CJC Prelim_H2_CHEM_P2_ans

9647/02/CJC JC2 Preliminary Examination 2016 CATHOLIC JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Higher 2 CANDIDATE NAME CLASS 2T CHEMISTRY 9647/02 Paper 2 Structured Questions Monday 22 August 2016 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class 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, or correction fluid. Answer all questions. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number o f marks is given in brackets [ ] at the end of each question or part question. This document consists of 18 printed pages. ANSWER SCHEME For Examiner’s Use Paper 1 40 Paper 2 Q 1 12 Q 2 15 Q 3 15 Q 4 15 Q 5 15 72 Paper 3 Q 1 20 Q 2 20 Q 3 20 Q 4 20 Q 5 20 80 Total 192

2 9647/02/CJC JC2 Preliminary Examination 2016 1 Planning (P) One method of studying the kinetics of a chemical reaction, to find the order of reaction with respect to a particular reactant , is to measure the initial rates, by using the ‘clock experiment’. To determine the initial rate, we can measure the time taken for a prominent visual change to occur in the course of a reaction. When aqueous sodium thiosulfate, Na 2S2O3, is added to a solution containing dilute hydrochloric acid, HC l, a fine, pale yellow precipitate of solid sulfur will be formed after a while. This is the prominent visual change that can be clearly identified. The overall equation for the reaction is represented as follows: S2O32-(aq) + 2H+(aq)  S(s) + SO2(g) + H2O(l) The initial rate of this reaction is determined by measuring the time taken for sufficient precipitate of sulfur to be formed to just obscure a ‘cross’ marked on a piece of white paper below a reaction container. A student carried out a series of preliminary experiments, using approximate volumes of the two reactants and each reaction mixture was made up to the sa me total volume with deionised water. The student found that the time taken for the pale yellow precipitate to appear doubled when the volume of hydrochloric acid added was halved. (a) (i) State the relationship between the initial rate of reaction and the time taken for the pale yellow precipitate to appear. ................................ ................................ ................................ .......................... ................................ ................................ ................................ ...................... [1] (ii) Explain why it is necessary to top up the reaction mixture with deionised water to the same total volum