2023 ASRJC H2 Chem Prelim P4 QP

ASRJC JC2 PRELIM 2023 9729/04/H2 ANDERSON SERANGOON JUNIOR COLLEGE 2023 JC 2 PRELIMINARY EXAMINATION NAME:________________________________ ( ) CLASS: 23 /____ CHEMISTRY 9729/04 Paper 4 Practical 24 August 2023 2 hours 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your name, class and index number 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. Quantitative Analysis Notes are printed on pages 19 and 20. Shift Laboratory 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. For Examiner’s Use 1 / 14 2 / 12 3 / 16 4 / 13 Total / 55 This document consists of 19 printed pages and 1 blank page.

2 ASRJC JC2 PRELIM 2023 9729/04/H2 Answer all the questions in the spaces provided. 1 Determination of the kinetics of a redox reaction Equation 1 presents the redox reaction between iodide ions and acidified hydrog en peroxide, H2O2, to form iodine. equation 1 H 2O2(aq) + 2I–(aq) + 2H+(aq)  I2(aq) + 2H2O(l) If [H+] and [I–] are kept constant, a simplified rate equation can be obtained. rate = k’ [H2O2]n where k’ is k [H+]a [I–]b When starch is added to the reaction mixture, a blue-black colour is immedi ately seen due to the formation of an iodine-starch complex. If a small but constant amount of sodium thiosulfate , Na2S2O3, is also present in the reaction mixture, the formation of the blue-black colour is delayed. T he Na2S2O3 reacts with the I2 as shown in equation 2. equation 2 I2 + 2Na2S2O3 2NaI + Na2S4O6 FA 1 is 0.100 mol dm–3 hydrogen peroxide, H2O2. FA 2 is 0.150 mol dm–3 potassium iodide, KI. FA 3 is 1.00 mol dm–3 dilute sulfuric acid, H2SO4. FA 4 is 0.050 mol dm–3 sodium thiosulfate, Na2S2O3. Starch solution You will perform a series of four experiments and add a fixed amount of sodium thiosulfate, FA 4, to each of the experiment. You will need to ensure that the same total volume of reaction mixture is used by adding deionised water as required. The rate of this reaction is studied by measuring the time taken , t, for the blue-black colour to appear with a constant quantity of sodium thiosulfate present. Y ou will then graphically analyse your results to determine the order of reaction with r espect to hydrogen peroxide. For each experiment, you will note the volume of FA 1 added, VFA 1, volume of H 2O, V 2HO and the time taken for the blue-black colour to appear. You will then calculate values for  1/t  lg( 1/t)  lg( VFA 1)

3 ASRJC JC2 PRELIM 2023 9729/04/H2 [Turn Over (a) Fill Table 1.1 on page 4 with  all volumes of FA 1, VFA 1 and water, V 2HO  all values of t, to the nearest second  all calculated values of 1/t, lg(1/t) and lg(VFA 1) recorded to 3 significant figures. Experiment 1 The end-point of the reaction is the first appearance of a blue-black colour. 1. Fill a burette with FA 4. 2. Transfer 10.00 cm3 of FA 4 from the burette into a 250 cm 3 conical flask. Place the conical flask on a white tile. 3. Using appropriate measuring cylinders, transfer to the 250 cm3 conical flask  25.0 cm3 of FA 2  25.0 cm3 of deionised water  3.0 cm3 of starch solution 4. Using appropriate measuring cylinders, transfer to a 100 cm3 beaker  50.0 cm3 of FA 1  20.0 cm3 of FA 3 5. Pour the mixture in the beaker rapidly into the conical flask. Start the stopwatch when half the contents of the beaker are added. 6. Mix the contents thoroughly by swirling the flask. 7. Stop the stopwatch when the blue –black colour appears. 8. Record the time taken , t, in Table 1.1. 9. Discard the reaction mixture. Carefully wash out the beaker and conical fl ask. Stand the conical flask upside down on a paper towel to drain. Experiment 2 to 4 Carry out three further experiments using a volume of 40.0 cm3, 30.0 cm3 and 25.0 cm3 of FA 1, respectively at point 4. In each case, you will need to ensure that the same total volume of reaction mixture is used by adding deionised water as required. You should alternate the use of the two conical flasks. Record all required volumes, time taken and calculated values in Table 1.1.

4 ASRJC JC2 PRELIM 2023 9729/04/H2 Results Table 1.1 Experiment VFA 1 / cm3 V 2HO / cm3 t / s 1/t / s-1 lg(1/t) lg(VFA 1) 1 2 3 4 [5] (b) Graphical determination of order of reaction In a series of experiments, where volume of one of the reactants added is changed, the total volume of the mixture is kept constant and the same end-poi nt (appearance of the dark blue colour) is timed:  1 time (1/t) can be used as a measure of rate  the volume of the reactant which is changed in each experiment can provide a measure of its concentration. In these experiments, only the concentration of hydrogen peroxide, [H 2O2], in the reaction mixture has been changed. The simplified rate equation is rate = k’ [H2O2]n  taking logarithms of the factors in this equation gives lg(rate) = n  lg([H2O2]) + lg(k’)  by substituting for rate and [H 2O2], as described above, we get lg(1/t) = n  lg(VFA 1) + lg(k’) By plotting a graph of lg( 1/t) on the y-axis against lg(VFA 1) on the x-axis, you will be able to draw a straight line of best fit, the gradient of which is the required order of reaction, n.

5 ASRJC JC2 PRELIM 2023 9729/04/H2 [Turn Over (i) Plot a graph of lg(1/t) on the y-axis against lg(VFA 1) on the x-axis on the grid in Fig. 1.1. Draw the best-fit straight line taking into account all of your plotted points. Fig. 1.1 [3]

6 ASRJC JC2 PRELIM 2023 9729/04/H2 (ii) Calculate the gradient of the line, showing clearly how you did this. Hence, deduce the order of reaction with respect to [H2O2]. gradient = .………………… order of reaction = .………………… [3] (c) Explain why it is necessary to use a burette to measure accurately the volume of FA 4. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. ……………………………………………………………………………………….…[1] (d) A student repeated the four experiments using the same reagents the next day under the same conditions. The student noticed that the time taken for t he first appearance of a blue-black colour was longer for each experiment. Give a possible reason for the longer timing. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. ……………………………………………………………………………………….….[1] (e) Point 9 of Experiment 1 in 1(a) instructs you to wash and drain the conical flask before it is used again. Deduce and explain the likely effect on time taken, t, if a student had failed to wash the conical flask but had drained it before starting another experiment. effect on t ………………………………………………………………………………… explanation ……………………………………………………………………………… …………………………………………………………………………………………….. …………………………………………………………………………………….…