VJC 2020 H2 Chem Prelim P4 Ans

1  VJC 2020 H2 PRELIMINARY EXAMINATION [Turn over This document consists of 17 printed pages and 1 blank page. CANDIDATE NAME CT GROUP VICTORIA JUNIOR COLLEGE PRELIMINARY EXAMINATION Higher 2 ……………………………………………….………….. …………………………….. CHEMISTRY 9729/04 Paper 4 Practical Candidates answer on the Question Paper. 31 Aug 2020 2 hours 30 minutes Additional Materials: As listed in the instructions below READ THESE INSTRUCTIONS FIRST Write your name and CT group 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 a 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 14 and 15. 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 / 12 2 / 13 3 / 17 4 / 13 Total / 55

2  VJC 2020 H2 PRELIMINARY EXAMINATION Answer all the questions in the spaces provided. 1 X is a solid sample of sodium ethanedioate, Na2C2O4 and ethanedioic acid, H2C2O4, a weak acid. FA 1 is prepared by dissolving 2.00 g of X in a 250 cm3 volumetric flask and making up to the mark with deionised water. FA 2 is 0.0100 mol dm3 sodium hydroxide, NaOH(aq). FA 3 is 0.0200 mol dm3 potassium manganate(VII), KMnO4(aq). Titration of FA1 against FA2 (a) 1. Fill a burette with FA 2. 2. Pipette 25.0 cm3 of FA 1 into a conical flask. 3. Add 2−3 drops of thymolphthalein indicator to the conical flask. 4. Titrate FA1 against FA 2 until a distinct colour change is observed. 5. Carry out as many titrations as you deem necessary to obtain consistent results. 6. Record in an appropriate form all of your burette readings and the volume of FA 2 added in each titration. Final burette reading / cm3 20.50 41.60 Initial burette reading / cm3 0.00 21.00 Volume of FA 2 used / cm3 20.50 20.60 [4] (b) (i) From your titration results, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. Average volume of FA 2 used = (20.50 + 20.60) ÷ 2 = 20.55 cm3 25.0 cm3 of FA 1 required ………………… cm3 of FA 1. [1] The equation for the reaction between sodium hydroxide and ethanedioic acid in this experiment is shown below: H2C2O4 + 2NaOH  Na2C2O4 + 2H2O (ii) Calculate the amount of ethanedioic acid in 25.0 cm3 of FA 1. Amount of NaOH required = 0.0100  20.55  10–3 = 2.06  10–4 mol Amount of H2C2O4 present in 25.0 cm3 = ½  no. of moles of NaOH required = ½  2.06  10–4 = 1.03  10–4 mol Amount of H2C2O4 in 25.0 cm3 of FA 1 = ………………… mol [1] 20.55 1.03  10–4 4

3  VJC 2020 H2 PRELIMINARY EXAMINATION [Turn over (c) When 25.0 cm3 of FA1 was titrated with FA 3, 25.05 cm3 of FA 3 was required to reach the end−point. (i) Calculate the amount of potassium manganate(VII), FA 3, required to react with 25.0 cm 3 of FA 1. Amount of KMnO4 required = 0.0200  25.05  10–3 = 5.01  10–4 mol Amount of KMnO4 = ………………… mol [1] (ii) The equation for the reaction between acidified manganate(VII) ions and ethanedioate ions is shown in equation 1 below. equation 1 2MnO4(aq) + 5C2O42(aq) + 16H+(aq)  2Mn2+(aq) + 10CO2(g) + 8H2O(l) Calculate the total amount of ethanedioate ions in 25.0 cm3 of FA 1. Total amount of C2O42 present in 25.0 cm3 = 5 2  no. of moles of MnO4 required = 5 2  5.01  10–4 = 1.25  10–3 mol total amount of C2O42 in 25.0 cm3 of FA 1 = ………………… mol [1] (iii) Use your answers from (b)(ii) and (c)(ii) to calculate the amount of ethanedioate ions which came from the sodium ethanedioate in 25.0 cm3 of FA 1. Amount of C2O42 from Na2C2O4 in 25.0 cm3 = (1.25  10–3)  (1.03  10–4) = 1.15  10–3 mol amount of C2O42 from Na2C2O4 in 25.0 cm3 of FA 1 = ………………… mol [1] (iv) Calculate amount of sodium ethanedioate in FA 1.  Amount of Na2C2O4 in FA 1 = 1.15  10–3  250 25.0 = 1.15  10–2 mol amount of Na2C2O4 in FA 1 = ………………… mol [1] (v) Hence, calculate the percentage by mass of sodium ethanedioate in sample X. [Ar: Na, 23.0; C, 12.0; O, 16.0] Mass of Na2C2O4 in FA 1 = 1.15  10–2  (223.0 + 212.0 + 416.0) = 1.15  10–2  134.0 = 1.54 g  Percentage by mass of Na2C2O4 in X = 1.54/2.00  100% = 77.0% Percentage by mass of sodium ethanedioate in X = ………………… [2] [Total: 12] 5.01  10–4 1.25  10–3 1.15  10–3 77.0% 1.15  10–2

