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,