ACJC 2026 H2 Prelim Paper 4 Answers

2 ACJC2026 9476/Preliminary Examinations/2026 [Turn over Answer all the questions in the spaces provided. 1 Identification of two cations and one anion (a) FA 1 and FA 2 contain a different cation and the same anion. The volumes given in Table 1.1 are approximate and should be estimated rather than measured. Test any gas evolved during the tests. If there is no observable change, write no observable change. (i) Carry out the tests described in Table 1.1. Carefully record your observations in Table 1.1. Table 1.1 test observations with FA 1 observations with FA 2 1 Add 1 cm depth of the FA solution into a clean test- tube. Test the FA solution using Universal Indicator paper. • UI paper turns orange. pH is 2 / 3 / 4. ✓ 2 Add 1 cm depth of the FA solution into a clean boiling tube. Add 1 cm depth of aqueous sodium hydroxide slowly, with shaking, until the aqueous sodium hydroxide is in excess. • Blue ppt Cu(OH)2 ✓ • Insoluble in excess NaOH ✓ • White ppt Al(OH)3 ✓ • Soluble (dissolve) in excess NaOH to form a colourless solution [Al(OH)3]– ✓ Gently warm the boiling tube. • Black / dark brown ppt CuO seen ✓ • No observable change ✓ 3 Add 1 cm depth of the FA solution into a clean test- tube. Add 1 cm depth of aqueous ammonia slowly, with shaking, until the aqueous ammonia is in excess. • Blue ppt Cu(OH)2 ✓ • Soluble (dissolve) in excess NH3 to form a dark blue solution [Cu(NH3)4(H2O)2]2+ ✓ • White ppt Al(OH)3 ✓ • Insoluble in excess NH3 ✓

3 ACJC2026 9476/Preliminary Examinations/2026 [Turn over 4 Add 1 cm depth of the FA solution into a clean test- tube. Add a few drops of aqueous silver nitrate, then • White ppt AgCl formed ✓ • White ppt AgCl formed ✓ add aqueous ammonia to the mixture, with shaking, until the aqueous ammonia is in excess. • Pale blue ppt formed Cu(OH)2 • Completely soluble in excess to form a dark blue solution ✓ Cu(OH)2(s) forms [Cu(NH3)4(H2O)2]2+(aq) and AgCl(s) forms [Ag(NH3)2]+ (aq) • Ppt is partially soluble in excess. ✓ AgCl(s) forms [Ag(NH3)2]+(aq) and Al3+(aq) forms Al(OH)3 (s) [7] (b) (i) Complete Table 1.2 to identify the cations and anion in FA 1 and FA 2. Table 1.2 FA 1 FA 2 cation present Cu2+ Al3+ anion present Cl– [3] (ii) Explain which test was used to identify the anion. Test ………… of FA …….. was used to identify the anion. In test 4, AgCl was formed which dissolved (completely soluble) in excess aq. NH3 to form [Ag(NH3)2]+ Evidence: ……………………………………………………………………………………... [1] (iii) Write an equation and explain the observation of FA 2 in test 1. [Al(H2O)6]3+(aq) [Al(H2O)5(OH)]2+(aq) + H+(aq) [Al(H2O)6]3+(aq) undergoes partial / substantial hydrolysis to form H+ ions, due to the high charge density / polarising power of the cation. ………………………………………………………………………………………………... [2] [Total: 13] 4 1

