SAJC 2026 H2Chem Prelim P4 solutions

1 1 Investigating the kinetics of the reaction between thiosulfate ions and acid FA 1 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. FA 2 is 1.00 mol dm–3 sulfuric acid, H2SO4 The thiosulfate ion, S2O32-, decomposes when an acid is added according to the equation shown. S2O32-(aq) + 2H+(aq) → S(s) + SO2(aq) + H2O(l) In this experiment, the rate of the reaction is studied by measuring the rate of formation of sulfur using a ‘clock’ method. You will vary the concentration of thiosulfate ions and measure the time taken for a fixed amount of sulfur to be produced. This will be indicated by how long it takes for the solid sulfur forming to obscure the print on the insert. You will perform a series of five experiments. For each experiment you will need to ensure that the same total volume of reaction mixture is used, by adding deionised water as require. For each experiment, you will measure the time, t, for the sulfur to obscure the print on the insert. Note: A small amount of sulfur dioxide gas may be formed in the experiment. It is very important that you avoid inhaling any fumes. As soon as each experiment is complete, add the reaction mixture to the waste beaker and rinse the beaker thoroughly. (a) Prepare a table in the space provided on page ?? in which to record, to an appropriate degree of precision: • the volumes of FA 1 (VFA 1) and deionised water used • all values of t • rate Experiment 1 1) Use the 50 cm 3 measuring cylinder to measure 40.0 cm 3 of FA 1 into the 100 cm3 beaker. 2) Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FA 2. 3) Pour the FA 2 into the same beaker, starting the stopwatch during this addition. 4) Stir the mixture once and place the beaker on the printed insert. 5) Stop the stopwatch when the print on the insert becomes obscured. 6) Record the time taken, to the nearest second, in your table. 7) Discard the reaction mixture immediately into the waste beaker. 8) Wash out the beaker thoroughly with tap water and then with deionised water. Stand the beaker upside down on a paper tower to drain. Experiment 2

2 1) Use the 50 cm 3 measuring cylinder to measure 20.0 cm 3 of FA 1 into the 100 cm3 beaker. 2) Use the same 50 cm 3 measuring cylinder to measure 20.0 cm 3 of deionised water into the same beaker. 3) Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FA 2. 4) Pour the FA 2 into the same beaker, starting the stopwatch during this addition. 5) Stir the mixture once and place the beaker on the printed insert. 6) Stop the stopwatch when the print on the insert becomes obscured. 7) Record the time taken, to the nearest second, in your table. 8) Discard the reaction mixture immediately into the waste beaker containing sodium carbonate solution. 9) Wash out the beaker thoroughly with tap water and then with deionised water. Stand the beaker upside down on a paper tower to drain. Experiments 3 - 5 Carry out three further experiments to investigate how using different volumes of FA 1 affects the reaction time. Do not use a volume of FA 1 that is less than 15.00 cm3. In each experiment, you should adjust the volume of deionised water in step 12 to ensure the same total volume of reaction mixture is used. You should alternate the use of the two 100 cm 3 beaker. You should keep FA 1 and FA 2 for use in Question 2. Record all your results in a table. You should include all volumes, the reaction time and the reaction rate for each of your five experiments in your table. The rate of reaction can be calculated using the following formula. rate= 1000 reaction time [5] Expt VFA 1 / cm3 Volume of water / cm3 t / s rate / s‒1 1 40.0 0.0 26 38.5 2 20.0 20.0 54 18.5 3 30.0 10.0 35 28.6 4 25.0 15.0 44 22.7 5 15.0 25.0 75 13.3 PDO Layout [M1]

3 • Tabulates VFA1, volume of FA 1, volume of water, time and rate • Table contain correct headers and units PDO Record [M2] Records all: • time to nearest s • volumes of FA 1 and water to 1 dp. PDO Manipul [M3] correctly calculates rate for Experiment 1 and 2 and 3 other experiments of varying amount of FA1 and shown to 3 sf. MMO Collect [M4] • 5 experiments done • 3 additional experiments with volume FA 1 ⩾ 15.00 cm3, < 40.00 cm 3 and no volume closer than 5.00 cm3 to another volume • Volumes of water chosen so that total volume = 40.00 cm3 for additional experiments. MMO Quality [ M5] Award when all times recorded increase with decreasing volume of FA 1. (b) (i) Plot a graph of rate on the y -axis against the volume of FA 1 on the x-axis on the grid in Fig. 1.1. Start each axis at the origin (0,0). Draw the best-fit line. [3]

4 Fig. 1.1 PDO Layout [M1] • Axes correct way round + correct unambiguous labels + units + scale. • Scales chosen so that plotted points occupy at least half graph grid in x axes • The scale must start at zero on both the x and y-axis. PDO Layout [M2] All points correctly plotted within ±½ small square. PDO Manipul [M3] Line of best fit drawn takes into account only the 5 points, not, straight line or smooth curve, that ignores anomalous results.

5 (ii) From your graph, what conclusion can you make about the relationship between the relative rate for the reaction and the volume of FA 1 used? Explain your answer. [2] If straight line: rate is directly proportional to volume of FA 1 [1] because it shows a straight line passing through the origin or shows a y = mx graph [1]. If curve: rate is directly proportional to square of the volume of FA 1 [1] because it shows a curve passing through the origin or shows a y = mx2 graph [1]. (iii) From your graph, determine the time it would take for the print to be obscured if 7.50 cm3 of FA 1, 32.5 cm3 of water and 10.0 cm3 of FA 2 had been used. Show clearly on the graph how you worked out your answer. time for printing to be obscured = …………………… s [2] 7 = 1000 / t t = 143 s PDO Manipul [M1] • Correct line(s) drawn • rate correctly read within half a small square. ACE interpret [1] • 1000/ rate, ecf from point read • correctly calculates answer to 3 sf. (c) (i) A student found that the reaction was first order with respect to [H+]. Predict and explain how the value of t in Experiment 1 would be affected if hydrochloric acid of the same concentration had been used instead of FA 2. [1] t will be doubled. HCl is monoprotic, while H2SO4 is diprotic. When HCl is used, [H+] is halved and rate of reaction is halved. ACE interpret (ii) Predict and explain how the value of t in Experiment 1 would be affected if a 250 cm3 beaker with larger base area was used instead of the 100 cm3 beaker. [1]

6 t will be larger. A larger amount of solid sulfur (wef) would need to be produced for the solution to become opaque due to the larger base area. ACE interpret [Total: 14]

7 2 Determination of the x of a iodate ion, IOx– Iodate ions contain iodine and oxygen. They have the formula IOx– where x is an integer. (a) In this experiment you will determine the value of x in an iodate. You will first react IOx– with an excess of iodide ions, I–, to form iodine, I2. The amount of iodine produced is then determined by titration with thiosulfate ions, S2O32–. I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq) FA 1 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. FA 2 is dilute sulfuric acid, H2SO4. FA 3 is a solution containing 0.0140 mol dm–3 IOx– ions. FA 4 is 0.500 mol dm–3 potassium iodide, KI. FA 5 is starch indicator. (i) Titration 1) Fill the burette with FA 1. 2) Use the pipette to transfer 25.0 cm3 of FA 3 into a 250 cm3 conical flask. 3) Use the 25 cm 3 measuring cylinder to transfer 10.0 cm 3 of FA 2 to the same conical flask. 4) Use the same measuring cylinder to transfer 10 .0 cm3 of FA 4 to the same conical flask. 5) Run FA 1 from the burette into t