2020 JPJC Prelim P3 answers

 Jurong Pioneer Junior College [Turn Over NAME CLASS 19S JURONG PIONEER JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION 2020 CHEMISTRY 9729/03 Higher 2 Paper 3 Free Response 22 September 2020 2 hours Candidates answer on question paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class and exam index number on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a HB pencil for any diagrams, graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Section B Answer one question. 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 of marks is given in brackets [ ] at the end of each question or part question. This document consists of 30 printed pages.

2  Jurong Pioneer Junior College 9729/03/J2 PRELIMINARY EXAM/2020 Section A Answer all the questions in this section. 1 (a) In a bromine “clock” reaction , solutions of bromate(V) ions, bromide ions, bromine and acid are mixed together with a few drops of red methyl orange. There are two reactions occurring in the reaction mixture. BrO3–(aq) + 5Br–(aq) + 6H+(aq) → 3Br2(aq) + 3H2O(l) Reaction I Br2 + phenol → brominated compound Reaction II When a small but constant amount of phenol is added to the reaction mixture, the bromine being slowly produced by reaction I will immediately react with phenol in reaction II, until all the phenol has been used up. At that point, free bromine will be pres ent in the solution , which will cause a sudden decolourisation of red methyl orange. The time taken for the red methyl orange to decolourise were recorded below. Expt Volume of 0.350 mol dm3 KBrO3 / cm3 Volume of 0.650 mol dm3 KBr / cm3 Volume of 0.250 mol dm3 H2SO4 / cm3 Volume of deionised water / cm3 Time taken for methyl orange to decolourise /s 1 5.0 5.0 10.0 10.0 82 2 10.0 5.0 10.0 5.0 42 3 10.0 10.0 5.0 5.0 83 4 5.0 5.0 20.0 0 21 For Examiner’s Use (i) Explain why the amount of phenol is kept constant in each experiment. [1] As we are measuring the time taken for a fixed same amount of Br 2 to be used up for the reaction with phenol, so the amount of phenol must be kept constant. This time is related to the relative rate, (ii) Deduce the order of reaction with respect to [Br–], [BrO3–] and [H+], explaining your reasoning clearly. [4] 1Rate time

3  Jurong Pioneer Junior College 9729/03/J2 PRELIMINARY EXAM/2020 [Turn Over Since total volume of reaction mixture is constant, volume of reactantconcentration of reactant in the reaction mixture. Eg. Volume of KBrO3