4 ACJC2026 9476/Preliminary Examinations/2026 [Turn over 2 Determination of the kinetics of a clock reaction FA 3 is 0.100 mol dm−3 potassium iodate(V), KIO3. FA 4 is 0.0500 mol dm−3 sodium hydrogensulfite, NaHSO3. You are also provided with: • starch solution • deionised water. Iodate(V) ions react with hydrogensulfite ions in aqueous solution according to the equation shown. 2IO3−(aq) + 5HSO3−(aq) I2(aq) + 5SO42−(aq) + 3H+(aq) + H2O(l) In this experiment, the rate of reaction is studied by measuring the time taken to form a fixed amount of iodine using a ‘clock’ method. In the presence of starch, a blue-black colour indicates the formation of iodine. The reaction proceeds through several steps, so the blue-black colour does not appear immediately. You will vary the concentration of iodate( V) ions and measure the time, t, taken for the blue-black complex to appear. You will perform a series of five experiments, in which the volume of FA 3 is varied while the volume of FA 4 is kept constant. In each experiment, you will adjust the volume of deionised water so that the total volume of the reaction mixture is 20.0 cm3. Experiment 1 1. Use a teat pipette to add 10 drops of starch solution to a clean 100 cm3 conical flask. 2. Use a 10 cm3 measuring cylinder to measure 2.0 cm3 of FA 3 into the conical flask. 3. Use a 25 cm 3 measuring cylinder to add 16.0 cm 3 of deionised water into the same conical flask. Swirl gently to mix. 4. Use a second 10 cm3 measuring cylinder to measure 2.0 cm3 of FA 4. 5. Pour FA 4 into the conical flask, starting the stopwatch during this addition. 6. Swirl the mixture thoroughly. Place the flask on a white tile. 7. Stop the stopwatch at the first appearance of a blue-black colour. 8. Record the time taken, t, to the nearest second, in your table. 9. Wash the conical flask thoroughly with tap water, then with deionised water. Stand it upside down on a paper towel to drain. Record all volumes, the time taken and the calculated values of 1/t , log(1/t) and log(VFA3) in the space provided on page 6. Experiments 2 to 5

5 ACJC2026 9476/Preliminary Examinations/2026 [Turn over Repeat experiment 1 four times, using 4.0 cm 3, 6.0 cm3, 8.0 cm3 and 10.0 cm3 of FA 3 respectively in step 2. In each experiment, you should adjust the volume of deionised water in step 3 to ensure that the total volume of reaction mixture is 20.0 cm3. You should alternate the use of two 100 cm3 conical flasks. (a) Results Expt V FA 3 / cm³ V FA 4 / cm³ V H2O / cm³ t / s 1/t / s–1 log(1/t) log(VFA 3) 1 2.0 2.0 16.0 57 0.0175 -1.76 0.301 2 4.0 2.0 14.0 26 0.0385 -1.41 0.602 3 6.0 2.0 12.0 16 0.0625 -1.20 0.778 4 8.0 2.0 10.0 14 0.0714 -1.15 0.903 5 10.0 2.0 8.0 11 0.0909 -1.00 1.00 Examiner’s Comments: • Good answers ensured that: o there are correct headers and units, correct values of VH2O o all volumes recorded to 1 decimal place, o t values recorded to the nearest second o log(1/t) (all negative values), 1/t and log(V FA 3) calculated correctly to 3 s.f. [5]

6 ACJC2026 9476/Preliminary Examinations/2026 [Turn over (b) Plot a graph of log(1/ t) on the y-axis against log(VFA3) on the x-axis for experiments 1 to 5 on the grid in Fig. 2.1. Draw the best-fit straight line. [3] • Good answers ensured that o There were correct axes + axes labels + units + appropriate scale o Scale is chosen so that plotted points occupy more than half the graph grid in both x and y directions. o All 5 points plotted correctly to within ±½ small square A best-fit straight line is drawn (with all points no more than 5 small squares away from the line)

7 ACJC2026 9476/Preliminary Examinations/2026 [Turn over (c) Calculate the gradient of your best-fit line to three significant figures, showing clearly how you did this. gradient = …………..…………………………[1] Gradient = 1.10 ( 1.70) 0.9 0.34 − − − − = 1.07 (d) The gradient of the line in (b) gives the order of reaction, n, with respect to [ IO3−]. State t he value of n. n = …………………………………….. [1] n = 1 (e) Explain why log(VFA3) may be plotted directly on the x-axis instead of log[IO3−] to determine the order of reaction with respect to [IO3−]. [2] Since the total volume of the reaction mixture is kept constant in