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 axes and relevant data in Table 1.1, plot a graph of ln [N 2O5] against time (in second), showing how the concentration of N 2O5 changes with time. [2] (iv) Using your graph, determine a value for the rate constant, k, for the decomposition of N2O5. State the units of k clearly. [2] (v) Hence determine a value for the half-life of the decomposition of N2O5. State the units clearly. [1] Using the points (100, -0.155) and (2850, -1.75) k = – gradient = – ቂ –0.155–(–1.75) 100–2850 ቃ = 5.80 x 10-4 s-1 t1/2 = In2 k = In2 5.80 x 10-4 = 1195 s 0 -1 -2 ln [N2O5] t / s 0 500 1000 1500 2000 2500 3000

4 9729/02/CJC JC2 Preliminary Examination 2018 (vi) Outline another experiment to determine the rate constant, k, for the decomposition of N2O5 in tetrachloromethane. You are provided with the same solution of N 2O5 used in the experiment described in (a). No details regarding use of specific glassware are required. ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ........................................................................................................................ [2] (b) The NO 2 produced in the decomposition reaction in (a) can exist in equilibrium with dinitrogen tetroxide (N2O4): N2O4(g) ⇌ 2NO2(g) An experiment was conducted at 25 oC by varying initial concentrations of N 2O4 and NO2 contained in a closed reaction vessel. The initial and equilibrium concentrations of the two gases are shown in Table 1.2. Experiment No Initial concentration / mol dm-3 Equilibrium concentration / mol dm-3 [N2O4] [NO 2] [N 2O4] [NO 2] 1 0.000 0.200 0.0898 0.0204 2 0.600 0.040 0.594 0.0523 3 0.500 0.030 0.491 0.0475 4 0.446 0.050 0.448 0.0457 5 0.670 0.000 0.643 0.0547 (i) State Le Chatelier’s Principle. ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ........................................................................................................................ [1] Table 1.2 Le Chatelier's Principle states that if a change (e.g. change in concentration, pressure and temperature) is made to a system in equilibrium, the system reacts in such a way as to tend to oppose the change, and a new equilibrium is formed. 1. Measure the volume of gases (NO 2 and O2) produced OR the colou r intensity of the brown NO 2 gas (using a colorimeter) at regula r time intervals from the start of reaction 2. Plot a graph of “volume of gases produced against time” OR “colour intensity against time” and determine the half-life from the graph. 3. Use the equation t1/2 = ૛ ࢑to determine the value of k.

5 9729/02/CJC JC2 Preliminary Examination 2018 [Turn over (ii) State what will be observed when the pressure in the reaction vessel is decreased. ............................................................................................................................ ........................................................................................................................ [1] (iii) Identify the experiment that gives the initial concentration of N 2O4 : NO2 in the ratio 15:1. ........................................................................................................................ [1] (iv) Based on the experiment identified in (b)(iii), calculate a value for the equilibrium constant, Kc. [1] Kc = ሾNO2ሿ2 ሾN2O4ሿ