CJC 2023 Prelim 9729 P4 MS with examiners comments

[Turn over 9729/04/CJC JC2 Preliminary Examinations 2023 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/04 Paper 4 Practical 23 August 2023 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 and class in the boxes above. 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. 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 21 and 22. 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. This document consists of 21 printed pages and 1 blank page. Instruction Catholic Junior College JC2 Preliminary Examinations Higher 2 Shift Laboratory For Examiner’s Use 1 / 11 2 / 18 3 / 16 4 / 10 Total / 55 MARK SCHEME

2 9729/04/CJC JC2 Preliminary Examinations 2023 Answer all the questions in the spaces provided. 1 Qualitative analysis of an inorganic solid mixture FA 8 is MnCO3 + Al(NO3)3 FA 8 is a solid mixture that contains two cations and two anions listed in the Qualitative Analysis Notes on pages 21 and 22. Unless otherwise stated, the volumes given below are approximate and should be estimated rather than measured. 1. Transfer the FA 8 provided into a boiling tube and add in 3 cm depth of deionised water. 2. Using a glass rod, stir the mixture well for one minut e until no more of the solid mixture can dissolve. 3. Filter the mixture into another clean boiling tube. While waiting, proceed to Question 2 first. 4. When the filtration is completed, place the filter funnel containing the residue on a conical flask and wash the residue with a little deionised water. Keep both filtrate and residue for tests (a) and (b) below. Test and identify any gases evolved. At each stage of any test, you are to record details of the following: • colour changes seen • the formation of any precipitate • the solubility of such precipitates in an excess of the reagent added If there is no observable change, write no observable change. Record your observations in Tables 1.1 and 1.2 below. No additional tests for ions present should be attempted. (a) Tests on the filtrate. Table 1.1 test observations (i) Test the filtrate using Universal indicator paper. Hence conclude pH of the filtrate. • Universal indicator paper turns yellow. pH 3 to 5 Make sure the colour of the UI paper corresponds to the pH given. pH 3 – orange pH 4 – yellow pH 5 – yellowish-green

3 [Turn over 9729/04/CJC JC2 Preliminary Examinations 2023 (ii) To 1 cm depth of the filtrate in a test- tube, add aqueous sodium hydroxide dropwise, with shaking, until no further change is observed. (b) Tests on the residue. Table 1.2 test observations (i) Place the funnel containing the residue (on the filter paper) into a clean boiling tube. Pour about 5 cm3 of dilute nitric acid onto the residue. Collect 1 cm depth of the solution in the boiling tube. Remove the funnel and return it to the conical flask. • Effervescence observed; gas gives white ppt with limewater. CO2(g) liberated. • Colourless filtrate and dark brown residue obtained. • Off-white / light brown ppt formed; insoluble in excess NaOH Solubility of ppt in excess NaOH must be included since question stated ‘until no further change is observed’. Test of gas and identity of gas must be included when effervescence is observed.

4 9729/04/CJC JC2 Preliminary Examinations 2023 (ii) To this solution in the boiling tube, add aqueous sodium hydroxid e dropwise, with shaking, until no further change is observed. Keep this resultant mixture for test (iii) below. (iii) To 1 cm depth of the mixture from (ii), carefully add 1 cm depth of aqueous hydrogen peroxide. [6] (c) (i) One of the cations in FA 8 is responsible for the nature of the filtrate in test 1(a)(i). It is formed from a Period 3 element. This cation present in FA 8 is: …………………………………………………………… [1] (ii) Write an equation to account for the nature of the filtrate in test 1(a)(i). ……………………………………………………………………………………………… [1] (d) (i) The second cation present in FA 8 is ……………… evidence ...…………………………………………………………………………………… ….……………………………………………………………………………………………… ………………………………………………………………………………………………...[1] • Off-white / beige / pale or light brown/cream ppt insoluble in excess NaOH(aq). • Ppt rapidly turns brown on contact with air / darkens on standing. • ppt turns brown / darker brown / brown-black. • Brisk effervescence: gas re-ignite a glowing splint; O2 gas liberated. Mn2+ In test 1b(ii), addition of NaOH (aq) results in formation of a off-white ppt that was insoluble in excess NaOH(aq). The ppt rapidly turned brown upon contact with air, indicating presence of Mn2+. [Al(H2O)6]3+ + H2O ⇌ [Al(H2O)5(OH)]2+ + H3O+ Al3+ Solubility of ppt in excess NaOH must be included since question stated ‘until no further change is observed’. Test of gas and identity of gas must be included when effervescence is observed. Double arrow must be used to show that partial hydrolysis is occurring. As the solution is acidic shown by UI paper (pH 3-5), it must be Al3+. Mg2+ is not accepted because the solution is almost neutral (pH 6.5) Reference to the test and observations must be included in the evidence to support the cation identified.

5 [Turn over 9729/04/CJC JC2 Preliminary Examinations 2023 (ii) One anion present in FA 8 is ………………. evidence ………………………………………………………………………………………. ……………………………………………………………………………………………….… ………………………………………………………………………………………………… ………………………………………………………………………………………….…… [1] (iii) Suggest what type of reaction takes place when hydrogen peroxide is added in test 1(b)(iii). ……………………………………………………………………………………………....[1] [Total: 11] CO32‒ In test 1b(i), addition of nitric acid results in brisk effervescence of CO2 gas which gave a white ppt with Ca(OH)2, indicating presence of CO32‒. Redox Reference to the test and observations must be included in the evidence to support the cation identified. The reaction that is occurring here Mn(OH)2 + H2O2 → MnO2 + 2H2O Off-white ppt brown-black ppt. Mn(OH)2 reacted with H2O2 & oxidised to form brown-black MnO2 solid. Redox is occurring because Mn2+ is oxidized to MnO2 while H2O2 is being reduced to H2O. Explanation for O2 liberated: The MnO2 product catalysed the disproportionation of H2O2, H2O2 ⇌ 2H2O + ½O2 resulting in formation of H2O(l) and O2(g) which explain for the effervescence observed.

6 9729/04/CJC JC2 Preliminary Examinations 2023 2 Determination of the molar enthalpy changes of two reaction by indirect methods. Sodium carbonate, Na 2CO3 reacts with water and carbon dioxide to form sodium hydrogencarbonate, NaHCO3, according to equation 1. equation 1 Na2CO3(s) + H2O(l) + CO2(g) → 2NaHCO3(s) H1 The enthalpy change of this reaction cannot be determined directly. Both Na2CO3(s) and NaHCO3(s) react with dilute sulfuric acid. equation 2 Na2CO3(s) + H2SO4(aq) → Na2SO4(aq) + H2O(l) + CO2(g) H2 equation 3 NaHCO3(s) + ½ H2SO4(a