Cambridge A Level Chemistry 9701 — 2015 May/June Paper 5 · Variant 1

READ THESE INSTRUCTIONS FIRST Write your Centre number, candidate number and name on all the work you hand in. 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. 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 9701/51 Paper 5 Planning, Analysis and Evaluation May/June 2015 1 hour 15 minutes Candidates answer on the Question Paper. No Additional Materials are required. Cambridge International Examinations Cambridge International Advanced Level This document consists of 9 printed pages and 3 blank pages. [Turn over IB15 06_9701_51/FP © UCLES 2015 *3274811489*

2 9701/51/M/J/15 © UCLES 2015 1 A saturated aqueous solution of magnesium methanoate, Mg(HCOO)2, has a solubility of approximately 150 g dm–3 at room temperature. Its exact solubility can be determined by titrating magnesium methanoate against aqueous potassium manganate(VII). During the titration, the methanoate ion, HCOO–, is oxidised to carbon dioxide while the manganate(VII) ion, MnO4 –, is reduced to Mn2+. You are supplied with: a saturated aqueous solution of Mg(HCOO)2 aqueous potassium manganate(VII), KMnO4, of concentration 0.0200 mol dm–3 (a) (i) Write the half equations for the oxidation of HCOO–(aq) to CO2(g) and the reduction of MnO4 –(aq) to Mn2+(aq) in acid solution. … … [2] (ii) Using the approximate solubility above, calculate the concentration, in mol dm–3, of the saturated aqueous magnesium methanoate and the concentration of the methanoate ions present in this solution. [Ar: H, 1.0; C, 12.0; O, 16.0; Mg, 24.3] [2] (iii) In order to obtain a reliable titre value, the saturated solution of magnesium methanoate needs to be diluted. Describe how you would accurately measure a 5.0 cm3 sample of saturated magnesium methanoate solution and use it to prepare a solution fi fty times more dilute than the saturated solution. … … … … [2]

3 9701/51/M/J/15 © UCLES 2015 [Turn over (iv) Before the titration is carried out, dilute sulfuric acid must be added to the magnesium methanoate. Explain why this is necessary and also whether the volume of sulfuric acid chosen will affect the result of the titration. … … … … [2] (v) The potassium manganate(VII) is added from a burette into the magnesium methanoate in a conical fl ask. Describe what you would see when you had reached the end-point of the titration. … … [1] (vi) 1 mol of acidifi ed MnO4 – ions reacts with 2.5 mol of HCOO– ions. 25.0 cm3 of the diluted solution prepared in (iii) required 25.50 cm3 of 0.0200 mol dm–3 potassium manganate(VII) solution to reach the end-point. Use this information to calculate the concentration, in mol dm–3, of HCOO– ions in the diluted solution. … mol dm–3 [1] (vii) Use your answer to (vi) to calculate the concentration, in mol dm–3, of the saturated solution of magnesium methanoate, Mg(HCOO)2. Give your answer to three signifi cant fi gures. … mol dm–3 [1]

4 9701/51/M/J/15 © UCLES 2015 (b) The solubility of magnesium methanoate can be determined at higher temperatures using the same titration. In an experiment to determine how the concentration of saturated magnesium methanoate varies with temperature, name the independent variable and the dependent variable. independent variable … dependent variable … [1] (c) The solubility of magnesium methanoate increases with temperature. What does this tell you about ∆H for the process below? Mg(HCOO)2(s) Mg2+(aq) + 2HCOO–(aq) Explain your answer. … … … … [2] (d) A student used the same titration method, this time to measure the concentration of a saturated solution of barium methanoate. Explain why the acidifi cation of the solution with dilute sulfuric acid might make the titration diffi cult to do. … … [1] [Total: 15]

5 9701/51/M/J/15 © UCLES 2015 [Turn over QUESTION 2 STARTS ON THE NEXT PAGE.

6 9701/51/M/J/15 © UCLES 2015 2 At high temperatures a mixture of iodine and hydrogen gases reacts to form an equilibrium with gaseous hydrogen iodide. H2(g) + I2(g) 2HI(g) (a) (i) Write an expression for the equilibrium constant, Kc, based on concentration, for this reaction. [1] (ii) If the starting concentration of both iodine and hydrogen was a mol dm–3 and it was found that 2y mol dm–3 of hydrogen iodide had formed once equilibrium had been established, write Kc in terms of a and y. [1] (b) The expression for the equilibrium constant from (a)(ii) can be re-written as shown below. y = a 2 K K + c c In an experiment, air was removed from a 1 dm3 fl ask and amounts of hydrogen and iodine gases were mixed together such that their initial concentrations were both a mol dm–3. This mixture was allowed to come to equilibrium at 760 K in the fl ask. The equilibrium concentration of iodine, (a – y) mol dm–3, was then measured. The experiment was repeated for various initial concentrations, a mol dm–3, and the results were recorded in the table below. (i) Complete the table to give the values of y mol dm–3 to three decimal places. a mol dm–3 (a – y) mol dm–3 y mol dm–3 0.200 0.022 0.178 0.500 0.050 0.800 0.252 1.000 0.200 1.500 0.365 2.100 0.570 2.800 0.652 3.400 0.700 3.800 0.867 4.200 0.868 4.900 1.150 [2]

