PJC H2 Chem 2013 Prelim P2 QP

1 © PJC 2013 9647/02/JC2 Prelim/2013 PIONEER JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION HIGHER 2 CANDIDATE NAME CT INDEX GROUP NUMBER CHEMISTRY 9647/02 Paper 2 Structured Questions 18 September 2013 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your Centre number, index number and name on all work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough workings. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions. 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. This document consists of 20 printed pages. FOR EXAMINER’S USE Paper 2 1 / 12 5 / 5 2 / 10 6 / 7 3 / 14 Penalty sf units 4 / 24 Total / 72 1 2 963

2 © PJC 2013 9647/02/JC2 Prelim/2013 1 Planning Heat is absorbed when potassi um iodide dissolves in water. Sucrose, on the other hand, does not produce a temperature change when added to water. By studying the temperature changes wh en known masses of potassium iodide dissolve, the percentage by mass of potassi um iodide in a mixture of potassium iodide and sucrose, FA 5 can be determined. You are provided with  a test tube of FA 1 which contains 2.00 g of potassium iodide  a test tube of FA 2 which contains 4.00 g of potassium iodide  a test tube of FA 3 which contains 6.00 g of potassium iodide  a test tube of FA 4 which contains 8.00 g of potassium iodide  an empty test tube labelled FA 5  FA 5, a mixture of potassium iodide and sucrose  a thermometer  a polystyrene cup  other common apparatus in the laboratory (a) The percentage composition of potassium iodide in the mixture, FA5, is estimated to be 50%. Suggest a suitable value for mass of FA 5 used. ……………………………………………………………………………………………. [1] (b) Describe, in a series of numbered steps, how you would measure the temperature changes by dissolving separate samples of FA 1 to FA 5 in water. Hence, determine the mass of potassium iodide in a mixture of FA 5. Your plan should include the following:  the apparatus for measurements  how you would take and record the various measurements  the tables for record ing the data measured  the graph to be plotted ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. 964

3 © PJC 2013 9647/02/JC2 Prelim/2013 ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. [5] 965

4 © PJC 2013 9647/02/JC2 Prelim/2013 (c) Sketch the graph you suggested in (b). Show clearly how the percentage by mass of potassium iodide in FA 5 can be determined. [2] (d) A second student conduct ed a similar experiment using the same apparatus and the same volume of water as in your plan in (b) but using twice the mass of FA 1 to FA 5 . Suggest whether your experim ent or the second student’s experiment would result in a more reli able determination of the percentage by mass of potassium iodide in FA 5. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. [2] (e) (i) A third student measured a temper ature change of 3 °C when he dissolved 12 g of potassi um iodide in 100 cm 3 of water. Assume that the specific heat capacity of t he final solution is 4.18 J g 1 K1, and that its density is 1.00 g cm 3, calculate the enthalpy change of solution of potassium iodide. 966

5 © PJC 2013 9647/02/JC2 Prelim/2013 (ii) The published value for the enthalpy change of solution of potassium iodide is +20.3 kJ mol 1. Suggest an explanation for the difference between the published value and the value calculated in (i). ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… [2] [Total: 12] 967

6 © PJC 2013 9647/02/JC2 Prelim/2013 2 In a typical iodine clock reaction, soluti ons of hydrogen peroxide, sulfuric acid, potassium iodide, starch and sodium thiosu lfate are mixed. There are two reactions occurring in the reaction mixture. In the first reaction, iodide ions are oxidised slowly by acidified hydrogen peroxide to form iodine. H2O2(aq) + 2I(aq) + 2H+(aq) → I2(aq) + 2H2O(l) reaction I In the second reaction, iodine is reduced back to iodide ions as they are generated. 2S2O3 2(aq) + I2(aq) → S4O6 2(aq) + 2I(aq) reaction II When all the sodium thiosulfate present has reacted, iodi ne will appear in the solution producing an immediate deep blue colour with starch indicator. A series of experiments was carried out us ing different volumes of the five reagents, each solution being made up to the same to tal volume with water. The following results were obtained. (a) Calculate the relative initial rate of reaction I for each of these experiments. [1] (b) Given that the order of reaction with respect to hydrogen peroxide is found to be one, use the results in (a) to deduce the order of reaction with respect to I and H+ in reaction I. Explain your reasoning. [2] expt no. volume of 0.10 mol dm -3 H2O2 / cm3 volume of 0.100 mol dm-3 KI / cm3 volume of 0.100 mol dm-3 HCl / cm3 volume of 0.050 mol dm-3 Na2S2O3 / cm3 volume of starch solution / cm3 volume of water / cm3 time for the appearance of deep blue colour / s 1 18 12 6 2 3 59 23 2 6 12 12 2 3 65 70 3 6 18 12 2 3 59 47 4 12 12 12 2 3 59 35 968

7 © PJC 2013 9647/02/JC2 Prelim/2013 (c) Hence write an overall rate equation for reaction I, stating the units of the rate constant. ……………………………………………………………………………………………. ……………………………………………………………………………………………. [1] (d) Suggest, with reasons, which of the following two mechanisms, A or B, fits the observed kinetics data. Mechanism A 2H+(aq) + 2I(aq) → 2HI (aq) (fast) 2HI(aq) + H2O2 (aq) → I2(aq) + 2H2O(l) (slow) Mechanism B H 2O2 (aq) + I(aq) → H2O(l) + OI(aq) (slow) H+(aq) + OI(aq) → HIO(aq) (fast) HIO(aq) + I(aq) + H+(aq) → I2(aq) + H2O(l) (fast) ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. ……………………………………………………………………………………………. [2] 969

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