NYJC 2022 Prelim P4

[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 / 17 2 / 16 3 /10 4 / 12 Total /55 This document consists of 20 printed pages and 0 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 oxidation 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] and test Relevant standard electrode potential data of some species are given in Table 1.1. Table 1.1 V3+(aq) + e– green Ý V2+(aq) purple Eʅ = −0.26 V VO2+(aq) + 2H+ + e– blue Ý V3+(aq) + H2O(l) green Eʅ = +0.34 V VO2+(aq) + 2H+ + e– yellow Ý VO2+(aq) + H2O(l) blue Eʅ = +1.00 V Zn2+ + 2 e– Ý Zn Eʅ = −0.76 V MnO4È(aq) + 8H+(aq) + 5e– Ý Mn2+(aq) + 4H2O(l) Eʅ = +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 , use 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 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 s wirling, 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 VO2+ and blue VO2+] Green solution turns blue (OR bluish- green OR greenish blue) [more VO 2+ formed] Blue solution turns green (OR dark green) [V3+ formed] Green solution turns purple (OR violet OR blue) [V2+ formed] • identified all colours • sequence of colours identified is correct (allow to skip colours) Effervescence of H 2 gas which extinguish lighted splint wisoumarking 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]

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] (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] • Zn. (iv) Suggest an explanation for your observations in test 3. explanation…………………………………………………………………………… ………………………………………………………………………………………[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]

5 H2 Chemistry 9729/04 NYJC J2/22 PX [Turn over (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. Your tests should be based on the Qualitative Analysis Notes on pages 19−20 and should use only the bench reagents provided. tests expected observations 1 To 1 cm depth of FA 1 in a test-tube, add aqueous silver nitrate dropwise. Solution remains colourless [1] 2 To 1 cm depth of FA 1 in a boiling tube, add 1 cm depth of aqueous sodium hydroxide and a piece of aluminium foil. Heat the resulting mixture. No gas evolved [1] 3 To 1 cm depth of FA 1 in a test-tube, add aqueous barium chloride dropwise. Then, add excess aqueous hydrochloric acid. White ppt formed; white ppt remains insoluble in excess HCl(aq) [1] [3] (ii) Perform your tests on FA 1 to confirm its identity. Any test requiring heating MUST be performed in a boiling tube. FA 1……………………………………. [1] H2SO4 [1] [To