2024 Prelims CJC H2 Chem P3 (Ans)

9729/03/CJC JC2 Preliminary Examination 2024 [Turn over CANDIDATE NAME CLASS 2T INDEX NUMBER CHEMISTRY 9729/03 Paper 3 Free Response 9 September 2024 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 calculat or is expected, where appropriate. At the end of 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 29 printed pages and 1 blank page. For Examiner’s Use Section A Q1 /20 Q2 /15 Q3 /25 Section B Q4 /20 OR Q5 /20 TOTAL 80 Catholic Junior College JC2 Preliminary Examinations Higher 2 WORKED SOLUTIONS WORKED SOLUTIONS

2 9729/03/CJC JC2 Preliminary Examination 2024 Section A Answer all the questions in this section. 1 (a) Electrocatalysts are a specific form of catalysts that participate in electrochemical reactions. An example is the direct -methanol fuel cells (DMFCs) where methanol is oxidised over an electrocatalyst consisting of a mixture of transition metals such as platinum and rhodium to carbon dioxide. The main reaction occurring at the anode in the DMFCs is shown below: Anode CH3OH(aq) + 6OH‒(aq) → 5H2O(l) + CO2(g) + 6e‒ However, methanoic acid can also be produced at the anode and it can be further oxidised to carbon dioxide. CH3OH + 4OH‒ → HCO2H + 3H2O + 4e‒ HCO2H + 2OH‒ → 2H2O + CO2 + 2e‒ Some standard electrode potentials are shown in Table 1.1. Table 1.1 E o / V HCO2H / CH3OH +0.03 CO2 / HCO2H +0.61 (i) Deduce the type of catalysis in DMFCs. Explain your answer. [1] (ii) Explain how transition metals such as platinum and rhodium can act as catalysts . [1] (iii) Draw a fully labelled diagram of the experimental set-up used to measure the standard electrode potential of the CO2 / CH3OH half-cell. [3] (iv) State the relationship between the standard Gibbs free energy change, ∆G o , and standard cell potential, E𝑐𝑒𝑙𝑙 𝑜 . Use this relationship, the data in Table 1.1 and an energy cycle to calculate the standard electrode potential for the CO2 / CH3OH half-cell. ∆G o can be used in the same manner as ∆H o in a Hess’ law cycle but E o cannot. [3] (v) Platinum(II) complexes are often coloured. Explain why complexes of transition metals are usually coloured. [3] …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… …………………………………………………….……………………………………………. .. …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… (i) The type of catalysis in the reaction used in the DMFCs is heterogeneous catalysis. This is because the catalysts, platinum and rhodium, exist in a different physical state / phase (solid state) as that of the reactants (liquid/gas).

3 9729/03/CJC JC2 Preliminary Examination 2024 [Turn over …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. …………………………………………………….…………………………………………….. (ii) Some transition metals are good catalyst because of the availability of low- lying partially filled 3d orbitals / 3d and 4s electrons to form temporary bonds with reactants molecules during heterogeneous catalysis. (iii) (iv) ΔGo = −nFEocell For HCO2H + 3H2O + 4e‒ → CH3OH + 4OH‒ Eo = +0.03V ΔG2o = −(4)(96500) (+0.03) = ‒11.6 kJ mol‒1 For 2H2O(l) + CO2(g) + 2e‒ → HCO2H + 2OH‒ Eo = +0.61V ΔG3o = −(2)(96500) (+0.61) = ‒117.7 kJ mol‒1 By Hess’s Law, ΔG1o = ΔG2o + ΔG3o = (‒11.6) + (‒117.7) = ‒129.3 kJ mol‒1 Hence, ΔG1o = −nFEocell (‒129300) = −(6)(96500)E Eo = +0.223V

4 9729/03/CJC JC2 Preliminary Examination 2024 …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… …………………………………………………….……………………………………………. .. …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… …………………………………………………….……………………………………………. .. …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… …………………………………………………….……………………………………………. .. …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… …………………………………………………….……………………………………………. .. …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… …………………………………………………….……………………………………………. .. …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… …………………………………………………….……………………………………………. .. …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… …………………………………………………….……………………………………………. .. …………………………………………………….……………………………………………… …………………………………………………….……………………………………………… (v) In the presence of ligands, the partially filled degenerate 3d orbitals are split into 2 groups of non-degenerate 3d orbitals with a small energy gap. When exposed to visible light, a d electron from the lower energy level absorbs a particular wavelength of light and is promoted to the higher energy level. This is known as d-d* electronic transition. The complementary colour of the wavelength absorbed is observed as the colour of the complex

5 9729/03/CJC JC2 Preliminary Examination 2024 [Turn over (b) Chloroacetophenone was for