2019 SAJC H2 Chem Prelim P4 QP

[Turn Over NAME Class ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION Chemistry (9729) Paper 4 Practical 27 Aug 2019 2.5 hours Candidates answer on the Question Paper Additional Materials : As listed in the Confidential Instructions : Insert READ THESE INSTRUCTIONS FIRST. Write your name and class on all the work you hand in. Give details of the practical shift and laboratory in the boxes provided above. 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 the 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. Shift Laboratory Qualitative Analysis Notes are printed on pages 21 and 22. The number of marks is given in the brackets [ ] at the end of each question or part question. For Examiner’s use: Question Practical Planning Total Marks 44 11 55 This document consists of 22 printed pages.

2 1 To standardise a 0.0 3 mol dm–3 of iron(III) chloride solution and determine the concentration of sodium thiosulfate solution. (a) Planning FA1 is a standard solution containing 0.03 mol dm–3 of FeCl3 solution. You are to design an experiment to show how FA1 is prepared using solid FeCl3. You are provided with : a solid sample of approximately 1.50 g of FeCl3 (Mr = 162.3), 250 cm3 volumetric flask and the usual laboratory apparatus In your plan, you should include details on:  the mass of FeCl3(s) you use with justification,  the apparatus you would use, and  the procedures which you would follow. Do not carry out this plan as FA1 is provided for you to continue with 1(b). ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… [3]

3 [Turn Over 1 (b) In addition to FA1, you are also provided with the following: FA2 is 0.060 mol dm–3 potassium iodide, KI. FA3 is aqueous sodium thiosulfate, Na2S2O3. When FA1 is combined with FA3, a complex, [Fe(S2O3)2]–, is formed. Reaction 1: Fe3+(aq) + 2S2O32–(aq) ⇌ [Fe(S2O3)2]–(aq) To 1 cm3 of FA3 in a test–tube, add 2–3 drops of FA1 and shake them thoroughly. State the colour of the complex formed. Colour : ………………………… [1] (c) A student aimed to determine the concentration of FA3 using volumetric analysis involving the use of FA1 and FA2. Reaction 2: 2Fe3+(aq) + 2I–(aq) → 2Fe2+(aq) + I2(aq) Reaction 3: I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq) Using the above reactions, the student planned and carried out the following procedure. 1. Fill a 50.00 cm3 burette with FA3 solution. 2. Using a 10 cm3 measuring cylinder, measure 5 cm3 of FA1 and place it in a 100 cm3 conical flask. 3. Using another 10 cm3 measuring cylinder, measure 5 cm3 of FA2 into the conical flask. 4. Run FA3 from the burette into the conical flask. Near the endpoint, when the brown solution becomes pale, add 2 drops of starch. 5. Continue adding FA3 slowly, the endpoint is reached when the solution first becomes colourless. 6. Record the titration results, to an appropriate level of precision, in an appropriate table and repeat the experiment once more.

4 1 (c) (i) Carry out the student’s procedure twice only to obtain two titration readings. Your titre values need not be consistent. Record your titration in an appropriate table in the space below. [2] (ii) Planning A teacher commented that it is appropriate for the student to use 5 cm 3 instead of 25 cm 3 of FA1 for this titration. Justify the teacher’s comment using suitable calculations. Assume the concentration of FA3 is 0.006 mol dm–3. [2] Note: After the end point is reached, you may observe the appearance of the coloured mixture again. There is no need to titrate any further.

5 [Turn Over 1 (c) (iii) Planning Justify whether the amount of FA2 that the student used in step 3 of the procedure in 1(c) is sufficient to determine the concentration of FA3. [1] (iv) The student did the titration several times but could not get consistent titre values of FA3. Based on your observation during titration, suggest a reason why. …………………………………………………………………………………… …………………………………………………………………………………… …………………………………………………………………………………… [1] (v) Suggest another reason why the results obtained are not reliable. …………………………………………………………………………………… …………………………………………………………………………………… …………………………………………………………………………………… [1]

6 1 (c) (vi) The student performed the titration and had a titration value of 18.50 cm3. Given t he errors (uncertainties) associated with each reading using a measuring cylinder and burette are 0.1 cm3 and 0.05 cm3 respectively, calculate the maximum total percentage error (uncertainty) from the apparatus in the titration. [3] [Total: 14 marks]

7 [Turn Over 2 To investigate the kinetics of the reaction between iron(III) ions and iodide ions In this experiment, you are investigating the rate of reaction between iron(III) ions and iodide ions. You are provided with: FA1 is 0.03 mol dm–3 of FeCl3 solution FA2 is 0.060 mol dm–3 potassium iodide, KI FA3 is sodium thiosulfate, Na2S2O3 The reaction is started by mixing a solution of iron(III) chloride with sodium thiosulfate, potassium iodide, and starch. The iodine, I2, produced in reaction 2 reacts immediately with thiosulfate ions, S2O32– in reaction 3. Reaction 1: Fe3+(aq) + 2S2O32–(aq) ⇌ [Fe(S2O3)2]–(aq) Reaction 2: 2Fe3+(aq) + 2I–(aq) → 2Fe2+(aq) + I2(aq) Reaction 3: I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq) When all the thiosulfate have been used, the iodine produced will turn starch indicator blue–black. The rate of the reaction can therefore be determined by finding the time for the blue–black colour to appear. You are advised to read the instructions before starting any practical work and draw a table, in an appropriate format, for your results in the space on page 10. For each of the five experiments, you will need to include the  volume of FA2, VFA2,  volume of water, Vwater,  calculated squared volume of FA2, (VFA2)2,  reaction time, t to nearest second, and  calculated rate in mol dm–3 s–1 Record all calculated values to 3 significant figures.

8 2 (a) Method Experiment 1 1. Use the measuring cylinders to measure the following:  20 cm3 of FA1  20 cm3 of FA3  1 cm3 of starch indicator 2. Using another measuring cylinder, measure 10 cm3 of FA2. 3. Pour the measured FA1 and starch into a dry 100 cm3 beaker. 4. Pour in the FA3, followed by FA2 immediately into the same beaker. Start the stopwatch on adding FA2. 5. Stir the mixture and place the beaker on the printed page on page 2 of the insert. 6. The mixture turns brown and then yellow before turning a blue–black colour. Stop the stopwatch when this blue –black colour appears and obscures the wordings. 7. Record the time to the nearest second in the space on page 10. 8. Wash the beaker thoroughly with water and carefully dry the beaker with paper towel. Experiment 2 9. Repeat step 1 in Experiment 1. 10. Using a measuring cylinder, add 2 cm3 of FA2. Make up the volume to 10 cm3 using deionised water using another measuring cylinder. 11. Pour the measured FA1, starch and deionised water into a dry 100 cm3 beaker. 12. Pour in the FA3, followed by FA2 immediat