2023 JPJC J2 H2 Prelim Paper 4 Qn Paper with Mark Scheme

 Jurong Pioneer Junior College [Turn Over NAME Mark Scheme CLASS 22S JURONG PIONEER JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION 2023 CHEMISTRY 9729/04 Higher 2 Paper 4 Practical 15 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 exam 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, 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 18 and 19. Shift At the end of the examination, fasten all your work securely together. Laboratory The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 2 3 4 Total This document consists of 19 printed pages.

2  Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2023 Answer all the questions in the spaces provided. 1 Determination of the kinetics of the reaction between iodide ions and peroxodisulfate ions FA 1 is 0.0200 mol dm–3 potassium peroxodisulfate, K2S2O8. FA 2 is 1.00 mol dm–3 potassium iodide, KI. FA 3 is 0.00500 mol dm–3 sodium thiosulfate, Na2S2O3. starch indicator. Equation 1 represents the reaction between peroxodisulfate ions, S 2O8 2, and iodide ions, I–. equation 1 S2O8 2(aq) + 2I–(aq)  2SO4 2(aq) + I2(aq) When starch is added to the reaction mixture, a blue-black colour is immediately seen due to the formation of an iodine-starch complex. If a small amount of sodium thiosulfate, Na2S2O3, is also present in the reaction mixture, the formation of the blue-black colour is delayed. The Na2S2O3 reacts with I2 as shown in equation 2. equation 2 I2(aq) + 2S2O3 2–(aq) → 2I–(aq) + S4O6 2–(aq) You will perform a series of five experiments. Then you will graphical ly analyse your results in order to determine the order with respect to the concentration of the peroxodisulfate ions, [S2O8 2]. For each experiment, you will note the volume of FA 1 added, VFA1, and the time taken, t, for the reaction mixture to turn blue-black. In each experiment, you will add a fixed amount of sodium thiosulfate, FA 3, to each of your experiments. You will need to ensure that the same total volume of reaction mixture is used by adding deionised water as required. You will then calculate values for  1/t,  lg( 1/t),  lg( VFA1) For Examiner’s Use Prepare a table in the space provided on page 4 in which to record, to an appropriate level of precision:  all volumes, except the volumes of FA 2, FA 3 and starch indicator,  all values of t,  all calculated values of 1/ t, lg(1/t), lg(VFA1) to 3 significant figures.

3  Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2023 [Turn Over BLANK PAGE

4  Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2023 (a) Experiment 1 1. Fill the burette labelled FA 1, with FA 1. 2. Run 20.00 cm 3 of FA 1 into a 100 cm3 beaker labelled Beaker A. 3. Using appropriate measuring cylinders, add the following to the second 100 cm3 beaker labelled Beaker B:  20.0 cm3 of FA 2  10.0 cm 3 of FA 3 4. Add 10 drops of starch indicator to Beaker B. 5. Add the contents of Beaker A to Beaker B and start timing immediately. 6. Stir the mixture thoroughly using the glass rod and place the beaker on a white tile. 7. Stop timing as soon as the solution turns blue-black. 8. Record the time taken, t, to the nearest 0.1 s in your table. 9. Wash out both beakers thoroughly with water and shake dry. Rinse and dry the glass rod. Experiments 2 to 5 Repeat experiment 1 four times, adding 18.00, 14.00, 10.00 and 6.00 cm 3 respectively, of FA 1 at point 2. In each case, you will need to ensure that the same total volume of the reaction mixture is used by adding deionised water as required using the second burette provided. Record all required volumes, time taken and calculated values in your table. Results Expt VFA1/ cm3 Vwater/ cm3 t/ s 1/t / s-1 lg(1/t ) lg(VFA1) 1 20.00 0.00 13.4 0.0746 -1.13 1.30 2 18.00 2.00 14.9 0.0671 -1.17 1.26 3 14.00 6.00 19.4 0.0515 -1.29 1.15 4 10.00 10.00 27.3 0.0366 -1.44 1.00 5 6.00 14.00 48.3 0.0207 -1.68 0.778 [1] single table with correct headers and units [1] correct precision of recording.  Recorded volumes to 2.d.p (nearest 0.05 cm3)  all t to nearest 0.1s  1/t, lg(1/t), lg(VFA1) correctly calculated to 3.s.f. [1] 5 sets of data + correct volumes of deionised water [4]

5  Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2023 [Turn Over  VFA1 as stated in the question  VFA1 + Vwater = 20.00 cm3 Accuracy: Deviation in ratio of time for 10.00 cm3 FA 1 / time for 20.00 cm3 FA 1 btw Supervisor and student’s titre, :   0.10 [1] (b) (i) Plot a graph of lg( 1/t) on the y-axis against lg( VFA1) on the x-axis. Draw

6  Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2023 the best-fit straight line taking into account all of your plotted points. [3] (ii) Calculate the gradient of the line to three significant figures, showing clearly how you did this. Hence, deduce the order of the reaction wit h [1] Axes correct way round + correct labels with units + proper scale (plotte d points occupy at least half of the given grid in both directions. Do not award this mark if student uses awkward scale (e.g. 1 for 3/7) [1] All points clearly plotted to within correct ½ small square. [1] Draw a line of best fit Do not award this mark if:  there is clearly a better line that could have been drawn through the points  the line is drawn through points “bunched” in less than 20  20 small 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 lg(1/t) lg(VFA1) (1.32, -1.1) (0.75, -1.71)

7  Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2023 [Turn Over respect to [S2O8 2]. Gradient = (−1.10)−(−1.71) 1.32−0.75 = 1.07 [1] construction lines drawn on the graph.  The hypotenuse of the constructed “triangle” should cover at least half of the length of the line drawn by the candidate  correctly reads to nearest ½ small square [1] calculates gradient correctly using the values read from the graph 1/t  rate, VFA1  [S2O8 2-], Rate = k’[S2O8 2-]n 1/t = k’(VFA1)n lg(1/t) = lg k’ + nlg(VFA1) lg(1/t) = nlg(VFA1) + lg k’ (y=mx+c graph) n= gradient of line. Order of reaction w.r.t. [S2O8 2] = 1 [1] [1] correct conclusion of order Gradient = ……………………………… Order = ……………………………… [3] (c) (i) A student thought that the experiment could be made more accurate by having longer reaction times. To do this, he repeated Experiment 1 w ith the same volumes but using 0.100 mol dm–3 sodium thiosulfate instead of

8  Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2023 FA 3. He found that the reaction never turned blue-black. Explain why. sodium thiosulfate is in excess . All the iodine reacts with the thiosulfate/ no iodine produced to turn blue-black). [1] (ii) Other than errors involving measurements of vol