2024 Prelims NJC H2 Chem P3 (Ans)

1 NJC/H2 Chem Preliminary Examination/03/2024 [Turn over NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on Question Paper. Additional Materials: Data Booklet 9729/03 17 September 2024 2 hours READ THE INSTRUCTIONS FIRST Write your subject class, registration number and name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. 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. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Section A 1 /20 2 /20 3 /20 Section B (*circle the question you attempted) 4 /20 5 /20 Paper 3 Total /80 This document consists of 32 printed pages.

2 NJC/H2 Chem Preliminary Examination/03/2024 Section A Answer all the questions in this section. 1 (a) Chlorinated phenols have seen growing applications as fungicides, herbicides, insecticides, and as starting materials in the production of various pesticides since the 1930s. It can be readily produced from compound A using chlorine in water. compound A (i) Write a balanced equation for the reaction between compound A and excess aqueous chlorine. [1] (ii) Compound A undergoes a series of reactions shown in the reaction scheme below. State the reagent and condition for Reaction I and II, and hence draw the structures of B and E. [4] (iii) Write a balanced equation for reaction III. [1] ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..………………

3 NJC/H2 Chem Preliminary Examination/03/2024 [Turn over ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… ………………………………………………………………………………..……………… 1(a)(i) Note: Similar to reaction of phenol with Br2(aq). Electrophilic substitution at 2,4,6 position w.r.t. phenol. 1(a)(ii) • Reaction I: K2Cr2O7, H2SO4(aq), heat with distillation • Reaction II: HCN, trace amount of NaCN, Cold ● B: OH C O H ● E: CH2Cl OH Note: For reaction of C to give D, NaOH causes alkaline hydrolysis of −CN and neutralisation of the acidic phenol −OH. Alcohol group is neutral and remains unchanged. For reaction of A to give E, alcohol −OH undergoes substitution by −C l, phenol −OH does not undergo substitution due to partial double bond of C−O 1(a)(iii) OH C CN H OH + 2NaOH C H O-Na+ COO-Na+ OH + NH3

4 NJC/H2 Chem Preliminary Examination/03/2024 (b) Vanadium is a transition element that exhibits variable oxidation states. (i) Explain why vanadium can form stable ions with varying oxidation states. [1] (ii) Write the full electronic configuration for V3+ . [1] (iii) Draw a fully labelled diagram of the experimental set -up used to measure the standard electrode potential of the VO2+ (aq)/V3+(aq) half-cell. [3] A Latimer diagram below summarises the standard electrode potential data of vanadium complexes in acidic medium. The standard electrode potential x for converting VO 3− to V 3+, is not the sum of +1.00 V and +0.34 V. Instead, it can be calculated from Gꝋ. (iv) Write the half-equation for the conversion of VO3− to V3+. [1] (v) Calculate the standard electrode potential x, given that Gꝋ for the conversion of VO3− to V3+ is −129 kJ mol-1. [1] …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… VO3− VO2+ V3+ +0.34 V +1.00 V x

5 NJC/H2 Chem Preliminary Examination/03/2024 [Turn over …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… …..…………………………………………………………………………………………… 1(b)(i) Vanadium can exhibit a variety of oxidation states due to close similarity in energy of the 3d and 4s electrons . Both 3d and 4s electrons can be removed to form stable ions of different oxidation states. 1(b)(ii) 1s2 2s2 2p6 3s2 3p6 3d2 1(b)(iii) Note: For VO2+/V3+ half-cell, H+ is required in the electrolyte. VO2+ + 2H+ + e− V3+ + H2O Minus 0.5m for each missing details 1(b)(iv) VO3- + 6H+ + 2e– → V3+ +3H2O 1(b)(v)Go = −nFEo, where n = 2 for this equation. E = (–129  103) / (−2  96500) = + 0.67 V Note: ∆G has t