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 sodium thiosulfate, Na 2S2O3, using starch indicator. The average titre for the titration was 14.50 cm3. (iii) Calculate the amount of I2 present in the 10.0 cm3 diluted bleach sample. [1] Cl2 + 2e‒ → 2Cl‒ Cl2 + 4OH‒ → 2ClO‒ +2e‒ + 2H2O Cl2 + 2OH‒ → Cl‒ + ClO‒ + H2O Chlorine had undergone disproportionation since it is simultaneously reduced, as the oxidation number decreases from 0 in Cl2 to –1 in Cl‒, and oxidised, as the oxidation number increases from 0 in Cl2 to +1 in ClO‒. amount of S2O 2– 3 used = 14.50 1000  0.100 = 0.00145 mol I2 + 2S2O 2– 3 → 2I‒ + S4O 2– 6 , I2 ≡ 2S2O 2– 3 , amount of I2 in 10.0 cm3 sample = 1 2  0.00145 = 0.000725 mol

3 © EJC 9729/02/J2PE/24 [Turn over For Examiner’s Use (iv) Hence, determine the [ClO‒] in the commercial bleach sample. [2] (b) Hydrogen chloride, HCl, is formed when Cl2 is mixed with hydrogen gas. HCl, like other Group 17 hydrides will decompose at high temperatures. Describe and explain the trend observed in the ease of decomposition of hydrides of chlorine, bromine and iodine. ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ [3] (c) 2-chlorobutane reacts with sodium hydroxide under different conditions to form four different organic products B, C, D and E. When 2-chlorobutane is warmed with aqueous sodium hydroxide, product B is formed. (i) Name the type of reaction occurring when B is formed from 2-chlorobutane. ................................ ................................ ................................ ......................... [1] (ii) Describe a simple chemical test to confirm the formation of B. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ......................... [2] I2 ≡ ClO‒, amount of ClO‒ in 10.0 cm3 sample = 0.000725 mol [ClO‒] in diluted sample = 0.000725 10/1000 = 0.0725 mol dm‒3 [ClO‒] in commercial bleach sample = 0.0725 × 250 25 = 0.725 mol dm‒3 When hydrogen halides decomposes, the H –X bond is broken. Down group 17, the size of halogen atom increase and the extent of orbital overlap between H and X atoms decreases. The bond energy of H–X bond thus decreases, and less energy is required to break the H –X bond . Hence, the ease of decomposition of hydrogen halides increases down the group. Nucleophilic substitution Add I2 (aq) and NaOH (aq) to the test tube and warm the mixture. If product B is present, brown I2 will decolourise and a yellow precipitate of CHI3 will form.

4 © EJC 9729/02/J2PE/24 For Examiner’s Use (iii) State and explain the relative rate of reaction of 2 -chlorobutane and of 2-bromobutane with NaOH(aq). ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ......................... [1] Compounds C, D and E are hydrocarbons. D and E are stereoisomers of each other. (iv) State the reagents and conditions needed to favour the formation of a mixture of products C, D and E, rather than B. ................................ ................................ ................................ ......................... [1] (v) Draw the skeletal formulae of C, D and E. C D E [2] Heating 2-chlorobutane with ethanolic sodium hydroxide The reaction between 2-bromobutane and NaOH will be faster. The C–Cl bond is stronger than the C –Br bond, hence the C –Br bond will be cleaved more easily for the reaction to occur.

5 © EJC 9729/02/J2PE/24 [Turn over For Examiner’s Use (d) Chlorinated phenols are good antiseptics. (i) State and explain how the acidity of a chlorinated phenol might differ from that of phenol itself. ................................ .............