MI 2020 Prelim Paper 4

Class Adm No Candidate Name: This question paper consists of 21 printed pages and 1 blank page. 2020 Preliminary Examinations Pre-University 3 H2 CHEMISTRY 9729/04 Paper 4 Practical 31 Aug 2020 2 hour 30 mins Candidates answer on the Question paper. READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission number on all the work you hand in. 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 at the back of the Question Paper. 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. Question 1 2 3 Total Marks 27 13 15 55 Shift Laboratory

2 1 Determination of the order of reaction with respect to H2O2 Hydrogen peroxide, H2O2, reacts with iodide ions, I-(aq), to form iodine, I2(aq). H2O2(aq) + 2H+(aq) + 2I–(aq) 2H2O(l) + I2(aq) The rate of this reaction can be measured by adding acidified hydrogen peroxide, H 2O2, to a mixture of iodide ions, I-, thiosulfate ions, S 2O32-, and starch indicator. The iodine, I2, produced from the reaction between acidified H2O2 and I- can be reacted immediately with thiosulfate ions, S2O32–. I2(aq) + 2S2O32–(aq) 2I–(aq) + S4O62–(aq) When all the thiosulfate has reacted, the iodine produced will then turn the starch indicator blue- black. The rate of the reaction can therefore be measured by the time taken for the reaction mixture to turn blue-black colour. FA 1 is 0.180 mol dm-3 hydrogen peroxide, H2O2. FA 2 is 0.200 mol dm-3 aqueous potassium iodide, KI. FA 3 is 0.0100 mol dm–3 sodium thiosulfate, Na2S2O3. FA 4 is 0.500 mol dm-3 aqueous sulfuric acid, H2SO4. starch indicator For Examiners’ Use (a) Procedure Experiment 1 1. Fill a burette with FA 1. 2. Run 20.00 cm3 of FA 1 from the burette into a 100 cm3 beaker. 3. Using a suitable measuring cylinder, measure 10 cm3 of FA 4 and transfer into the same 100 cm3 beaker. 4. Using suitable measuring cylinders, transfer the following into another 100 cm3 beaker.  10 cm3 FA 2  20 cm3 FA 3  5 cm3 of starch indicator 5. Add the contents of the second beaker to the first beaker and start the stopwatch. 6. Place the beaker on a white tile and stir the mixture once using a glass rod. 7. Observe the solution and stop the stopwatch when the solution turns blue-black. 8. Record the time taken to the nearest second. 9. Wash both beakers thoroughly with water and drain.

3 [Turn over Experiment 2 10. Fill the other burette with distilled water. 11. Run 6.00 cm3 of FA 1 into a 100 cm3 beaker. 12. Run 14.00 cm3 of distilled water into the same beaker. 13. Add 10 cm3 of FA 4 into the same beaker. 14. Using suitable measuring cylinders, transfer the following into another 100 cm3 beaker.  10 cm3 FA 2  20 cm3 FA 3  5 cm3 of starch indicator 15. Add the contents of the first beaker to the second beaker and start the stopwatch. 16. Place the beaker on a white tile and stir the mixture once using a glass rod. 17. Observe the solution and stop the stopwatch when the solution turns blue-black. 18. Record the time taken to the nearest second. 19. Wash both beakers thoroughly with water and drain. Experiment 3 – 5 Carry out three further experiments to investigate how the reaction time changes with different volumes of FA 1, keeping the volumes of FA 2, FA 3, FA 4 and starch indicator constant. Do not use a volume of FA 1 that is less than 6.00 cm3 and the total volume of the reaction mixture must always be kept at 65 cm3 by adding distilled water. For each Experiments 1 – 5, you are to record all your results in a single table. You should include the volume of FA 1 used, the volume of distilled water used and the time taken for the blue-black colour to appear. Results M1 M2 M3 M4 M5

4 (b) (i) Calculate the amount of thiosulfate ions, S2O32– used in each experiment. Amount of S2O32– =……………………… M6 (ii) Calculate the amount of H2O2 that were used to produce the amount of iodine that reacted with the amount of S2O32– in (i). Amount of H2O2 =……………………… M7 (iii) Using your answer to (ii), calculate the change in concentration of H2O2 up to the time of appearance of the blue-black colour using the following formula: Change in concentration of H2O2 = amount of H2O2 reacted total volume of reaction mixture, in dm3 Change in concentration of H2O2 =……………………… M8

5 [Turn over (iv) The following formula can be used as a measure of the ‘rate of reaction’. ‘rate of reaction’ = change in concentration of H2O2 time taken for solution to turn blue-black ×105 Complete the table in (a) to include the rate in Experiments 1 – 5. M9 (c) On the grid, plot the rate against the volume of FA 1. Draw a line of best fit through the points. M10 M11 M12

6 (d) Deduce the order of reaction with respect to H2O2. Order of reaction with respect to H2O2 =…………………….. M13 (e) Explain why a fixed amount of sodium thiosulfate is required. ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… M14 (f) Instead of washing and draining the conical flask as required in step 9 of (a), another student simply just poured away the reaction mixture. There was some leftover reaction mixture in the flask when he continued using it for Experiment 2. State and explain the effect on time, t, in his Experiment 2. ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… M15

7 [Turn over (g) Planning The concentration of a coloured chemical species can be determined by spectrometry where a small volume of the solution is placed inside a machine called spectro photometer. This machine measures the amount of light that is absorbed when a specific wavelength of visible light is shone through a coloured solution, held in a glass sample holder called a cuvette. The amount of light absorbed is expressed as an absorbance value. The more concentrated the solution, the higher the absorbance value. Based on Be er-Lambert’s Law, the absorbance values, A, is directly proportional to the concentration of absorbing species, c. The general Beer-Lambert’s Law is usually written as A = cl where  is the molar extinction coefficient and l is the path length, which is usually 1.0 cm. This equation can be used to calculate the absorbance value when the concentration of iodine solution is known. You may assume that you are provided with the following in the su bsequent parts of the question.  Iodine solid  FA 5: unknown concentration of aqueous iodine  access to a spectrometer  the apparatus and chemicals normally found in a school or college laboratory.

8 This technique can be used to determine the concentration of a solution of aqueous iodine. A series, of known, but different concentrations of aqueous iodine is prepared. A spectrometer is used to measure the absorbance of each solution. A graph of absorba