2023 TJC H2 Chem Prelim P3 (Ans)

9729 / TJC PRELIM/ 2023 DO NOT WRITE IN THIS MARGIN [Turn over TEMASEK JUNIOR COLLEGE 2023 JC2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME WORKED SOLUTIONS CG / 22 SUBJECT TUTOR’S NAME CHEMISTRY 9729/03 Paper 3 Free Response 14 Sep 2023 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, CG and subject tutor’s name 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 shown clearly. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. 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 ?? printed pages and ?? blank page. For Examiner’s Use Section A Q1 / 20 Q2 / 22 Q3 / 18 Section B (Circle one) Q4 Q5 / 20 TOTAL / 80

2 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC PRELIM/ 2023 DO NOT WRITE IN THIS MARGIN Section A Answer all the questions in this section. 1 The transition metals such as iron and rhodium are known for their catalytic activity. (a) The reaction between S 2O82− ions and I− ions is very slow. If a small amount of aqueous iron(II) ions is added to the mixture, the rate of reaction increases. (i) Write the electronic configuration for iron(II) ions. [1] Fe2+: 1s22s22p63s23p63d6 [1] (ii) Explain why the reaction between S2O82− ions and I− ions is very slow. [1] In the uncatalysed reaction, there is a direct reaction between two similarly charged ions. [1] The electrostatic repulsion between the two negatively charged ions partly causes the reaction to have a high activation energy. (iii) Explain why the iron(II) ions can be described as a homogeneous catalyst. [2] Fe2+ in the same physical state/same phase as the reactants [1], and it is not consumed/used up OR the Fe2+ is regenerated at the end of the reaction [1] (iv) State the property, typical of transition metals, which allows iron(II) ions to behave as a catalyst in this reaction. Include relevant chemical equations to support your answer. [3] [1] The catalytic activity of transition metals in homogenous catalysis depends on their ability to exist in variable oxidation states. The catalysed pathway involves the following two steps with lower Ea. Step 1: 2Fe2+(aq) + S2O82–(aq) → 2SO42–(aq) +