2023 ACJC H2 Chem Prelim Paper 3 Suggested Solutions

1 © ACJC2023 9729/03/Prelim/2023 Anglo-Chinese Junior College JC2 Preliminary Examination Higher 2 CANDIDATE NAME SUGGESTED SOLUTIONS FORM CLASS TUTORIAL CLASS INDEX NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/03 28 August 2023 2 hours READ THESE INSTRUCTIONS FIRST Write your index number and 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 que stion number must be clearly shown. Section A Answer all questions. Section B Answer one question. Circle the number of the question you have attempted. 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. For Examiners’ use only Section A 1 / 17 2 / 18 3 / 25 Section B 4 / 5 / 20 Total / 80 This document consists of 24 printed pages.

2 © ACJC2023 9729/03/Prelim/2023 Section A Answer all the questions in this section. 1 Ibuprofen belongs to a class of drugs called non-steroidal anti-inflammatory drugs (NSAIDs) used to reduce inflammation, pain and fever. Pain, fever, and inflammation are caused by the release in the body of chemicals called prostaglandins. Ibuprofen block s the enzyme that makes prostaglandins resulting in lower levels of prostaglandins. The following diagram shows the first step in the synthesis route to form ibuprofen from compound A. (a) A series of experiments were carried out at 25 °C to find the effect of initial concentration of hydrogen cyanide, cyanide ions and A on the initial rate of formation of B. Table 1.1 experiment initial concentration / mol dm−3 initial rate of formation of B / mol dm−3 s−1 [A] [HCN] [CN−] 1 0.020 0.060 0.060 1.00 2 0.020 0.050 0.050 0.833 3 0.020 0.050 0.060 1.00 4 0.025 0.050 0.050 1.042

3 © ACJC2023 9729/03/Prelim/2023 (i) Write a balanced equation for the reaction that produces compound B. [1] (ii) Use the data in Table 1.1 to deduce the order of reaction with respect to each of the reactants. [3] Comparing Expt 1 and 3: [A] and [CN−] were unchanged. Initial rate of formation of B remained unchanged when [HCN] was increased by 1.2 times. The reaction is zero order with respect to HCN. Comparing Expt 2 and 4: When [ A] increased by 1.25 times while [H CN] and [CN−] were unchanged, initial rate of formation of B increased by 1.25 times. The reaction is first order with respect to A. Comparing Expt 2 and 3: When [CN −] increased by 1.2 times while [H CN] and [A] were unchanged, initial rate of formation of B increased by 1.2 times. The reaction is first order with respect to CN−. (iii) Write the rate equation for this reaction. Hence, calculate the value of the rate constant using the data in experiment 2. Include its units. [2] Rate = k[A][CN−] Substituting values from expt. 2 0.833 = k(0.02)(0.05) k = 833 mol‒1 dm3 s−1

4 © ACJC2023 9729/03/Prelim/2023 (iv) Describe a mechanism that is consistent with the rate equation deduced in (a)(iii) and indicate which step in the mechanism is the rate determining step. Explain your reasoning. [3] Since H CN does not appear in rate equation, it is not involved in slow/rate determining step OR Only 1 CN ‒ and 1 A is involved in the rate determining step/slow step as indicated by their order of reaction in the rate equation.

5 © ACJC2023 9729/03/Prelim/2023 (b) Explain, with the aid of a labelled Boltzmann distribution diagram, the effect on rate when the temperature is increased. [3] • When temperature increases, there is an increase in the number of particles with kinetic energy  Ea (larger shaded area under the curve at T2 than T1). • This leads to an increase in the frequency of effective collisions between reactant particles. • Hence the rate of reaction increases.

6 © ACJC2023 9729/03/Prelim/2023 (c) Propose a 3 -step synthesis of ibuprofen starting from the compound 1-isobutyl-4-vinylbenzene. Suggest the reagents and conditions, and draw the structure of the organic intermediate product in each step. [5] Step 1: HBr(g) or HCl(g) Step 2: alcoholic KCN or alcoholic NaCN, heat under reflux Step 3: dilute H2SO4, heat under reflux Intermediate from step 1 Intermediate from step 2 [Total: 17]

7 © ACJC2023 9729/03/Prelim/2023 2 (a) Describe and explain the general trend in the first ionisation energy across Period 3. [3] • There is an increasing nuclear charge due to increase in number of protons. • There is an approximately constant shielding effect as additional electrons are added to the same valence shell and provides negligible shielding effect. • Effective nuclear charge increases. OR Valence electrons thus experiences stronger nuclear attraction. • Therefore, more energy required to remove the valence electron. Hence, first I.E. increases across the period. (b) (i) Describe and explain the reactions, if any, of the chlorides NaCl, AlCl3 and PCl5 with water. Write equations for all reactions that occur and suggest the pH of the resulting solutions. [4] NaCl - giant ionic structure. Solid readily soluble. Hydration: NaCl(s) + aq → Na+(aq) + C l−(aq) ( 100% ionic  no hydrolysis). Na+(aq) ion does not undergo hydrolysis as it has a low charge density (low polarising power), thus the solution is neutral, pH=7. AlCl3 – Readily soluble with substantial hydrolysis Hydration: AlCl3(s) + 6H2O(l) → [Al(H2O)6]3+(aq) + 3Cl−(aq) Substantial hydrolysis: [Al(H2O)6]3+(aq) [Al(H2O)5(OH)]2+(aq) + H+(aq) Substantial hydrolysis of AlCl3, is due to the high polarising power of the small, highly charged A l3+ in solution, distorts the electron cloud of its surrounding water molecules and weakens the O −H bond in the complexing water molecules. It forms an acidic solution of pH≈3. PCl5 is a simple covalent compound. It undergoes complete hydrolysis to give a strongly acidic solution, pH=2. P has low-lying, vacant 3d-orbitals a