EJC 2024 JC2 Prelim P4 Solutions

© EJC 9729/04/J2Prelim/24 [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2024 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 2 3 – INDEX NUMBER CHEMISTRY Paper 4 Practical 9729/04 15 August 2024 2 hour 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidental Instructions READ THESE INSTRUCTIONS FIRST Write your name, civics group and registration number on 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 paper clips, highlighters, 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. 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. Shift Laboratory For Examiner’s Use 1 / 9 2 / 11 3 / 22 4 / 13 Total / 55 This document consists of 19 printed pages and 1 blank page.

2 © EJC 9729/04/J2Prelim/24 For Examiner’s Use Answer all the questions in the spaces provided. 1 Investigation of some reactions of ascorbic acid Ascorbic acid, more commonly known as vitamin C, is a water-soluble vitamin with molecular formula, C 6H8O6. Ascorbic acid is a well -known antioxidant that is reasonably stable in the solid form but oxidised quite rapidly by oxygen in air once dissolved in water. FA 1 is a solution of ascorbic acid, C6H8O6. FA 2 is a solution of copper(II) sulfate, CuSO4 FA 6 is 1.0 mol dm–3 hydrochloric acid, HCl Carry out the following test. Carefully record your observations in Tables 1.1 and 1.2. Unless otherwise stated, the volumes given below are approximate and should be estimated rather than measured. Table 1.1 test observations (a) Add 2 cm depth of aqueous silver nitrate to a clean dry boiling tube. Add 1 cm depth of aqueous sodium hydroxide slowly to the same tube. Brown ppt formed. Add aqueous ammonia slowly, with shaking until the precipitate just dissolves. You may use a clean glass rod to help dissolve the precipitate. Colourless solution obtained. Add 1 cm depth of FA 1 to this mixture and shake the tube. Place the boiling tube in the test -tube rack and leave it for 3 minutes. Grey ppt formed. [1] Silver mirror formed on side of boiling tube/Grey ppt remains /no further change [1] Important: After about 3 minutes, pour the mixture down the sink and wash out the boiling tube several times with tap water. [2]

3 © EJC 9729/04/J2Prelim/24 [Turn Over For Examiner’s Use Table 1.2 test observations (b) (i) Add 2 cm depth of FA 2 to a clean dry boiling tube. Add 1 cm depth of FA 1 to the same tube and shake the tube. Gently heat the boiling tube until the liquid boils. Place the boiling tube in the test -tube rack and leave it to stand. Blue FA 2 turned green/greenish- blue. [1] Pink/Reddish-brown ppt/solid formed [1] in a blue solution (ii) Add 2 cm depth of FA 2 to a clean dry boiling tube. Add 10 drops of FA 6 to the same tube. Add 1 cm depth of FA 1 to the same tube and shake the tube. Gently heat the boiling tube until the liquid boils. Place the boiling tube in the test -tube rack and leave it to stand. FA 2 remains blue. White ppt [1] formed in a blue solution [3] (c) In (a), an organic product with molecular formula C 6H6O6 is obtained from ascorbic acid. (i) Name the type of reaction that ascorbic acid undergoes in (a). ................................ ................................ ................................ ......................... [1] (ii) State and explain the chemical change the reagent undergoes during the reaction in (a). ................................ ................................ ................................ ............................. ................................ ................................ ................................ ......................... [1] Reduction. The oxidation state of silver decreases from +1 in [Ag(NH3)2]+ complex to 0 in silver metal. [1] accept based on reaction of ascorbic acid Oxidation [1]

4 © EJC 9729/04/J2Prelim/24 For Examiner’s Use (iii) Explain why ascorbic acid is not expected to react with the reagent in (a). ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ......................... [1] (d) The Cu2+ in (b) undergoes the same chemical change identified in (c)(ii). From the appearance of the copper-containing product, state • the oxidation state of copper in the copper-containing product in (b)(i) ................... • the oxidation state of copper in the copper-containing product in (b)(ii) .................. [1] [Total : 9] Tollens’ reagent / ammoniacal silver nitrate / the reagent in (a) is used to test for the presence of aldehyde functional group. However, there is no aldehyde functional group in ascorbic acid [1] 0 +1 [1] for both

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6 © EJC 9729/04/J2Prelim/24 For Examiner’s Use 2 Determination of the percentage by mass of ascorbic acid in a tablet As ascorbic acid is readily oxidised, it is easier to analyse ascorbic acid using a redox titration rather than an acid-base titration. When iodate ions, IO – 3, are added to an acidic solution containing iodide ions, I–, a redox reaction occurs to produce iodine, I2. equation 1 IO – 3 + 5I– + 6H+ → 3I2 + 3H2O The I2 formed by this reaction is able to oxidise ascorbic acid to dehydroascorbic acid. equation 2 C6H8O6 + I2 → 2I– + 2H+ + C6H6O6 Due to this reaction the I2 formed is immediately reduced to I– as long as there is any ascorbic acid present. Once all the ascorbic acid has reacted, the excess I2 is free to react with the starch indicator, forming the dark blue starch-iodine complex. This is the end-point of the titration. In 2(a), you will perform titrations to determine the percentage by mass of ascorbic acid present in a vitamin C tablet. FA 3 is a powdered vitamin C tablet, C6H8O6 FA 4 is 0.0020 mol dm–3 potassium iodate, KIO3 FA 5 is 0.200 mol dm–3 potassium iodide, KI FA 6 is 1.0 mol dm–3 hydrochloric acid, HCl Solution S is starch indicator (a) (i) Preparation of standard solution of FA 3 1. Weigh the capped bottle containing solid FA 3. Record the mass in Table 2.1 on page 7. 2. Transfer all the solid FA 3* into a 250 cm3 beaker. 3. Reweigh the empty capped container. Record this mass in Table 2.1 on page 7. 4. Dissolve the solid in about 70 cm3 of deionised water. 5. Transfer all the solution into a 250 cm3 volumetric flask. 6. Make up the solution to 250 cm3 with deionised water and mix thoroughly. You need FA 3 solution for use in 3(a) as well. Do not pour away after the titration. * Gently tap the capped bottle on the benchtop to loosen the solid in the bottle before transfer and ensure that as much of solid FA 3 is transferred into the beaker as possible without using any other aids.

7 © EJC 9729/04/J2Prelim/24 [Turn Over For Examiner’s Use Table 2.1 mass of capped container and solid FA 3 / g 3.187 mass of emptied capped container / g 2.944 (ii