2024 Prelims RI H2 Chem P3 (Ans)

© Raffles Institution 2024 9729/03/S/24 2024 Y6 H2 Chemistry Preliminary Exams Paper 3 – Suggested Solutions Section A 1(a)(i) Kc = [SO3]2 [SO2]2[O2] mol−1 dm3 1a(ii) Kc = [SO3]2 [SO2]2[O2] = (4.60 1.00)2 (0.500 1.00 )2(0.100 1.00 ) = 846.4 mol−1 dm3 = 846 mol−1 dm3 (3 s.f.) 1(a)(iii) pV = nRT p = (0.5+0.1+4.6)(8.31)(273+450) 1 1000 = 31.24 x 106 Pa p = 31.2 MPa 1(a)(iv) Partial pressure of SO3 = 4.60 0.5+0.1+4.6 × (31.24 ×106) = 27.6 MPa 1(a)(v) Let y be the amount of O2 added into the system 2 SO2 + O2 ⇌ 2 SO3 Initial amt / mol 0.500 0.1 + y 4.6 Change in amt / mol −0.1 −0.05 +0.1 Equilibrium / mol 0.400 0.05 + y 4.7 Kc is a constant as temperature remained constant At equilibrium, (4.70 1.00)2 (0.400 1.00 )2(0.05+y 1.00 ) = 846.4 mol−1 dm3 4.72 = 846.4(0.4)2(0.05 + y) 0.05 + y = 0.1631 y = 0.113 mol 1(b)(i) When temperature increases, equilibrium position shifts left to favour the backward endothermic reaction to counteract the increase in temperature. Hence, the amount of SO2 will increase.

© Raffles Institution 2024 9729/03/S/24 1(b)(ii) 1(c)(i) 800 K 1(c)(ii) As temperature increases, ∆Gr of reaction 1 becomes more positive. Thus, position of equilibrium of reaction 1 lies more to the left, causing the positions of equilibrium for both reactions 2 and 3 to shift to the left, which result in a lower proportion of H2SO4. 1(d)(i) - An ideal gas consists of particles of negligible volume . The size of the gas particles is negligible compared to the volume of the container. - The gas particles exert negligible attractive forces on one another. - Collisions between gas particles are perfectly elastic. 1(d)(ii) At moderately high pressure, the gas particles come closer together and intermolecular attractive forces between the gas particles become significant. OR At very high pressure, the gas particles are much closer together and the gas occupies a smaller volume. As such, the volume of the gas particles is not negligible as compared to the volume of the container. 1(e)(i) Above Tc, kinetic energy of the gas particles is so high such that the intermolecular forces of attraction are overcome at all pressures. 1(e)(ii) The intermolecular forces of attraction present in steam are stronger hydrogen bonds compared to the weaker instantaneous dipole-induced dipole interactions (id-id) between carbon dioxide molecules.

© Raffles Institution 2024 9729/03/S/24 More energy is required to overcome the stronger hydrogen bonds as compared to weaker id-id, hence steam has a higher critical temperature. 2(a)(i) Standard enthalpy change of formation is the energy change when 1 mole of the pure substance in a specified state is formed from its constituent elements in their standard states under standard conditions of 1 bar and 298 K. 2(a)(ii) By Hess’ Law, (+10