2024 Prelims SAJC H2 Chem P2 (Ans)

1 [TURN OVER ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS HIGHER 2 CANDIDATE NAME CLASS 2 3 S CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/02 27 August 2024 2 hours READ THESE INSTRUCTIONS FIRST Write your name and class on all the work that you hand in. Write in dark blue or black pen. You may use a 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 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. This document consists of 22 printed pages (including this cover page). For Examiner’s Use Q1 8 Q2 17 Q3 11 Q4 14 Q5 25 Total 75

2 [TURN OVER 1 Hydrogen sulfide, H2S is a toxic gas which is commonly formed as a by -product in oil and gas refining industries. The Claus process has been widely used to remove H2S. It involves two main reactions as shown. Reaction 1: 2H2S(g) + 3O2(g) → 2SO2(g) + 2H2O(l) ΔHθ1 Reaction 2: 2H2S(g) + SO2(g) → 3 8S8(s) + 2H2O(l) ΔHθ2 = − 108 kJ mol−1 (a) (i) Use the data in Table 1.1 to calculate the standard enthalpy change of reaction for Reaction 1, ΔHθ1. Table 1.1 Compound ΔHθf / kJ mol−1 H2S(g) −20.2 SO2(g) −296.8 H2O(l) −285.8 [2] Ho1 = [2(–296.8) + 2(–285.8)] – (2 x – 20.2) [1] = – 1120 kJ mol–1 (to 3 sf) [1] (ii) The standard Gibbs free energy, Go2, for Reaction 2 is −89.6 kJ mol−1. Calculate the entropy change, So2, for Reaction 2 and explain the significance of its sign with respect to the process that is occurring. [2] Gro = Hro – TSro Sro = (−89.6 + 108)/(–298) = - 0.0617 kJ mol–1 K–1 = - 61.7 J mol–1 K–1 [1 with correct units] ∆S< 0 . The system becomes less disordered as t he number of gaseous particles decreases from 3 to 0. [1] (b) Besides the Claus process, organic solvents like methanol can also be used to remove H2S.

3 [TURN OVER With reference to the intermolecular forces, explain why methanol is a good solvent to remove H2S. [2] Energy released from the permanent dipole -permanent dipole interactions between H 2S and methanol molecules is sufficient to overcome the hydrogen bonds between methanol molecules and the permanent dipole-permanent dipole interactions between H2S molecules. Hence, H2S is soluble in methanol. [2] – 4 points [1] – 2, 3 points (c) One of the key advantages of the Claus process is the recovery of S 8, which is a valuable product that can be used for other purposes. S8 consists of 8 sulfur atoms bonded to each other in a crown confo