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 volume. ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ ...................... [1] timerate 1 OR Rate of reaction is inversely proportional to time, the time taken for the pale yellow precipitate to appear is low/short if the rate is high. ionconcentratvolume or To ensure that the total volume of the resultant reaction mixture remains the same in all experiments so that the volume of each reactant is proportional to its concentration in the resultant solution . When the volume of that reactant added doubles, its concentration in the resultant solution doubles.

3 9647/02/CJC JC2 Preliminary Examination 2016 (b) Hence, based on the results of the student’s preliminary experiments, predict the order of the reaction with respect to H+ ions. ................................ ................................ ................................ ................................ ... ................................ ................................ ................................ ................................ ... ................................ ................................ ................................ ................................ ... ................................ ................................ ................................ ................................ ... ................................ ................................ ................................ ............................... [1] (c) You are to design an experiment to study the kinetics of the reaction between S2O32- ions and H+ ions. The following data in tabulated form is shown below. experiment volume of 1.00 mol dm-3 Na2S2O3 / cm3 volume of 1.00 mol dm-3 HCl / cm3 volume of deionised water / cm3 time taken for the ‘marked cross’ to be obscured / s 1 10 20 20 t1 2 20 20 10 t2 3 10 10 30 t3 (i) Fill in the blanks in the table above, appropriate volumes of the reac tants and deionised water to be used in experiments 2 and 3 such that the order of reaction with respect to S2O32- and H+ ions can be determined and verified. [1] Appropriate volumes of reactants and deionised water stated correctly in experiments 2 and 3. (ii) Outline in a series of numbered steps, how experiment 1 could be carried out. Your plan should include:  the apparatus used to measure the various volumes,  the order that the various solutions are mixed,  how the time to determine the rate of reaction is measured, and  other experimental details to ensure the consistency of the experiment. Based on the table in (c)(i), e xplain how the results of any two of the three experiments can be used to determine the order of reaction with respect to S2O32- ions. ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... The time taken for the pale yellow precipitate to appear doubled when the volume of hydrochloric acid added was halved. Since ionconcentratvolume  and timerate 1 , when the concentration of hydrochloric acid added was halved, the rate was halved. ∴ rate of reaction  [H+] or the reaction is first order with respect to H+.

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