2024 MI PU3 MYE P4 (Answers)

Class Adm No Candidate Name: This question paper consists of 18 printed pages. 2024 Mid-Year Examination Pre-University 3 H2 CHEMISTRY 9729/04 Paper 4 Practical 25 June 2024 2 hours 30 minutes 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 in the spaces at the top of this page. 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 on pages 17 and 18. 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 4 Total Marks 15 13 12 15 55 Class Adm No

2 Answer all the questions in the spaces provided. 1 Determination of the concentration of a glucose solution using a graphical method Glucose in solution reduces purple, acidified potassium manganate( VII) solution, FA 3, to a colourless or slightly yellow solution at end-point. MnO4 + 8H+ + 5e → Mn2+ + 4H2O In this experiment, the rate of this reaction is studied by measuring the time taken for the reaction mixture to reach the end-point. You will perform a series of five experiments. Then you will graphically analyse your results to determine the order of reaction with respect to the concentration of glucose. For each experiment, you will note the time taken, t, for the reaction mixture to reach end-point. You will then calculate values of 1/t and record them to an appropriate level of precision. The experiment is then repeated using a solution of glucose of unknown concentration. By comparing the time taken for the reaction mixture to reach the end-point with the graph , the concentration of glucose in the unknown solution can be determined. FA 1 is 20% by mass of glucose solution. (i.e. there is 20 g of glucose in 100 cm3 of water) FA 2 is 1 mol dm-3 sulfuric acid, H2SO4. FA 3 is 0.0025 mol dm-3 potassium manganate(VII), KMnO4. FA 4 is a sample of glucose solution that has concentration between 30-40%. Experiment volume of FA 1 /cm3 volume of water /cm3 % glucose in solution X t /s 1/t /s1 1 10.0 0.0 20% 35 0.0286 2 8.0 2.0 16% 46 0.0217 3 6.0 4.0 12% 57 0.0175 4 4.0 6.0 8% 81 0.0123 5 2.0 8.0 4% 135 0.00741 Table 1.1 (a) Varying volumes of water are added to FA 1 to form 10 cm 3 of solution X, which contains different concentrations of glucose. State the volume of water to be added for each experiment in Table 1.1 above. [1] 1m for all correct volumes of water

3 [Turn over (b) Explain why the total volume of solution X needs to be kept constant for all experiments. [1] This is so that the concentration of FA 1 in the experiment will be proportional to the volume of FA 1 used. Candidate must include volume of FA1, cannot just say proportional to the volume. (c) Experiment 1 1. Add 10 cm3 of FA 1 into a boiling tube using a measuring cylinder. 2. Add 5 cm3 of FA 2 into the same boiling tube using the second measuring cylinder. 3. Swirl the mixture to mix well. 4. Add 2 cm3 of FA 3 using the third measuring cylinder and start the stopwatch. 5. Swirl the mixture gently and continuously until the purple colour is discharged. 6. When the purple colour is discharged, stop the stopwatch and record the time to the nearest second in Table 1.1 on page 2. 7. Discard the reaction mixture immediately down the sink. Wash out the boiling tube and stand it upside down on the rack to drain. Experiment 2 1. Add 8 cm3 of FA 1 into another boiling tube using a measuring cylinder. 2. Add the volume of water you stated in Table 1.1 using the same measuring cylinder. 3. Add 5 cm3 of FA 2 into the boiling tube using the second measuring cylinder. 4. Swirl the mixture to mix well. 5. Add 2 cm3 of FA 3 using the third measuring cylinder and start the stopwatch. 6. Swirl the mixture gently until the purple colour is discharged. 7. When the purple colour is discharged, stop the stopwatch and record the time to the nearest second in Table 1.1 on page 2. 8. Discard the reaction mixture immediately down the sink. Wash out the boiling tube and stand it upside down on a rack to drain. Repeat the procedure above for experiments 35. You should alternate the use of the two boiling tubes provided.

4 (d) Plot a graph of 1/t on the y-axis against % of glucose on the x-axis on the grid in Fig 1.1. Draw the best fit line taking account all of your plotted points. Fig. 1.1 [4] 1m completed Table 1.1 (5 readings) 1m Table 1.1 to correct precision (t to nearest second, 1/t to 3sf) 1m axis with units, appropriate scale (more than 50% of plot area occupied) 1m points plotted correctly + best fit line drawn (shape not penalised) (e) Explain why the rate of reaction is proportional to 1/t. [1] A fixed amount of purple KMnO4 is used for each experiment. Since the time taken is for a fixed amount of purple KMnO4 to disappear, rate of reaction is proportional to 1/time. (f) Use your graph to deduce the order of reaction with respect to glucose. Since a straight line is obtained, rate of reaction is directly proportional to concentration of glucose [1]. Thus order of reaction is first order [1] with respect to glucose. 1m traight line + rate proportional to conc of glucose 1m conclude first order (must be substantiated) order of reaction with respect to glucose = 1 [2] 5 10 15 20 25 17 % glucose 0.0100 0.0200 0.0300 0.0400 0.0250 1/t /s1     

5 [Turn over (g) You are to dilute FA 4 by half by mixing 5 cm 3 of FA 4 with 5 cm3 of water as in Table 1.2 below before conducting the experiment. Table 1.2 Experiment volume of FA 4 /cm3 volume of water /cm3 t /s 1/t /s1 6 5.0 5.0 40 0.0250 Explain why it is necessary to dilute FA 4 by half before conducting the experiment. [1] The concentration of glucose in FA 4 is too high and is beyond the plotted points on the graph. An extrapolation of the best -fit line has to be made, and values obtained from this extrapolation will not be as accurate. OR The concentration of glucose is too high in FA 4, hence the time would be too short, leading to a high % error in the time measurement. (h) Carry out your experiment according to the procedure in Experiment 2 and record your results in Table 1.2. Use the graph to determine the % of glucose in FA 4. You are required to show your working on the graph and calculations clearly. From the graph, when 1/t is 0.0250, % of glucose is 17% Hence % of glucose in FA 4 = 17.0 x 2 = 34.0% 1m results in Table 1.2 1m use of graph with construction lines to obtain % of glucose 1m multiply by 2 to obtain % glucose in FA 4 % glucose in FA 4 = 34.0% [3] (i) When you performed the experiments in 1(c), you were instructed to wash and drain the boiling tube before using it again. State and explain the likely effect on t of not draining the boiling tube before it is reused. effect on t increase [1] explanation residual water will dilute the re