2024 Prelims EJC H2 Chem P2 (Ans)

© EJC [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2024 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 2 3 – INDEX NUMBER CHEMISTRY Paper 2 Structured Questions 9729/02 10 September 2024 2 hours Candidates answer on the Question Paper Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, civics group, index number 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. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. 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 Examiner’s Use Paper 2 1 / 19 2 / 20 3 / 19 4 / 17 Total / 75 This document consists of 20 printed pages.

2 © EJC 9729/02/J2PE/24 For Examiner’s Use 1 This question is about different compounds of chlorine. (a) Sodium chlorate, NaC lO, is commonly found in household cleaning products like bleach. It is produced when chlorine, Cl2, is mixed with sodium hydroxide, NaOH. Both Cl‒(aq) and ClO‒(aq) ions are produced in the reaction. (i) By writing appropriate reduction and oxidation half equations, write an overall balanced ionic equation for the reaction. reduction: ................................ ................................ ................................ .............. oxidation: ................................ ................................ ................................ .............. overall: ................................ ................................ ................................ .................. [2] (ii) Explain, in terms of oxidation numbers, what happens to chlorine in this reaction. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ......................... [1] The concentration of C lO‒ ions in bleach can be determined by carrying out a redox titration. 25.0 cm3 of a commercial bleach sample was diluted with deionised water and made up to exactly 250 cm3 in a volumetric flask. 10.0 cm3 of the diluted bleach sample was pipetted out into a conical flask and an excess of acidified potassium iodide, K I, was added to it. The solution turns brown as iodine, I2 was liberated. ClO‒ + 2I‒ + 2H+ → I2 + Cl‒ + H2O The mixture obtained in the conical flask was titrated with 0.100 mol dm ‒3 s