2023 ACJC H2 Chem Prelim Paper 2 - Suggested Solutions

1 © ACJC2023 9729/02/Prelim/2023 [Turn over Anglo-Chinese Junior College JC2 Preliminary Examinations Higher 2 CANDIDATE NAME SUGGESTED SOLUTIONS FORM CLASS TUTORIAL CLASS INDEX NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 9729/02 22 August 2023 2 hours READ THESE INSTRUCTIONS FIRST Write your index number and name in the spaces at the top of this page. Write in dark blue or black pen. You may use an HB pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiners’ use only 1 / 15 2 / 9 3 / 21 4 / 14 5 / 16 Total / 75 This document consists of 19 printed pages and 1 blank page.

2 © ACJC2023 9729/02/Prelim/2023 [Turn over 1 This question examines the chemistry of Group 17 elements and their halides. (a) A2 and B2 are halogens, and they are known to be more soluble in organic solvents. In an experiment, excess A2 was mixed with Na2S2O3(aq). When cyclohexane was added, two immiscible layers were observed which were later separated using a separatory funnel. An orange-red organic layer was obtained, and the aqueous layer was divided into two portions. To one portion of the aqueous layer, a solution of Ba(NO3)2 was added and a white precipitate formed which is insoluble in excess dilute nitric acid. To another portion of the aqueous layer, B2(aq) was added and shaken. When CHCl3 was added, a purple organic layer was obtained. (i) State the identities of A2 and B2. A2: ………………………… B2: ………………………… [1] (ii) Write a balanced ionic equation for the reaction between Na2S2O3 and A2. …………………………………………………………………………………………. [1] (iii) Explain why halogens are more soluble in organic solvents than in water, in terms of the energy changes involved. ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ………………………………………………………………………………………….. [2] (b) In another reaction, halogens can react with alkanes to form mono-substituted products. An example is the reaction of chlorine with 2-methylpentane. (i) Alkanes are generally unreactive. Explain why