28   10 The progress of the reaction between iodine and propanone with an acid catalyst can be followed in an experiment using a titrimetric method. Procedure Step 1 Mix 25 cm3 of 1 mol dm–3 aqueous propanone with 25 cm3 of 1 mol dm–3 sulfuric acid in a beaker. Both these reactants are in excess. Step 2 Start the stop clock as 50 cm3 of 0.02 mol dm–3 iodine solution is added to the beaker. Mix the reactants thoroughly. Step 3 Withdraw a 10.0 cm3 sample of the reaction mixture, using a pipette, and transfer it to a conical flask. Step 4 Add a spatula measure of sodium hydrogencarbonate, noting the exact time. Step 5 Titrate the iodine present in the 10.0 cm3 sample with 0.01 mol dm–3 sodium thiosulfate solution, using starch indicator. Step 6 Continue to withdraw 10.0 cm3 samples about every two minutes, repeating Steps 4 and 5 with each sample. (a) (i) Explain why sodium hydrogencarbonate is added in Step 4. 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(ii) Write the ionic equation for the reaction that takes place during Step 4. State symbols are not required. (1) Turn over 29   (b) Some data from the experiment are shown. Time sodium hydrogencarbonate is added / min 2.0 5.0 6.5 8.0 10.5 12.0 Volume of sodium thiosulfate / cm3 19.2 15.5 14.0 12.1 9.5 7.2 (i) Plot a graph of the volume of sodium thiosulfate against the time the sodium hydrogencarbonate is added. (2) 30   (ii) Explain how the graph of volume of thiosulfate against time confirms the reaction is zero order with respect to iodine, I2. 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Turn over 31   (c) The overall rate equation for the reaction is rate = k[H+(aq)][CH3COCH3(aq)]. A student researching the mechanism for the reaction found this example. Step 1 Step 2 O CH3 H3C C Step 3 OH CH3 H2C C OH CH3 H2C C O + CH3 H2 H I C C O + CH3 H2 H I C C + H+ + I– + H+ + H+ O CH3 H2 I C C + I2 O CH3 H3C C O CH3 IH2C C acidic conditions Overall reaction + H+ + I– + I2 (i) Predict which of the three steps is the rate‑determining step. Justify your answer. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 32   (ii) The student stated that ‘The hydrogen ions cannot be acting as a catalyst. One hydrogen ion is a reactant in Step 1 but two hydrogen ions are formed as products in Steps 1 and 3.’ Explain whether or not this statement is valid. 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(Total for Question 10 = 12 marks) TOTAL FOR PAPER = 120 MARKS 33   BLANK PAGE 34   BLANK PAGE 35   BLANK PAGE 36  