ACJC H2 CHEM Chem P4 Ans Prelim

2 © ACJC 2017 9729/04/Prelim/2017 [Turn over Question 1 In this experiment, you are required to find the concentration in mol dm -3 of sodium thiosulfate, Na2S2O3, in solution FA 1. You are provided with: FA 1 contains sodium thiosulfate, Na2S2O3. FA 2 is potassium manganate (VII) containing 1.83 g dm-3 KMnO4. FA 3 is 1 mol dm-3 sulfuric acid, H2SO4. FA 4 is 10% potassium iodide containing 4 g dm-3 KI. You are also provided with starch indicator. For Examiner’s Use Dilution of FA 2 (a) By using a burette measure between 41.00 cm3 and 42.00 cm3 of FA 2 into the 250 cm3 graduated flask. Record your burette readings and the volume of FA 2 added to the flask in the space below. Final burette reading / cm3 41.50 Initial burette reading / cm3 0.00 Volume of FA2 used / cm3 41.50 [2] Make up the contents of the flask to the 250 cm 3 mark with distilled water. Stopper and mix the contents thoroughly by slowly inverting the flask a number of times. Label this solution FA5. Titration Filll a second burette with FA 1, the solution containing sodium thiosulfate. Use a measuring cylinder to transfer 10 cm 3 of FA 3 and 10 cm 3 of FA 4 into a conical flask. Pipette 25.0 cm 3 of FA 5 into the conical flask containing the mixture of FA 3 and FA 4. The potassium manganate (VII) oxidises potassium iodide to iodine, I2. Titrate the liberated iodine with FA 1 as follows. Run the solution from the burette into the conical flask until the initial brown colour of the iodine becomes pale yellow. Then add 1 cm 3 of the starch indicator and continue to add FA 1 drop by drop until the blue - black colour of the starch-iodine complex disappears, leaving a colourless solution. This is the end-point of the titration. Record your titration results in the space below. Make certain that your recorded results show the precision of your working. Repeat your experiment to obtain consistent results. 1 2 Final burette reading / cm3 24.20 24.20 Initial burette reading / cm3 0.00 0.00 Volume of FA1 used / cm3 24.20 24.20 [6]

3 © ACJC 2017 9729/04/Prelim/2017 [Turn over (b) From your titration results obtain a suitable volume of FA 1 to be used in your calculations. Show clearly how you obtained this volume. Average titre = (24.20 + 24.20) / 2 = 24.20 cm3 Volume of FA1 = ………………… [1] For Examiner’s Use Calculations Show your working and appropriate significant figures in all of your calculations. (c) Calculate how many moles of KMnO 4 are contained in the FA 2 run into the graduated flask. [Ar: K, 39.1; O, 16.0; Mn, 54.9] No. of moles of KMnO4 added into the graduated flask = 𝟒𝟏.𝟓𝟎 𝟏𝟎𝟎𝟎 x 𝟏.𝟖𝟑 𝟏𝟓𝟖.𝟎 = 4.81 x 10-4 No. of moles of KMnO4 added into the graduated flask = ……………………… [1] (d) Calculate how many moles of KMnO 4 are then pipetted from the 250 cm 3 graduated flask into the titration flask. No. of moles of KMnO4 pipetted into the titration flask = 4.81 x 10-4 x 𝟐𝟓.𝟎 𝟐𝟓𝟎 = 4.81 x 10-5 No. of moles of KMnO4 pipetted into the titration flask = ……………………… [1] (e) Use this answer to calculate how many moles of iodine molecules, I2 are formed when the manganate (VII) ions react with an excess of iodide ions in the titration flask. MnO4- + 8H+ + 5e → Mn2+ + 4H2O I- → ½ I2 + e No. of moles of I2 formed = 4.81 x 10-5 x 𝟐. 𝟓 = 1.20 x 10-4 No. of moles of iodine molecules, I2 formed = ………………………. [1]

4 © ACJC 2017 9729/04/Prelim/2017 [Turn over (f) Use this answer to calculate how many moles of sodium thiosulfate, Na 2S2O3 react with the iodine molecules formed. 2S2O32- → S4O62- + 2e ½ I2 + e → I- No. of moles of Na2S2O3 reacted = 1.20 x 10-4 x 2 = 2.40 x 10-4 No. of moles of thiosulfate ions reacted = ………………………. [1] For Examiner’s Use (g) Calculate the concentration in mol dm-3 of the sodium thiosulfate, Na2S2O3, in FA 1. Concentration of Na2S2O3 in FA1 = 2.40 x 10-4 x 𝟏𝟎𝟎𝟎 𝟐𝟒.𝟐𝟎 = 9.93 x 10-3 mol dm-3 Concentration of sodium thiosulfate = ………………………. [1] [Total: 14 marks]

5 © ACJC 2017 9729/04/Prelim/2017 [Turn over Question 2 The derivation of rate equations is an important part of the process of dis covering a reaction mechanism. The experiment detailed in this question is part of an investigation into the mechanism of the iodination of propanone: CH3COCH3(aq) + I2(aq) → CH3COCH2I(aq) + H+(aq) + I-(aq) The reaction is catalysed by hydrogen ions and hence the rate equation can be expressed as Rate = k [CH3COCH3]a [H+]b [I2]c where a, b and c are the orders with respect to the species shown in the rate equation. FA 1 contains sodium thiosulfate, Na2S2O3 (from Question 1) FA 3 is 1 mol dm-3 sulfuric acid, H2SO4 (from Question 1) FA 6 is 1 mol dm-3 aqueous propanone FA 7 is 0.02 mol dm-3 I2 in KI solution (5 g of I2 and 33 g of KI in 1 dm3 solution) FA 8 is 1 mol dm-3 NaHCO3 For Examiner’s Use Procedure 1. Pipette 25.0 cm3 of FA 3 into a conical flask. 2. Using the same pipette deliver 25.0 cm3 of FA 6 into the same conical flask. 3. Using a measuring cylinder, collect 50 cm3 of FA 7. 4. Transfer the FA 7 into the conical flask containing the mixture of FA 3 and FA 6 as quickly as possible and at the s ame time start the stop watch. Swirl the mixture in the conical flask to ensure complete mixing. 5. When the reaction has been going on for 3 minutes, pipette 10.0 cm 3 of the reaction mixture into another conical flask containing 10 cm 3 of FA 8, noting the exact time at which this was done. 6. Mix the solution thoroughly and titrate the mixture against FA 1 from a burette until the solution turns yellow. Using the plastic dropper add 1 cm 3 of starch and continue titration until the solution becomes colourless. 7. Repeat steps 5 – 6, at times of 6, 9, 12 and 15 minutes. 8. Record yours results in the table below: Results Time when reaction mixture was added to NaHCO3 Final Volume / cm3 Initial Volume / cm3 Vol. of FA1 used / cm3 [6]

6 © ACJC 2017 9729/04/Prelim/2017 [Turn over (a) Plot a graph of the volume of FA 1 used against time.  Labelled axes of appropriate scale  Straight line (best fit) [2] For Examiner’s Use (b) What is the purpose of adding the reaction mixture to FA 8 in Step 5. Explain. NaHCO3 neutralises the acid and hence stops the reaction so that the amount of iodine remaining at that instant can be determined [1] (c) The acid (FA 3) and the propanone (FA 6) are used in large excess in the reaction. Explain why this is necessary. To ensure that the concentration of propanone and H + remains approximately constant during the duration of the reaction [1] For Examiner’s Use

7 © ACJC 2017 9729/04/Prelim/2017 [Turn over (d) Based on your graph,