7 9701/51/M/J/15 © UCLES 2015 [Turn over (ii) Plot a graph to show how y mol dm–3 varies with initial concentrations of hydrogen and iodine, a mol dm–3. 5.0 4.0 3.0 2.0 1.0 0 0 1.0 2.0 3.0 a / mol dm–3 y / mol dm–3 4.0 5.0 [1] (iii) Use your points to draw a line of best fi t. [1]

8 9701/51/M/J/15 © UCLES 2015 (c) (i) Determine the slope of your graph. State the co-ordinates of both points you used for your calculation. Record the value of the slope to three signifi cant fi gures. co-ordinates of both points used … … slope = … [2] (ii) Use the value of your slope and the equation in (b) to calculate the value of Kc. Your working must be shown. [2] (d) Explain why, for safety reasons, it is necessary to remove air from the 1 dm3 fl ask. … … [1] (e) One of the experiments in (b) was repeated in a 500 cm3 fl ask instead of the 1 dm3 fl ask. What effect, if any, would this have on the rate of reaction and the value of Kc measured? … … … … … [2]

9 9701/51/M/J/15 © UCLES 2015 [Turn over (f) The reaction of hydrogen and iodine to form hydrogen iodide is exothermic. H2(g) + I2(g) 2HI(g) ∆H = –9.6 kJ mol–1 (i) On your graph, draw and label the line you would expect if the experiment was performed at 1000 K instead of 760 K. [1] (ii) What effect, if any, would the higher temperature have on the value of Kc? … … [1] [Total: 15]

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® IGCSE is the registered trademark of Cambridge International Examinations. CAMBRIDGE INTERNATIONAL EXAMINATIONS Cambridge International Advanced Level MARK SCHEME for the May/June 2015 series 9701 CHEMISTRY 9701/51 Paper 5 (Planning, Analysis and Evaluation), maximum raw mark 30 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 2015 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 A Level – May/June 2015 9701 51 © Cambridge International Examinations 2015 Question Statement Expected Answer Mark 1 (a) (i) M10 HCOO– (aq) ––––> CO2(g) + H+(aq) + 2e– MnO4 –(aq) + 8H+(aq) + 5e– ––––> Mn2+(aq) + 4H2O(l) [1] [1] (ii) M6 Magnesium methanoate is 1.312 mol dm–3 [HCOO–(aq)] = 2.624 mol dm–3 [1] [1] (iii) M6 Use volumetric apparatus (to measure 5.0 cm3 / saturated (magnesium) methanoate solution). Make (the above) up to the mark (with water) in a 250 cm3 volumetric / graduated flask [1] [1] (iv) M3/P4 H+ is needed for the reaction with manganite Provided the acid is in excess / sufficient / enough, the volume does not matter [1] [1] (v) M5 A pale pink colour [1] (vi) M10 0.051 mol dm–3 [1] (vii) M10 1.28 mol dm–3 [1]

Page 3 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2015 9701 51 © Cambridge International Examinations 2015 Question Statement Expected Answer Mark (b) P1/P2 (Independent) Temperature (Dependent) Concentration of magnesium methanoate [1] (c) P3 ∆H is positive (An increase in temperature) will favour / promote / increase / a movement in the direction of the endothermic change / reaction [1] [1] (d) P3 Precipitate is formed / barium sulfate is insoluble / insoluble product [1] [15] 2 (a) (i) D1 [HI]2 Kc = ––––––– [H2][I2] [1] (ii) D1 4y2 Kc = ––––––– (a – y)2 [1]

Page 4 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2015 9701 51 © Cambridge International Examinations 2015 Question Statement Expected Answer Mark (b) (i) D3 a mol dm–3 a – y mol dm–3 y mol dm–3 0.200 0.022 0.178 0.500 0.050 0.450 0.800 0.252 0.548 1.000 0.200 0.800 1.500 0.365 1.135 2.100 0.570 1.530 2.800 0.652 2.148 3.400 0.700 2.700 3.800 0.867 2.933 4.200 0.868 3.332 4.900 1.150 3.750 All results for y are to 3 decimal places All values for y are correct [1] [1] (ii) D1 All points plotted correctly [1] (iii) E5 Appropriate straight line drawn through the origin [1]

Page 5 Mark Scheme Syllabus Paper Cambridge International A Level – May/June 2015 9701 51 © Cambridge International Examinations 2015 Question Statement Expected Answer Mark (c) (i) D3/C1 Co-ordinates read correctly from the line Slope of the graph calculated correctly and given to three significant figures with no units. [1] [1] (ii) D3/C1 Uses



= gradient (value or y / a) and provides working Gives value of Kc [1] [1] (d) P4 The hydrogen with air / oxygen is explosive at 760K / raised temperature [1] (e) E4 Faster reaction / increased rate The value of Kc would be unaffected [1] [1] (f) (i) E4/C2 The line drawn on the graph has a less steep gradient [1] (ii) The equilibrium constant will be smaller [1] [15]