4  VJC 2020 H2 PRELIMINARY EXAMINATION 2 To study the kinetics of the Mn2+ catalysed reaction between MnO4− and C2O42− FA 3 is 0.0200 mol dm3 potassium manganate(VII), KMnO4(aq). FA 4 is 0.200 mol dm3 Na2C2O4(aq). FA 5 is 2.00 mol dm3 sulfuric acid, H2SO4. FA 6 is 0.200 mol dm3 MnSO4(aq). FA 7 is 0.400 mol dm3 KI(aq). FA 8 is 0.00400 mol dm3 Na2S2O3(aq). In this experiment, you will determine the order of reaction with respect to MnO 4− for the reaction between MnO4− and C2O42−, catalysed by Mn2+ ions. The mechanism for the catalysed reaction is shown by step 1 and step 2 below: Step 1: 8Mn2+(aq) + 2MnO4−(aq) + 16H+(aq) → 10Mn3+(aq) + 8H2O(l) Step 2: 10Mn3+(aq) + 5C2O42− (aq) → 10CO2(g) + 10Mn2+(aq) (a) (i) With reference to the mechanism above and the uncatalysed reaction in equation 1 , explain why the addition of Mn2+ will speed up the reaction.  The steps in the catalysed reaction have lower activation energy as they involved oppositely charged species unlike the uncatalysed reaction which involved two anions which will experience electrostatic repulsion . Hence, Mn2+ can provide a pathway with lower activation energy. [1] Preparation and titration of the reaction mixture Notes: Read through the instructions carefully before starting any practical work. You will perform each titration once only. Great care must be taken that you do not overshoot the end-point. Once you have started the stopwatch, it must continue running for the duration of the experiment. You must not stop it until you have finished the experiment. You should aim not to exceed a maximum reaction time of 15 minutes for this experiment. 1. Fill a burette with FA 8. 2. Use a measuring cylinder to place the following in a 250 cm3 conical flask.  50 cm3 of FA 4  5 cm3 of FA 5  15 cm3 of FA 6  45 cm3 of deionised water 3. Use a measuring cylinder to add 50 cm 3 of FA 3 into the conical flask in step 2 . Start the stopwatch simultaneously and swirl the mixture thoroughly to mix its contents. This is the reaction mixture. 4. Use a measuring cylinder to add 10 cm3 of FA7 into a 100 cm3 conical flask. 5. Use a teat pipette (dropper) to withdraw a 10 cm3 aliquot (portion) of the reaction mixture and then transfer it to a 10 cm3 measuring cylinder. 6. Immediately pour this aliquot of the reaction mixture into the 100 cm 3 conical flask and vigorously swirl the mixture. Read and record the time of transfer in minutes and seconds, to the nearest second, when the aliquot is added.

5  VJC 20