NYJC 2024 H2 Chem P4 Solutions

H NANYANG JUNIOR COLLEGE JC2 PRACTICAL PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS CENTRE NUMBER S INDEX NUMBER CHEMISTRY 9729/04 Paper 4 Practical 14 August 2024 2 hour 30 minutes Candidates answer on the Question Paper Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your Centre number, index number, 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 23 and 24. 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 / 16 2 / 16 3 /12 4 / 11 Total /55 This document consists of 20 printed pages and 4 blank pages.

2 H2 Chemistry 9729/04 NYJC J2/24 PX Answer all questions in the space provided. 1 An investigation of a gemstone Spinel minerals form crystals that are often used as gemstones. The general formula of a spinel mineral is AB2O4, where A and B are different metal ions. The presence of different combinations of metal ions accounts for the range of colours found in these gemstones. One example of a spinel gemstone is hercynite, which is blue-green in colour. As they are essentially composed of metal oxides, spinel crystals may be reacted with strong acids to form solutions containing salts of the metal ions present. The rarity and high cost of spinel gemstones, however, prohibit the preparation of such solutions for this task. The FA 1 solution you are supplied with is a mixture of two salts. It is intended to duplicate a solution that could have been produced by reacting hercynite crystals with a strong mineral acid. Clearly, non-metal ions are also present in FA 1. FA 1 is a solution containing the metal ions, A and B, together with non-metal ions. FA 2 is sodium carbonate, Na2CO3. Solid FA 3 is granulated zinc. You will perform tests to identify the metal ions, A and B, present in FA 1. You are not expected to identify the anions. You are advised to consider the general formula of the spinel minerals, AB2O4, and the likely oxidation states of the metal ions, A and B, before you start this experiment. Unless otherwise stated, the volumes given below are approximate and should be estimated rather than measured. Test and identify any gases evolved. (a) Carry out the following tests. Carefully record your observations in Table 1.1 Table 1.1 test observations 1 Test the FA 1 solution using Universal Indicator paper. Obs 1. pH 2–4 Or Yellow universal indicator paper changes to orange/red (1) 2 Using a measuring cylinder, transfer 10 cm3 of FA 1 to a clean boiling tube. Use a dropping pipette to transfer 1 cm3 of FA 2 to the boiling tube. Mix the contents of the boiling tube thoroughly. Repeat the transfer of 1 cm 3 of FA 2 with thorough mixing of the contents in Obs 2. (slight) white ppt formed (1) Obs 3. effervescence/bubbles but not ‘gas evolved’ (1) Obs 4. gas evolved gives a white ppt with limewater (1) gas evolved is CO2 Obs 5. green/grey-green/blue-green/grey-green precipitate (1) (indicates initial precipitate becomes darker) Obs 6. then turns brown (this is observed at the inner side of the boiling tube) (1)

3 H2 Chemistry 9729/04 NYJC J2/24 PX the boiling tube until all of the FA 2 is used up. 3 Place about 2 cm 3 of FA 1 into a test-tube. Carefully add sodium hydroxide, dropwise with shaking, until no further changes are seen. Swirl and filter the mixture, collecting the filtrate in a clean boiling tube. Keep the filtrate for use in Test 6. Wash the residue thoroughly with deionised water. Discard the washings. Keep the residue for use in Test 4. Obs 7. white ppt formed (soluble in excess NaOH) (1) Obs 8. then green/grey-green/blue-green/dirty green/light blue ppt formed (1) Obs 9. (green) ppt insoluble in excess NaOH (1) Obs 10. green residue/ppt (1) Obs 11. residue then then turns brown (1) Obs 12. colourless/pale yellow filtrate (1) Note: When aqueous potassium manganate(VII) is added in Test 4, the endpoint is a permanent pale pink colour. 4 Transfer a spatula load of the residue collected in Test 3 in to a clean test-tube. Carefully add hydrochloric acid, dropwise with shaking, until no further changes are seen. Add aqueous potassium manganate(VII), dropwise with shaking, until the endpoint is reached. Record the number of drops of aqueous potassium manganate( VII) you have added to reach the endpoint. Keep this mixture for use in Test 5. Obs 13. solid dissolves in acid to form an orange/orange- brown/brown/yellow solution (1) Obs 14. after  3 drops, it turns pink/endpoint is reached (1) 5 To the final mixture in Test 4, add one piece of FA 3 . Gently warm the mixture and set aside for a few minutes. From time to time, observe the mixture. While you are waiting, continue with Test 6. When no further changes are observed, decant the solution into a second test-tube. Obs 15. the solution gradually lightens in colour/becomes colourless/orange solution turns yellow (1) Obs 16. many bubbles/effervescence but not ‘gas evolved’ (1) Obs 17. ‘pops’ with a lighted splint (1) gas evolved is H2 Obs 18. decolourisation of the purple KMnO4(aq) (1) Obs 19. after a few drops, the solution darkens or turns orange/orange-brown/brown (1)

4 H2 Chemistry 9729/04 NYJC J2/24 PX Add a few drops of aqueous potassium manganate( VII) to the second test-tube, with shaking. 6 Place about 1 cm 3 of the filtrate collected in Test 3 in a clean test-tube. Carefully add hydrochloric acid, dropwise, until no further change is seen before you shake the mixture. The mixture is then use for Test 7. Obs 20. white ppt forms (1) Obs 21. that dissolves in excess HCl (1) to form colourless solution Note: You should NOT do the following test. Observations have been recorded for you. 7 Place about 1 cm 3 of the final mixture in Test 6 in a clean test-tube. Add aqueous ammonia, dropwise with shaking, until no further change is seen. white ppt. insoluble in excess [8] Obs points marks Observation points See the responses listed below. There are 2 1 observational points identified in Tests 1 to 6, each indicated by (1). Record the total number of observational points awarded in the “Obs points” box at the end of question 1. Use the scaling grid below to determine the number of marks awarded. Write this number in the “marks” box at the end of question 1. >= 18 = 8 16 – 17 = 7 13 – 15 = 6 10 – 12 = 5 7 – 9 = 4 5 – 6 = 3 3 – 4 = 2 1 – 2 = 1

5 H2 Chemistry 9729/04 NYJC J2/24 PX (b) By c onsider your observations in Tests 1 and 2 , s uggest an explanation for the observations in Test 2 in terms of the chemistry involved. ……………………………………………………………………………………………................................. ………………………………………………………………………………………………………………... [2] Test 1 indicates the presence of acidic cation in FA 1. The acidic cation (which is Al3+) has high charge density, hence hydrolyses in water to produce acidic solution OR acidic cation has high charge density and polarises electron cloud of H 2O which weakens the O –H bond wh