2024 MI PU3 H2 Chem EOY P2 student (final)

Class Adm No Candidate Name: This question paper consists of 22 printed pages and 2 blank pages. 2024 Preliminary Examination Pre-University 3 H2 CHEMISTRY 9729/02 Paper 2 Structured Questions 10 September 2024 2 hours Candidates answer on the Question paper. Additional materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission number 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. 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. Question 1 2 3 4 5 Total Marks 15 17 15 13 15 75

2 1 (a) Table salt can be fortified with iodine to supplement the diet. Common iodine chemicals used to fortify salt are sodium or potassium iodate(V). Aqueous acidified iodate(V) ions, IO3–, react with iodide ions, as shown. IO3– + 6H+ + 5I– → 3I2 + 3H2O The initial rate of this reaction is investigated. Table 1.1 shows the results obtained. Table 1.1 Expt [IO3–] / mol dm–3 [H+] / mol dm–3 [I–] / mol dm–3 initial rate / mol dm–3 min–1 1 0.0400 0.0150 0.0250 4.20 × 10–2 2 0.0200 0.0150 0.0250 2.10 × 10–2 3 0.0200 0.0150 0.0500 8.40 × 10–2 It is known that the reaction is second order with respect to hydrogen ions. (i) Determine the order of the reaction with respect to IO3– and I– ions. Explain your reasoning. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] (ii) Hence, write the rate equation for the reaction. ………………………………………………………………………………………………… [1]

3 [Turn over The reaction is then repeated at the same temperature but at different pH. Table 1.2 lists the concentrations of IO3–, I– and the pH at which the experiments are carried out. Table 1.2 Expt [IO3–] / mol dm–3 [I–] / mol dm–3 pH [H+] / mol dm–3 4 0.0400 0.0250 1.0 5 0.0400 0.0250 2.0 (iii) Complete Table 1.2 with the concentrations of H+ ions in experiments 4 and 5. [1] (iv) Calculate the value of rate at pH 1.0 rate at pH 2.0. rate at pH 1.0 rate at pH 2.0 = …………………….……..[1]

4 (b) In an aqueous solution, iron(III) ions react with iodide ions as shown. 2Fe3+ + 2I– → 2Fe2+ + I2 The kinetics of this reaction were investigated and below are some information about the reaction  The reaction is first order with respect to Fe3+ and second order with respect to I–.  The mechanism for this reaction has three steps.  Each step involves only two ions reacting together. The first step of the mechanism has been written for you. Suggest equations for the remaining two steps of this me chanism and identify the rate-determining step. step 1: Fe3+ + I– ⇌ FeI 2+ step 2: .……………………………………………………………………………………………….. step 3: .……………………………………………………………………………………………….. rate-determining step: …………………. [3] (c) Fumaric acid is a white solid that occurs widely in nature and has a fruit -like taste and hence used as a food additive. Maleic acid is a stereoisomer of fumaric acid. fumaric acid maleic acid (i) State the type of isomerism that fumaric acid and maleic acid shows. ………………………………………………………………………………………………… [1]

5 [Turn over (ii) Fumaric acid has a melting point of 286 °C while maleic acid has a melting point of 131 °C. With the aid of a suitable diagram, suggest why maleic acid has a lower melting point than fumaric acid. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] (iii) In the industry, maleic acid is obtained by reacting maleic anhydride, C 4H2O3, with water. Suggest a possible structure of maleic anhydride. [1]

6 The atom economy of a reaction is a measure of the amount of starting materials that end up as useful products. It is important for sustainable development and for economic reasons to use reactions with high atom economy. The percentage atom economy of a reaction is calculated with the following equation. atom economy = mass of desired product total mass of products ×100% Maleic anhydride can be obtained industrially by the oxidation of butane (reaction 1.1) or benzene (reaction 1.2). reaction 1.1 C4H10 + 3½ O2 → C4H2O3 + 4H2O reaction 1.2 C6H6 + 4½ O2 → C4H2O3 + 2H2O + 2CO2 (iv) Calculate the atom economy for each reaction and suggest which reaction is preferred. [2] (v) Suggest another reason why the reaction you selected in (iv) is preferred. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………[1] [Total: 15]

7 [Turn over 2 The Andrussow process involves heating a mixture of methane, ammonia and oxygen over a platinum catalyst at 1200 °C to generate hydrogen cyanide. 2CH4 + 2NH3 + 3O2 → 2HCN + 6H2O (a) (i) Draw a dot-and-cross diagram of HCN and state its shape. shape = ………………………………………. [2] (ii) Given that the oxidation number of hydrogen remains at +1 throughout the reaction, complete Table 2.1 with the oxidation numbers of carbon, nitrogen and oxygen in the reactants and products. Table 2.1 carbon nitrogen oxygen oxidation number of element in reactant oxidation number of element in product [2] (b) Hydrogen cyanide is a weak acid in aqueous solution. (i) State what is meant by the term weak acid. …………………………………………………………………………………………………… ………………………………………………………………………………………………… [1]

8 (ii) The pH of a solution of 0.10 mol dm–3 HCN is 5.10. Calculate the acid dissociation constant, Ka, of HCN. [2] (c) The reaction between propanone and HCN produces 2-hydroxy-2-methylpropanenitrile. (CH3)2CO + HCN → (CH3)2C(OH)CN (i) Describe the mechanism for this reaction between propanone and HCN. Show all charges and relevant lone pairs and show the movement of electron pairs by using curly arrows. [3]

9 [Turn over (ii) State the hybridisation state of the carbonyl carbon in propanone. ………………………………………………………………………………………………… [1] (iii) Suggest whether 2-hydroxy-2-methylpropanenitrile produced through this reaction will rotate plane-polarised light. …………………………………………………………………………………………………… ………………………………………………………………………………………………… [1] (d) Sodium thiosulfate, Na 2S2O3, is administered as a treatment of cyanide poisoning. It converts the cyanide ion to the less toxic thiocyanate ion . This process is facilitated by the enzyme rhodanese found in the kidney and liver via a two-step process. In the first step, thiosulfate is reduced by the thiol group present on cysteine -247 of rhodanese to form a persulfide. In the second step, the persulfide reacts with cyanide to produce thiocyanate. (i) Explain why the reaction between thiosulfate and cyanide ions occurs slowly in the absence of rhodanese. …………………………………………………………………………………………………… …………………………………………………………………………………………………… ……………………………………………………………………………………