EJC 2020 JC2 Prelim H2 Chemistry Paper 4 QP

© EJC 9729/04/J2PE/20 [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2020 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 1 9 – INDEX NUMBER CHEMISTRY Paper 4 Practical 9729/04 01 September 2020 2 hours 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. Qualitative Analysis Notes are printed on pages 19 and 20. 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 / 34 3 / 9 Total / 55 This document consists of 17 printed pages and 3 blank pages.

2 © EJC 9729/04/J2PE/20 For Examiner’s Use Answer all the questions in the spaces provided. 1 Standardisation of KMnO4 FA 1 is approximately 0.02 mol dm–3 potassium manganate(VII), KMnO4 FA 2 is 0.200 mol dm–3 potassium ethanedioate, K2C2O4 FA 3 is 1.0 mol dm–3 sulfuric acid, H2SO4 KMnO4 is not a primary standard as it cannot be obtained in a sufficiently pure state to allow its concentration to be determined by preparing a solution from a known mass of solid KMnO4. However, it can be standardised by using potassium ethanedioate, K2C2O4. Acidified MnO4– ions oxidises C2O42– ions as shown in equation 1. The Mn2+ ions produced in equation 1 act as a catalyst for the reaction. This is an example of ‘autocatalysis’. equation 1 2MnO4–(aq) + 5C2O42–(aq) + 16H+(aq)  2Mn2+(aq) + 10CO2(g) + 8H2O(l) In this question, you will perform a titration. The data from this titration will be used to determine the concentration of MnO4– ions in FA 1. (a) Titration of FA 2 against FA 1 In this titration, FA 1 is run from the burette into the conical flask containing FA 2 and FA 3. Initially, the colour of the FA 1 will take some time to disappear. After some FA 1 has been added, sufficient Mn 2+(aq) ions will be present to allow the reaction to occur faster. The end-point is reached when a permanent pale pink colour is obtained. (i) 1. Fill the burette labelled FA 1 with FA 1. 2. Using a pipette, transfer 10.0 cm3 of FA 2 into the conical flask. 3. Using an appropriate measuring cylinder, transfer 50.0 cm 3 of FA 3 to the same conical flask. 4. Heat this solution to about 65 ºC. 5. Turn off your Bunsen burner. 6. Run FA 1 from the burette into this flask until a permanent pale pink colour is obtained. 7. Record your titration results in the space provided on page 3. Make certain that your recorded results show the precision of your working. 8. Repeat points 1 to 6 as necessary until consistent results are obtained.

3 © EJC 9729/04/J2PE/20 [Turn Over For Examiner’s Use Results [6] (ii) From your titrations, obtain a suitable volume of FA 1 to be used in your calculations. Show clearly how you obtained this volume. volume of FA 1 = ................................ ................. [1] (b) (i) The equation for reaction 1 between ethanedioate ions and manganate(VII) ions is given on page 2. Calculate the amount of manganate( VII) ions, MnO4–, reacted with the C 2O42– in 10.0 cm3 of FA 2. amount of MnO4– reacted = ................................ ................. [1] (ii) Determine the concentration, in mol dm–3, of MnO4– in FA 1. concentration of MnO4– in FA 1 = ................................ ................. [1]

4 © EJC 9729/04/J2PE/20 For Examiner’s Use (c) For the titration in (a)(i) between ethanedioate ions C 2O42– and manganate(VII) ions MnO4–, the solution needs to be at least 65 ºC at the start. As cold FA 1 is added, the temperature of the mixture decreases. However, this decrease in temperature does not cause the rate of reaction between C2O42– ions and MnO4–to decrease. Suggest an explanation for this. ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ [1] (d) A student performed the experiment in (a)(i) using a solution of Na2C2O4 of lower concentration instead of FA 2. The student obtained a mean titre value of 21.05 cm3. Determine the percentage apparatus error in the student’s titre value and your titre value in (a)(ii). State and explain which titre value is more reliable. ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ [2] [Total: 12]

5 © EJC 9729/04/J2PE/20 [Turn Over For Examiner’s Use 2 To investigate the reaction between manganate(VII) ions and ethanedioate ions. In this experiment you will investigate the kinetics of the reaction between MnO 4– and C2O42– in equation 1. FA 4 is 0.0100 mol dm–3 sodium thiosulfate, Na2S2O3 FA 5 is 0.100 mol dm–3 potassium iodide, KI You will need access to the FA 1, FA 2 and FA 3 solutions you used earlier. You are also provided with a starch indicator. You will add a measured volume of FA 1 to a mixture consisting of measured volumes of FA 2, FA 3 and deionised water and, at timed intervals, you will transfer aliquots (portions) of the reaction mixture. The concentration of MnO4– ions in each aliquot will be determined after adding the aliquot to an excess of potassium iodide solution and titrating the iodine produced (equation 2) against sodium thiosulfate (equation 3). equation 2 2MnO4–(aq) + 10I–(aq) + 16H+(aq)  2Mn2+(aq) + 5I2(aq) + 8H2O(l) equation 3 2S2O32–(aq) + I2(aq)  2I–(aq) + S4O62–(aq) (a) (i) Preparation and titration of the reaction mixture Notes: 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 until you have finished this experiment. Leaving all of the titrations to be performed until after all the aliquots have been collected may cause you time problems. In an appropriate format in the space provided on page 6, prepare a table in which to record for each aliquot  the time of transfer, t, in minutes and seconds,  th