DHS H2 Chem 2013 Prelim P3 Soln

This question paper consists of 17 printed pages and 1 blank page. © DHS 2013 [Turn over Name: Mark Scheme Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY 9647/03 Paper 3 24 September 2013 Paper 2 2 hours Additional Materials: Data Booklet, Graph Paper & Writing Paper INSTRUCTIONS TO CANDIDATES 1 Answer any four questions. 2 Begin each question on a fresh sheet of paper. 3 At the end of the examination: • Staple or fasten all your work securely together with the Cover Sheet on top. • Hand in the question paper separately. INFORMATION FOR CANDIDATES The number of marks is given in brackets [ ] at the end of each question or part question. You are advised to show all workings and calculations. You are reminded of the need for good English and clear presentation in your answers. 534

2 © DHS 2013 9647/03 [Turn Over Answer any four questions. Begin each question on a fresh sheet of paper. 1 Dinitrogen pentoxide, N2O5, can be produced by the following reaction sequence in the car engine: I N2(g) + O2(g) → 2NO(g) ∆HӨ = +180 kJ mol-1; ∆SӨ = +24.8 J mol-1 K-1 II NO(g) + 1/2O2(g) → NO2(g) ∆HӨ = –57.0 kJ mol-1 III 2NO2(g) + 1/2O2(g) → N2O5(g) ∆HӨ = –110 kJ mol-1 (a) (i) Explain why reaction I does not take place at room temperature but occurs in car engine. ∆GӨ = ∆HӨ – T∆SӨ ∆GӨ (+172.6 kJ mol -1) is positive at 298 K, hence reaction I is not spontaneous at room temperature The high temperature in the car engine causes the T ∆SӨ to become more positive (or – T ∆SӨ to become more negative). This will lead to a more negative ∆GӨ and hence reaction I becomes spontaneous. (ii) Predict in reaction II, with reasons, the sign of ∆SӨ. The amount of gases decreases from 1 1/2 mol to 1 mol, hence there are lesser number of ways to arrange the gaseous particles in the system. ∆SӨ is negative. (iii) Nitrogen dioxide is a pollutant that is often produced in car engine. Suggest how the pollutant can be removed in the car engine. Pt 2NO2(g) → N2(g) + 2O2(g) The nitrogen dioxide is converted to nitrogen and oxygen gas in the catalytic convertor (with Pt as catalyst) in the car engine. (iv) By drawing a suitable energy cycle and using the data above, calculate the standard enthalpy change of formation of dinitrogen pentoxide. N 2(g) + 5/2O2(g) N2O5(g) 2NO(g) + 3/2O2(g) 2NO2(g) + 1/2O2(g) By Hess’ law, ∆H f Ө(N2O5) = +180 + 2(–57.0) – 110 = –44.0 kJ mol-1 +180 2(–57.0) –110 ∆Hf Ө(N2O5) 535

3 © DHS 2013 9647/03 [Turn Over (b) The rate of reaction for reaction II was investigated and the following kinetics data was obtained. Time/minutes Exper