2024 MI PU3 H2 Chem EOY P2 answers (final) w EC

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. C ommon 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. For Examiners’ Use (i) Determine the order of the reaction with respect to IO3– and I– ions. Explain your reasoning. [2] Using Expt 1 and 2, where [H+] and [I–] are kept constant, When [IO3–] doubles, the initial rate doubles. Hence the reaction is 1st order with respect to IO3–. [1] Using Expt 2 and 3, where [H+] and [IO3–] are kept constant, When [I–] doubles, the initial rate quadruples. Hence the reaction is 2nd order with respect to I–. [1] Examiners’ Comments: - Majority of students were able to solve this question, with most using the easier “by observation” method. Students using this method are advised to use ‘increases’ (e.g. doubled / 2 times) instead of ‘decreases’ (e.g. halved / ½ times) as it becomes confusing when the ratio is not a nice 2x or 4x. - Students who did not make reference to relevant experiments were penalised. (ii) Hence, write the rate equation for the reaction. [1] rate = k [IO3–][H+]2[I–]2 [1] Examiners’ Comments: - Majority of students were able to solve this question. The most common mistake was omitting H + from the equation due to not recognising the significance of it being in Table 1.1, and also not reading the question properly.

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 0.100 5 0.0400 0.0250 2.0 0.0100 (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. [1] rate at pH 1.0 rate at pH 2.0 = (0.100)2 (0.0100)2 = 100 [1] with working, ecf from (iii) Examiners’ Comments: - Majority of students were able to solve this question . Some students however, went through the unnecessarily longer method of determining the rate via a ratio method using Expt 1, not recognising that the other values would all cancel out using Expts 4 & 5.

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 mechanism and identify the rate-determining step. [3] step 1: Fe3+ + I– ⇌ FeI 2+ step 2: FeI2+ + I– → Fe2+ + I2– OR FeI2+ + I– → FeI2+ [1] step 3: Fe3+ + I2– → Fe2+ + I2 OR Fe3+ + FeI2+ → 2Fe2+ + I2 [1] rate-determining step: 2 [1] Examiners’ Comments: - Majority of students were unable to solve this question, with many failing to utilise the information that “each step involves only two ions reacting together”. Many students also did not ensure that their atoms and charges for each step were balanced, and that their overall reaction equation matched the one given in the question stem. (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] cis-trans isomerism [1] Examiners’ Comments: - Question was well done.

5 [Turn over [1] (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] Intramolecular hydrogen bonds form between the two COOH groups due their proximity to each other. [1] diagram + explanation This results in less extensive intermolecular hydrogen bonds forming and hence maleic acid has a lower melting point. [1] Examiners’ Comments: - Majority of students were unable to recognise that intramolecular hydrogen bonding was the reason for the huge difference in melting point. - The weakest students explained using reasons that resulted in fumaric acid having a lower melting point instead. (iii) In the industry, m aleic acid is obtained by reacting maleic anhydride, C 4H2O3, with water. Suggest a possible structure of maleic anhydride. [1] Examiners’ Comments: - Majority of students were unable to obtain the correct structure, with many having structures with more H atoms than in the given molecular formula. Many students also had a charged structure, which is incorrect. - For the students who attempted to draw a s tructure with the correct molecular formula, many did not consider if the reaction was viable based on their knowledge of OC reactions. This reaction is the hydrolysis of ester, adding “H2O” to the anhydride’s molecular formula.

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.