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) + 2Fe3+(aq) [1] Step 2: 2Fe3+(aq) + 2I–(aq) → I2(aq) + 2Fe2+(aq) [1] (b) Ferrocene is a highly stable compound in which the iron( II) ion is located between two cyclopentadienyl anion rings. Studies have shown that the carbon−iron bonding in ferrocene is capable of free rotation. Ferrocene

3 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 [Turn over (i) Suggest an alternative structure of ferrocene. [1] [1] The methylene hydrogen atoms shown in cyclopentadiene below are acidic. Cyclopentadiene can dissociate a proton to form the cyclopentadienyl anion. cyclopentadiene (ii) Write an expression for the acid dissociation constant, Ka, for cyclopentadiene. [1] [1] Ka = (iii) Suggest the number of delocalised  electrons in the cyclopentadienyl anion. [1] [1] 6 Comment: (iv) Similar to benzene, ferrocene undergoes electrophilic substitution with acetic anhydride and phosphoric acid to form the compound W.

4 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 compound W The steps of the mechanism are as described. Step 1: Acetic anhydride and phosphoric acid react to form the acylium ion. Step 2: Addition of the acylium ion to one of the carbon atoms on the cyclopentadienyl anion ring in ferrocene forms an intermediate. Step 3: Loss of a proton from the intermediate to form W. Suggest the mechanism for steps 2 and 3. Show all charges and relevant lone pairs and show the movement of electron pairs by using curly arrows. [2] [1] Step 2: [1] Step 3:

5 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 [Turn over (c) Hydrogenation of alkenes can be catalysed by the rhodium (Rh) -containing catalyst, Wilkinson’s catalyst, RhL3Cl, where L is the triphenylphosphine ligand. triphenylphosphine (i) Explain using bonding why triphenylphosphine is insoluble in water. [2] Triphenylphosphine has simple molecular structure. Energy produced from [✓] instantaneous dipole - induced dipole attraction (id- id) between triphenylphosphine and water is [✓] insufficient to overcome the [✓] id-id between triphenylphosphine and [✓] hydrogen bonds between water. 2[✓] = [1] The following steps take place when Wilkinson’s catalyst is used to carry out the hydrogenation of ethene. Hydrogen is reduced to hydride in step 1. Step 1: Step 2: Step 3: Step 4: Step 5:

6 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 With consideration of the oxidation state at the Rh metal centre, (ii) identify the step which involves an oxidative addition. [1] [1] Step 1 (iii) identify the step which involves a reductive elimination. [1] [1] Step 5 (iv) Step 4 of the reaction is the rate-determining step. Suggest why the rate of reaction is slower when 2,3 -dimethylbut-2-ene is used in place of ethene. [1] 2,3-dimethylbut-2-ene The rate of reaction will be slower as there is [1] greater steric hindrance around the metal center for the more highly substituted 2,3-dimethylbut-2-ene. (d) Triphenylphosphine, PPh3, is used as nucleophile in the Wittig reaction. In the Wittig reaction, a carbonyl compound reacts with a halogenoalkane to form an alkene. The conversion is shown in the following unbalanced equation. Suggest a three -step synthesis involving the Wittig reaction to convert cyclohexene to compound Q. [3] 5 = 3 mark 3-4 = 2 marks 2 = 1 mark [Total: 20]

7 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 [Turn over 2 This question involves various components found in different types of aerosol propellants and sprays. (a) (a) Dimethylether, CH3OCH3, is a colourless gas that is used as an aerosol propellant. Dimethylether can react with carbon dioxide to form carbon monoxide and hydrogen, as shown by the equation below. CH3OCH3(g) + CO2(g) ⇌ 3CO(g) + 3H2(g) ∆H = +245 kJ mol–1 (i) Write an expression for the equilibrium constant, Kp, for this reaction. Give the units for Kp. [2] Kp = (PCO)3 (PH2 )3 PCH3OCH3. PCO2 [1] atm4 or Pa4 [1] In an experiment conducted at 550 K, a mixture of CH3OCH3 and CO2 was introduced into in a 2 m 3 sealed vessel. The initial total pressure was 12 atm at this temperature. The reaction was allowed to reach dynamic chemical equilibrium. (ii) At equilibrium