ASRJC 2026 J2 H2Chem Prelim P4 ANSWER

ASRJC JC2 PRELIM 2026 9476/04/H2 [Turn over ANDERSON SERANGOON JUNIOR COLLEGE 2026 JC2 PRELIMINARY EXAMINATION NAME:______________________________ ( ) CLASS: 26 / _____ CHEMISTRY 9476/04 Paper 4 Practical SUGGESTED SOLUTIONS

2 ASRJC JC2 PRELIM 2026 9476/04/H2 Do not write in margin 1 (a) To determine the order of reaction with respect to iodine in an alpha halogenation reaction of a carbonyl compound Aldehydes and ketones can be halogenated at their  positions by reaction with halogens like iodine in acidic solution. A general equation is as shown in reaction 1. equation 1 C O C H I2 C O C I HI  FA 1 is 1.00 mol dm–3 of an unknown carbonyl compound FA 2 is 1.00 mol dm–3 sulfuric acid, H2SO4 FA 3 is an aqueous solution of iodine, I2 FA 4 is 0.0100 mol dm–3 sodium thiosulfate, Na2S2O3 FA 5 is 0.50 mol dm–3 sodium hydrogencarbonate, NaHCO3 You are also provided with Solution S, 1% starch solution. This reaction is first order with respect to both the carbonyl compound and H+ ions. You are to investigate the order of reaction with respect to I2. To do this, you will prepare a reaction mixture containing the carbonyl compound, I2 and H+. At timed intervals, you will remove aliquots (portions) of this reaction mixture and add an excess of NaHCO3 to the aliquots. The remaining amount of I2 at different times can then be determined by titration with Na2S2O3 using starch indicator. I2 reacts with Na2S2O3 as shown in the equation in reaction 2 equation 2 I2(aq) + 2S2O32–(aq) 2I–(aq) + S4O62–(aq) The required order of reaction will be obtained by graphical analysis of your results. (i) Preparation and titration of reaction mixture Notes: You will perform each titration once only. Great care must be taken that you do not overshoot the end-point. Once you have started the stopwatch , it must continue running for the duration of the experiment. You must not stop it until you have collected all of your aliquots. You should transfer your first aliquot and treat it with FA 5 at approximately four minutes after the reagents were mixed. You will then remove four further aliquots, at time intervals of your choice, up to a maximum time of 25 minutes. In an appropriate format in the space provided on page 4, prepare a table to record for each aliquot • the actual time of transfer, t, in minutes and seconds, • the decimal time, t d, in minutes, to 0.1 min, for example if t = 4 min 33s, then td = 4 + 33/60 min = 4.6 min, • the burette readings and the volume of FA 4 added.

3 ASRJC JC2 PRELIM EXAM 2026 9476/04/H2 [Turn over Do not write in margin Safety notes: • Keep the conical flask (reaction mixture) stoppered except when removing aliquots. • FA 1 is flammable. Transfer your titrated solutions into the waste bottle for disposal. Keep this bottle stoppered when not in use. 1. Fill a burette with FA 4. 2. Using a measuring cylinder, add the following to a 100 cm3 beaker. • 25.0 cm3 of FA 1 • 25.0 cm3 of FA 2 3. Using another burette, transfer 50.00 cm3 of FA 3 into the 250 cm3 conical flask, labelled reaction mixture. 4. Pour the contents of the 100 cm3 beaker into this 250 cm3 conical flask. Start the stopwatch, insert the stopper and swirl the mixture thoroughly. 5. Using a 10 cm 3 pipette, transfer a 10.0 cm 3 aliquot of the reaction mixture into a second 250 cm3 conical flask. Replace the stopper in the flask. 6. Using a 10 cm3 measuring cylinder, transfer about 10 cm3 of FA 5 to the 10.0 cm3 aliquot in the second conical flask. Swirl the conical flask thoroughly. Record the time of transfer, t, in minutes and seconds, to the nearest second, when FA 5 is added to the aliquot. 7. Immediately, titrate the iodine in the second conical flask with FA 4. When the colour of the solution turns pale yellow, add about 1 cm3 of starch indicator using a teat pipette. The solution will turn dark blue/black. Continue titration until the dark blue/black colour just disappears. Record your titration results. 8. Empty the contents of the second conical flask into the waste bottle. Wash this conical flask thoroughly with water. 9. Repeat steps 5 to 8 for four further aliquots, removing each at time intervals of your choice, up to a maximum time of 25 minutes. Continue until a total of five aliquots have been titrated and their results recorded.

