PJC H2 Chem 2013 Prelim P2 Soln

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 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 Suggested Answers 1 2 983

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. 4.00 < mass of FA 5 (x) < 16.00 [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 1. Using a wei ghing balance, weigh out accurately x g of FA 5 in a test tube labelled FA 5. Record the mass in the table below. Mass of test tube + FA 5 / g Mass of empty test tube / g Mass of FA 5 used / g x 2. Measure 50 cm 3 of deionised water using a (50 / 100 cm 3) measuring cylinder into a styrofoam cup. 3. Using a (0.2 oC) thermometer measure and record the initial temperature of the water in the cup, T1 oC. 984

3 © PJC 2013 9647/02/JC2 Prelim/2013 4. Tip the content of the test tube labelled FA 1 into the water in the cup. Stir the mixture with a thermometer and record the lowest temperature reached, T2 oC. 5. Discard the solution, rinse and dr y the cup. Repeat steps 2 to 4 for FA 2 to FA 5. 6. Record all measurements and results in the table below. Test tube Mass of KI / g Initial temperature / °C Minimum temperature / °C ∆T / oC FA 1 2.00 FA 2 4.00 FA 3 6.00 FA 4 8.00 FA 5 7. Plot a graph of ∆T or l∆Tl against mass of KI. [5] (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] Percentage by mass of KI in FA 5 = (y /mass of FA 5 used) x 100% (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. When larger masses of FA 1 to FA 5 are used, a larger l ∆Tl is obtained. This results in a smaller percentage error in temperature measurement in the student’s experiment. Thus the second student’s experiment would give a more reliable determination of percentage by mass of KI in FA 5. [2] 985

4 © PJC 2013 9647/02/JC2 Prelim/2013 (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. Heat absorbed = 100 x 4.18 x 3 = 1254 J Amount of K I dissolved = 12/166.1 = 0.0722 mol ∆H solution = +100 x 4.18 x 3 = +17.4 kJmol-1 (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). The smaller/lower magnitude of ∆H solution of KI compared to the published value is due to heat gained / abs orbed from the surroundings and the Styrofoam cup will cause lTl to be smaller. [2] [Total: 12] 986

5 © 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, iodine 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. Since the total volume of reac tion mixture is constant (100 cm 3), the volume of each reagent used is directly proportional to its concentration. 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 Experiment Number Time for the appearance of blue-black colour / s Relative rate  1/t 1 23 0.0435 2 70 0.0143 3 47 0.0213 4 35 0.0286 987

6 © PJC 2013 9647/02/JC2 Prelim/2013 Comparing expt 2 & 3: [H2O2] and [H+] are kept constant. When [I-] increases by 1.5 times, the initial rate al so in