2020 JPJCPrelim Paper 4 suggested solutions

 Jurong Pioneer Junior College [Turn Over NAME CLASS 19S JURONG PIONEER JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION 2020 CHEMISTRY 9729/04 Higher 2 Paper 4 Practical 2 September 2020 2 hours 30 minutes Candidates answer on the Question paper. Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your name, class and exam index number on all the work you hand in. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use a HB pencil for any diagrams, graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. Do not spend more than 40 minutes on Q1 and Q2. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 18 and 19. Shift At the end of the examination, fasten all your work securely together. Laboratory The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 /8 2 /9 3 /22 4 /16 Total /55 This document consists of 19 printed pages.

2  Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2020 Answer all the questions in the spaces provided. 1 Inorganic Qualitative Analysis FA 1 is a solid sample which contains two cations and one anion. One of the cations is Fe3+. Perform the tests described in Table 1.1 and record your observations in the table. Test and identify any gases evolved. If there is no observable change, write no observable change. For Examiner’s Use Table 1.1 tests observations (a) (i) Add one spatula full of FA 1 into a test-tube. Place about 5 cm depth of deionised water into the test -tube and shake the mixture until all the solid has dissolved. Use this FA 1 solution for tests (ii) and (iii). Lilac solid dissolves to give yellow / orange / brown solution. () (ii) To a 2 cm depth of FA 1 solution in a test -tube, add a small spatula of solid sodium carbonate. Red-brown/brown/orange-brown ppt. () Effervescence () observed. CO2 () gas gives a white ppt. with limewater. () (iii) To a 1 cm depth of FA 1 solution in a boiling tube , add aqueous sodium hydroxide dropwise, with shaking, until it is in excess. Gently warm the mixture. Then filter the mixture, collecting the filtrate in a clean test-tube. Keep the filtrate for test (iv). Red-brown/brown/orange-brown ppt. () insoluble in excess NaOH. () Pungent NH3 () gas turns damp red litmus paper blue. () Red-brown/brown/orange-brown residue () and colourless filtrate () (iv) To a 1 cm depth of the filtrate from test (iii) in a test-tube, add BaCl2(aq) dropwise. Then add dilute hydrochloric acid until no further change. White ppt. () ppt. insoluble in excess dilute hydrochloric acid. () [5]

3  Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2020 [Turn Over (b) (i) By considering the observations made in Table 1.1 and making reference to the Qualitative Analysis Notes, identify the other cation and the anion present in FA 1. The other cation in FA 1 is NH4+ The anion in FA 1 is SO42- [2] (ii) By considering the i ons present in FA 1 , suggests an explanation for observations made in test (a)(ii). Fe3+(aq) and NH4+(aq) undergo hydrolysis to give H +(aq)/acidic solution which reacts with Na2CO3 to liberate CO2. Or, NH4+ is a conjugate acid of NH 3 and undergoes partial dissociation to give H+(aq)/acidic solution which reacts with Na2CO3 to liberate CO2. Or, Due to high charge density of Fe 3+, Fe3+(aq) undergoes partial hydrolysis to give H +(aq)/acidic solution which reacts with Na 2CO3 to liberate CO2. NH4+ + H2O ⇌ NH3 + H3O+ Or, NH4+ ⇌ NH3 + H+ Fe(H2O)63+ + H2O ⇌ [Fe(H2O)5(OH)]2+ + H3O+ Or, Fe(H2O)63+ ⇌ [Fe(H2O)5(OH)]2+ + H+ [1] [Total: 8]

4  Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2020 2 Organic Qualitative Analysis In this question, you will deduce the structure of an organic compound, FA 2. The molecular formula of FA 2 is C4H8O2. Perform the tests described in Table 2.1, and record your observations in the table. Test and identify any gases evolved. Some observations have been recorded. Do not use Bunsen burner for heating. Use the hot water provided. Table 2.1 tests observations (a) (i) Place about 2 cm depth of aqueous sulfuric acid in a test-tube. To this test -tube, add 10 drops of FA 2, followed by 1 drop of aqueous potassium manganate(VII). Warm the mixture in the hot water bath for two minutes. Purple KMnO4 decolourised. (ii) Place about 2 cm depth of aqueous silver nitrate in a test-tube. To this test -tube, add 10 drops of aqueous sodium hydroxide, followed by aqueous ammonia until a clear solution is formed. To this test -tube, add 10 drops of FA 2. Warm the mixture in the water bath for two minutes. Brown ppt. dissolves in NH3(aq) to give a colourless solution. No silver mirror (iii) Place about 1 cm depth of deionised water in a test-tube. To this test-tube, add 10 drops of FA 2 and add 10 drops of aqueous sodium hydroxide. Now add iodine solution dropwise, until a permanent yellow/orange colour is obtained. Warm the mixture in the water bath for two minutes. Pale yellow ppt. formed. (iv) Place about 1 cm depth of FA 2 in a test-tube. To this test -tube, cautiously add a small piece of sodium metal. Effervescence observed. H2 gas produced extinguishes a lighted splint with a “pop” sound. (v) Place about 1 cm depth of FA 2 in a test-tube. To this test-tube, add 2,4-dinitrophenylhydrazine dropwise. Orange ppt. formed [3]

5  Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2020 [Turn Over (b) (i) Using the observations in (a)(ii) and (v) in Table 2.1, identify one functional group present in FA 2. Give evidence(s) to support your conclusion. Functional group: ketone Evidence: It undergoes condensation with 2,4-DNPH to give an orange ppt. but it does not undergo oxidation/react with Tollens’ reagent to give a silver mirror. [2] (ii) Using the observations in other tests in Table 2.1, identify the other functional group present in FA 2. Give evidence(s) to support your conclusion. Functional group: primary or secondary alcohol Evidence: It undergoes oxidation with acidified KMnO4 and decolourises purple KMnO4. It undergoes a redox reaction with Na to give H2 gas. [2] (iii) Suggest two possible structures of FA 2. [2] [Total: 9]

6  Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2020 3 Investigation of reaction between manganate(VII) ions and ethanedioate ions. For Examiner’s Use FA 3 is 0.0100 mol dm–3 sodium thiosulfate, Na2S2O3 FA 4 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4 FA 5 is 0.200 mol dm–3 ethanedioic acid, H2C2O4 FA 6 is 1.0 mol dm–3 sulfuric acid, H2SO4 FA 7 is 0.100 mol dm–3 potassium iodide, KI You are also provided with a starch indicator. Acidified potassium manganate(V II) oxidises ethanedioate ions, C 2O42 as shown in reaction 1 . The Mn 2+ ions produced in reaction 1 act as a catalyst for the reaction. This is an example of ‘autocatalysis’. 2MnO4–(aq) + 5C2O42(aq) + 16H+(aq)  2Mn2+(aq) + 10CO2(g) + 8H2O(l) reaction 1 You are to investigate how the rate of reaction changes with [MnO 4]. To do this, you wi