CJC Prelim P2_Answers

9729/02/CJC JC2 Preliminary Examination 2018 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/02 Paper 2 Structured Questions Friday 17 August 2018 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, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. At the end of 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 19 printed pages. For Examiner’s Use Paper 1 20 Paper 2 15 Paper 3 Q1 /20 Q2 /20 Q3/4 /20 60 Units/ Significant Figures TOTAL 95 OVERALL (100%) Grade Catholic Junior College JC2 Preliminary Examination Higher 2 WORKED SOLUTIONS

2 9729/02/CJC JC2 Preliminary Examination 2018 Answer all the questions in the spaces provided. 1 In atmospheric chemistry, NO x is a generic term for the nitrogen oxides that are most prevalent for air pollution, namely nitric oxide (NO) and nitrogen dioxide (NO 2). These gases contribute to the formation of smog and acid rain, as well as tropospheric ozone. In the atmosphere, dinitrogen pentoxide (N 2O5) is an important reservoir of the NO x species that are responsible for ozone depletion. (a) In the laboratory, the kinetics involving the decomposition of N 2O5 into NO 2 and O 2 can be investigated by dissolving it in an organic solvent such as tetrachloromethane, CCl4. The decomposition of N2O5 was found to be a first-order reaction. 2N2O5 → 4NO2(g) + O2(g) Table 1.1 below shows the variation of [N2O5] with time. (i) State the rate equation for the decomposition of N 2O5. ........................................................................................................................ [1] (ii) The rate equation for the decomposition of N 2O5 can also be expressed as: ln [N2O5]t = –kt + ln [N2O5]o where [N2O5]o is the initial concentration of N2O5 and [N2O5]t is the concentration of N2O5 at time, t. Using relevant data in Table 1.1, calculate the values for ln [N2O5] and complete Table 1.1. [1] Time / s [N 2O5] / mol dm-3 ln [N2O5] 0 0.910 -0.0943 300 0.750 -0.288 600 0.640 -0.446 1200 0.440 -0.821 3000 0.160 -1.83 Rate = k[N2O5] Table 1.1

3 9729/02/CJC JC2 Preliminary Examination 2018 [Turn over (iii) Using the following a