DHS Prelim P3_Ans

Dunman High School 2017 Year 6 H2 Chemistry Preliminary Examination Paper 3 (Answer Scheme) Section A 1 (a) (i) 1. The gas particles have negligible volume compared to the volume of the container. 2. There are no intermolecular forces of attraction between gas particles. Its electron cloud size is larger hence volume of NO 2 molecules is significant compared to the volume of the gas, unlike H2. Both NO 2 and H 2 have simple molecular structure. Since NO 2 has stronger permanent dipole -permanent dipole interaction than instantane ous dipole - induced dipole interactions in H2 OR NO2 has a larger electron cloud size than H2, the electron cloud of NO 2 is more polarisable and hence stronger intermolecular forces of attraction. (ii) 𝑝V = nRT = (m/M)RT 𝑝 = (m/V)(RT/M) 𝑝 ρ = RT/M At 100 °C and very low pressure, RT/M = 110.3 M = (8.31 x 373) / 110.3 = 28.1 g mol–1 Mr = 28.1 (iii) CO (b) NO2 + SO2 → SO3 + NO NO + ½ O2 → NO2 NO2 catalyses by oxidising SO 2 to SO3, while itself is reduced to NO. NO is rapidly re-oxidised to NO2 by oxygen, regenerating the catalyst, NO2. SO3 + H2O → H2SO4 OR SO3 dissolves in the water vapour in the atmosphere/ rain to form sulfuric acid, which causes acid rain. (c) (i) Order w.r.t [O2] is zero as the graph of [O2] against time is a downward sloping straight line / the gradient of the line, i.e. rate of reaction, is constant with changing [O2]. When [SO2] = 0.8 mol dm-3, r1, rate of reaction = 0.04 0.05 88  = 1.13 x 10-4 mol dm–3 s–1 When [SO2] = 1.2 mol dm-3, r2, rate of reaction = 0.030 0.05 80  = 2.50 x 10-4 mol dm–3 s–1 r2/r1 = 2.21 ≈ 2.25 When [SO2] x 1.5 times, rate of reaction x 2.25 times, reaction is second order w.r.t [SO2].

2 © DHS 2017 9729/03 [Turn Over (ii) rate = k[SO2]2 When [SO2] = 0.8 mol dm-3, k = 1.77 x 10-4 mol-1 dm3 s-1 (k = 1.74 x 10-4 mol-1 dm3 s-1, if [SO2] = 1.2 mol dm-3) (iii) [SO2] / mol dm-3 (iv) (iii) 2 (a) For hexane / cyclohexane mixture,  Energy produced from instantaneous dipole – induced dipole (id -id) interactions between hexane and cyclohexane molecules after mixing  is just sufficient to overcome id -id interactions between hexane molecules and between cyclohexane molecules before mixing.  Thus no heat change is observed. For hexane / ethanol mixture,  Energy produced from weak id -id interactions between hexane and ethanol molecules after mixing  is insufficient to overcome stronger hydrogen bonds between ethanol molecules before mixing.  Thus heat is absorbed from the surroundings for mixing to occur. (b) (i) bonds broken bonds formed H / kJ mol–1 step 1 1 C=C 1 C–C 1 C–I (–350 – 240) +