4 ASRJC JC2 PRELIM 2026 9476/04/H2 Do not write in margin Results Time of transfer Decimal time, td / min Initial burette reading / cm3 Final burette reading / cm3 Volume of S2O32- used / cm3 4 min 4.0 0.00 18.30 18.30 8 min 8.0 18.00 34.90 16.60 12 min 30 s 12.5 2.00 17.20 15.20 17 min 17.0 20.00 33.25 13.25 20 min 30 s 20.5 0.00 12.45 12.45 [1] Tabulates initial and final burette readings, volume added, time of transfer, decimal time, td. Tables must have correct headers and units. [1] Records all • volumes to 0.05 cm3 • correctly calculate volume of S2O32- used • actual times to consistent precision i.e. times in minutes and seconds • correctly calculates decimal values of t and records to 1 d.p [1] • 5 sets of titration results with decreasing volume of S2O32- used • First aliquot is taken at between 3 to 5 min • Well-spaced values of time of transer, where the longest time is ≤ 25 min [3]

5 ASRJC JC2 PRELIM EXAM 2026 9476/04/H2 [Turn over Do not write in margin (ii) Plot a graph of volume of FA 4, on the y-axis, against decimal time, td, on the x-axis on the grid in Fig. 1.1 Draw the most appropriate best-fit line taking into account all of your plotted points. Extrapolate (extend) this curve to td = 0.0 min. Fig. 1.1

6 ASRJC JC2 PRELIM 2026 9476/04/H2 Do not write in margin [3] [1] • Axes correct way round with correct labels and units and appropriate scale. • The scale must be chosen so that the y intercept would fall within the scale range; and, that the plotted points occupy at least half the graph grid in the x direction and the plotted points and the y intercept together occupy at least half the graph grid in the y direction. [1] Plotted points within ±½ small square. [1] Graph line is straight and is the best-fit line and extrapolated to include the point at 0 min. (iii) Deduce the order of reaction with respect to I2 for the reaction in equation 1. Explain your answer. [1] Order of reaction with respect to I2 is zero. Since the graph shows a constant gradient, the rate of reaction is constant. Hence, rate of reaction is independent of the [I2]. [1]

7 ASRJC JC2 PRELIM EXAM 2026 9476/04/H2 [Turn over Do not write in margin (iv) Calculate the gradient of the line, showing clearly how you did this. Gradient = (19.7−12.0) (0−21.0)  - 0.367 cm3 min-1 (units is not required) gradient = ……….…………[1] • Clear indication of correct co-ordinates from graph (measured to ± ½ small square) • Gradient correctly calculated. ` (v) The y–intercept of your line gives the volume of sodium thiosulfate required to completely react with the iodine present if an aliquot is taken at td = 0.0 min. Read and record th is volume of FA 4 , Vmax. Use this value to calculate the concentration of iodine in FA 3. Vmax = y–intercept = 19.7 cm3 [1] correctly read from intercept to within ± ½ small square Amount of thiosulfate = 19.7 × 10–3 × 0.0100 = 1.97 × 10–4 mol Amount of iodine (in 10 cm3) = (1.97 × 10–4) ÷ 2 = 9.85 × 10–5 mol [1] Amount of iodine in 100 cm3 of reaction mix