CJC_H2_CHEM_P3_QP Prelim

CATHOLIC JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response Thursday 27 August 2015 2 hours Additional Materials: Data Booklet Answer Paper READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen on both sides of the paper. [PILOT FRIXION ERASABLE PENS ARE NOT ALLOWED] You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. Write your answers on the answer paper provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. This document consists of 14 printed pages and 0 blank page. [Turn over 9647/03/CJC JC2 Preliminary Exam 2015

2 9647/03/CJC JC2 Preliminary Exam 2015 Answer any four questions. 1 Butanone is an industrial organic solvent used in the manufacture of plastics, lacquers and varnishes. (a) Butanone reacts with iodine in the presence of an acid according to the equation: CH3CH2COCH3 + I2  CH3CH2COCH2I + HI In a series of experiments, the reaction was carried out with different concentrations of reagents, and the following initial rates were obtained: Expt. [CH3CH2COCH3] / mol dm–3 [I2] / mol dm–3 [H+] / mol dm–3 Initial rate / mol dm–3 min–1 1 0.010 0.010 0.010 8.31 x 10–8 2 0.010 0.010 0.013 10.8 x 10–8 3 0.013 0.010 0.013 14.0 x 10–8 4 0.018 0.013 0.018 26.9 x 10–8 (i) Using the data given, deduce the order of reaction with respect to each of the three reagents, clearly showing how you arrive at your answers . Hence, write a rate equation for the reaction. (ii) Explain how the rate of reaction would change if the solution was diluted with an equal volume of inert solvent, using r to represent the original rate before dilution. (iii) The reaction can also be carried out in the presence of a strong base instead of an acid. State what would be observed when a strong base is used and w rite an equation for the reaction that occurs. [7] (b) Elemental iodine, I2, has limited solubility in water. Its solubility is greatly increased in a solution containing iodide ion, I–, due to the formation of triiodide ion, I3 –, as shown: I2(aq) + I–(aq) ⇌ I3 –(aq) (i) Write an expression for the equilibrium constant, Kc, of this reaction, stating its units. (ii) Experiments have shown that when [ I– ] = 1.4 x 10–3 mol dm–3, the concentrations of I2 and I3 – are equal. Use this information to calculate a value for Kc. (iii) Using your answer from (b)(ii), calculate the concentration of iodide ions necessary for 99 % of the iodine to be dissolved. [4]

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