NYJC 2021 H2 Chemistry 9729 P3

[Turn Over NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS TUTOR’S NAME CHEMISTRY 9729/03 Paper 3 Free Response 14 September 2021 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. If additional space is required, you should use the pages at the end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. 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. For Examiner’s Use 1 /22 2 /18 3 /20 4 /20 5 /20 Total /80 This document consists of 31 printed pages..

2 H2 Chemistry 9729/03 NYJC J2/2021 PRELIM. Section A Answer all the questions in this section. 1 Phosgene, COCl2, is a colourless gas that has the odour of freshly cut hay. It is a major industrial chemical used to make plastics and pesticides. It is highly toxic and was used extensively during World War I as a choking agent. (a) (i) Draw a dot-and-cross diagram of the phosgene molecule. [1] (ii) The actual value of the Cl-C-Cl bond angle is 117 o. Using the principles of the VSEPR theory, x deduce the expected Cl-C-Cl bond angle. x compare the two values and suggest a reason for the discrepancy. [2] …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………...........

3 H2 Chemistry 9729/03 NYJC J2/2021 PRELIM. [Turn Over (b) As phosgene is highly toxic and its odour may not be n oticeable at low concentration, reactions involving phosgene must be carried out in a fume hood equipped with a method of monitoring phosgene concentration. A simple method is to tape phosgene indicator papers, which changes colour upon exposure to gaseous phosgene, to the fume hood sill. The detection limit of one such indicator paper is 1.87 ppm (by volume). One part per million (ppm) denotes one part per 106 parts. Calculate the concentration of phosgene, in mg dm–3, that can be detected in the air by the phosgene indicator paper at room temperature and pressure. [2] …………………………………………………………………………………………………………. …………………………………………………………………………………………………………. …………………………………………………………………………………………………………. …………………………………………………………………………………………………………. …………………………………………………………………………………………………………. …………………………………………………………………………………………………………. …………………………………………………………………………………………………………. …………………………………………………………………………………………………………. …………………………………………………………………………………………………………. (c) Phosgene can be converted into urea, CO(NH2)2, which can be used to remove oxides of nitrogen from the flue gases of power stations where hydrocarbon fuels are burnt. (i) After conversion, urea only acts as a base when reacted with strong acids. Suggest an explanation. [1] (ii) Explain why it is important to remove oxides of nitrogen from the flue gases. [1] …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………...........

4 H2 Chemistry 9729/03 NYJC J2/2021 PRELIM. (d) Phosgene was first prepared in 1811 by the photochemical reaction of carbon monoxide and chlorine in the presence of a catalyst at 150 oC. CO(g) + Cl2(g) COCl2(g) 'Hreaction (i) Calculate a theoretical value for 'Hreaction using bond energies from the Data Booklet. [2] (ii) The actual value of 'Hreaction is −107.6 kJ mol−1. The two values are different as bond energies used in calculation of the theoretical value in (d)(i) are average values. Identify which bond accounts for why the actual value is more exothermic. Explain your reasoning. [1] (iii) By considering the entropy change during the preparation of phosgene, suggest how the Gibbs free energy change of the reaction varies with temperature. Hence, justify fully the conditions used for the reaction. [3] (iv) The mechanism for the formation of phosgene is thought to involve three steps: Cl2 2 Cl Cl + CO COCl COCl + Cl2 COCl2 + Cl Suggest the rate equation for the formation of phosgene that would result from the given mechanism. [1] (v) Define, with the aid of an equation, the term standard enthalpy change of formation of liquid phosgene. [2] (vi) Use the following data, together with actual value of 'Hreaction given in (d)(ii), to calculate the standard enthalpy change of formation of liquid phosgene. value / kJ mol−1 Standard enthalpy change of formation of CO(g) −110.5 Standard enthalpy change of vaporisation of COCl2(l) +33.3 [2] …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... …………………………………………………………………………………………........... Fast Fast Slow

5 H2 Chemistry 9729/03 NYJC J2/2021 PRELIM. [Turn Over …………………………………………………………………………………………........... ………………………………………………………………………………………….......