Decisions Cream ppt (not off white) with AgNO3 (partially soluble/insoluble in aq NH3) 1 [1] [Total: 16]