NYJC 2022 Prelim P4 Answers

[Turn over NANYANG JUNIOR COLLEGE JC2 PRACTICAL PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS 2 1 TUTOR CENTRE NUMBER S INDEX NUMBER CHEMISTRY 9729/04 Paper 4 Practical 18 August 2022 2 hour 30 minutes Candidates answer on the Question Paper Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your name and class 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 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. Shift 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 19 and 20. Laboratory 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 1(a), (b)(i) [7] 2(d) [4] 1 / 17 1(b)(ii)-(v) [6] 2(e), (f) [3] 2 / 16 1(c) [4] 3(a),(c)-(d) [5] 3 /10 2(a) [5] 3(b) [5] 4 / 12 2(b), (c) [4] 4(a) & (b)-(c) [12] Total /55 This document consists of 18 printed pages and 2 blank pages.

2 H2 Chemistry 9729/04 NYJC J2/22 PX Answer all the questions in the spaces provided. 1 An investigation of the chemistry of some vanadium ions FA 1 is a solution of a dilute acid. FA 2 contains vanadate(V) ions, VO3−, of concentration 0.50 mol dm–3. Ammonium vanadate(V), NH4VO3, is a crystalline solid that is slightly yellow in colour. It has a relatively low solubility in water at room temperature. Vanadium, like all transition metals, is able to exhibit variable oxidat ion states in its compounds. You are provided with a solution, FA 2, which was produced by reacting NH4VO3 with aqueous sodium hydroxide. In this reaction, the anion remains unreacted. (a) Write an equation for the reaction between ammonium vanadate(V) and sodium hydroxide to produce FA 2. Describe how, using a simple chemical test, you can confirm that this reaction occurred during the preparation of FA 2. DO NOT carry out this test. equation ................................ ................................ ................................ ........... test ................................ ................................ ................................ ................... ................................ ................................ ................................ ..................... [1] NH4VO3 + NaOH → NaVO3 + H2O + NH3 or NH4+ + OH → H2O + NH3 Gas evolved turns damp red litmus paper blue. NH3 is evolved. 1 mark for both equation and test Relevant standard electrode potential data of some species are given in Table 1.1. Table 1.1 V3+(aq) + e– green ⇌ V2+(aq) purple EO = −0.26 V VO2+(aq) + 2H+ + e– blue ⇌ V3+(aq) + H2O(l) green EO = +0.34 V VO2+(aq) + 2H+ + e– yellow ⇌ VO2+(aq) + H2O(l) blue EO = +1.00 V Zn2+ + 2 e– ⇌ Zn EO = −0.76 V MnO4È(aq) + 8H+(aq) + 5e– ⇌ Mn2+(aq) + 4H2O(l) EO = +1.51 V

3 H2 Chemistry 9729/04 NYJC J2/22 PX [Turn over (b) (i) When FA 2 is acidified and reacted with zinc powder, VO 3– ions are reduced stepwise. Carry out the following tests. Carefully record your observations in Table 1.2. Test and identify any gases evolved. Table 1.2 tests observations 1 Using a measuring cylinder, transfer 1 cm3 of FA 2 into a boiling tube. To this same boiling tube , us e a measuring cylinder to add 4 cm3 of FA 1 and stir the mixture. This is solution W. Retain solution W for use in test 2. • colourless solution turns yellow on acidification • beaker felt warm 2 To solution W in the boiling tube, add a spatula of zinc powder using a metal spatula. Swirl the mixture gently and record your observations. Continue to add more zinc powder one spatula at a time with swirling, until about 4 spatulas of zinc powder is added. Record all colour changes observed throughout the initial 10 minutes. • Yellow solution turns green [mixture of yellow VO 2+ and blue VO2+] • Green solution turns blue (OR bluish-green OR greenish blue) [more VO2+ formed] • Blue solution turns green (OR dark green) [V3+ formed] • Green solution turns purple (OR violet OR blue) [V2+ formed] • boiling tube felt warm • Effervescence of H 2 gas which extinguished lighted splint with a ‘pop’ sound. (not a marking point as it is not observable) When no further changes are seen, decant the reaction mixture into a test-tube. This is solution X which contains one of the reduction product of the vanadium ion originally present in W. Retain solution X for use in test 3. 3 Pour 1 cm3 of X into a test-tube. Observe this solution for at least 5 minutes. • Solution turns blue - green/blue/green [6] • 8-9 pts - [6]; 6-7 pts – [5]; 5 pts – [4]; 4 pts – [3]; 3 pts – [2]; 2 pts – [1]

4 H2 Chemistry 9729/04 NYJC J2/22 PX (ii) State the ion formed when FA 1 is added to FA 2 in test 1. Write an ionic equation for this reaction. ion formed ................................ ................................ ................................ ....... ionic equation ................................ ................................ ............................... [1] VO2+ VO3È(aq) + 2H+(aq) → VO2+(aq) + H2O(l) [1] (iii) Suggest which ion is responsible for the final colour observed in test 2. Explain your answer in terms of the Eʅ values given in Table 1.1. ion……………………………………………………………………………………… explanation……………………………………………………………………………. ………………………………………………………………………………………..... ……………………………………………………………………………………….[2] • V2+ • VO2+ will first be reduced to VO2+ by Zn. E0cell = +1.00 − (−0.76) = +1.76 V, > 0, hence reaction is feasible • VO2+ will then be further reduced by Zn to V3+. E0cell = +0.34 − (−0.76) = +1.10 V, > 0, hence reaction is feasible • V3+ will also be further reduced by Zn to V2+. E0cell = −0.26 − (−0.76) = +0.50 V, > 0, hence reaction is feasible Hence the ion responsible for the final colour is likely to be V2+. 4 pts – [2]; 2 pts – [1] (iv) Suggest an explanation for your observations in test 3. explanation…………………………………………………………………………… ………………………………………………………………………………………[1] aerial oxidation by O2 from V2+ to V3+ or VO2+ [1] (v) When aqueous sodium hydroxide was added dropwise till excess into a solution containing the complex ion [V(H2O)6]3+, a brown precipitate is observed. State the type of reaction and write an equation to explain the observation. ................................ ................................ ................................ ........................ ................................ ................................ ................................ .................... [2] [V(H2O)6]3+ + 3OH– → [V(H2O)3(OH)3] + 3H2O [1]

5 H2 Chemistry 9729/04 NYJC J2/22 PX [Turn over The ion of vanadium in Y has a high charge density. Hence, it undergoes hydrolysis to produce H +, which reacts with NaOH to give an insoluble hydroxide of vanadium. OR Acid-base reaction with OH− to form an insoluble hydroxide of vanadium. [1] Do not allow explanations in terms of ligand exchange. (c) The identity of FA 1 could be hydrochloric acid, nitric acid or sulfuric acid. (i) Devise a test that will positively identify hydrochloric acid, nitric acid, and sulfuric acid respectively. For each of the possible acids, you should indicate the expected observations in the table below. Yo