Edexcel Jan 2012 Paper 6 Q3 (with explained solutions) |

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Edexcel Jan 2012 Paper 6 Q3

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PMT 3 The equation for the reaction of iodine with propanone is CH3COCH3(aq) + I2(aq) (cid:111) CH3COCH2I(aq) + H+(aq) + I(aq) An experiment was carried out to find the order of reaction with respect to iodine. 50 cm3 of iodine solution, concentration 0.020 mol dm3, was added to 25 cm3 of sulfuric acid, concentration 2.0 mol dm3, in a conical flask. 25 cm3 of propanone solution, concentration 2.0 mol dm3, was added to the mixture and a timer started. A 10.0 cm3 sample was removed after one minute. Further 10.0 cm3 samples were removed every three minutes. Immediately, each sample was added to 20 cm3 of sodium hydrogencarbonate solution (an excess). Each sample was then titrated with sodium thiosulfate solution, concentration 0.010 mol dm3. (a) (i) Show, by calculation of the number of moles, whether propanone or iodine was in excess. (ii) What would be the most suitable piece of apparatus for removing a 10.0 cm3 sample from the mixture? (2) (1)(iii) What would be the most suitable piece of apparatus for measuring 20 cm3 of the sodium hydrogencarbonate solution? (1)*P35988A0616*<br />
PMT (iv) Suggest why each sample was added to sodium hydrogencarbonate solution. Explain your answer. (2)(b) (i) What colour change would you expect to see as the reaction takes place? (2) Fromto(ii) To make the end-point of the titration easier to see, an indicator can be added. Name the indicator and state the colour change you would expect to see. (2) IndicatorColour change fromto(iii) At what stage in the titration should this indicator be added? (1)*P35988A0716* Turn over<br />
PMT (c) The following results were obtained in the experiment. Time / min Volume of sodium thiosulfate solution / cm3 1 4 7 10 13 16 19 19.1 15.9 13.0 9.9 7.1 3.9 1.0 (i) Explain why these results can be used to determine the order of the reaction directly, without calculating the corresponding concentrations of iodine in the solution. (1)*P35988A0816*<br />
(ii) Plot a graph of the volume of sodium thiosulfate solution on the vertical axis against time on the horizontal axis. (iii) Use your graph to deduce the order of the reaction with respect to iodine. Explain how you arrived at your answer. PMT (2) (2)*P35988A0916* Turn over<br />
PMT (d) The following rate-determining step for the reaction between propanone and iodine is suggested. CH3COCH3 + H+ (cid:111)(cid:3)CH3C+ CH3 OH Explain why your order of reaction with respect to iodine is consistent with this rate-determining step. (1)(Total for Question 3 = 17 marks) 10 *P35988A01016*<br />
PMT BLANK PAGE *P35988A01116* 11 Turn over<br />

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Question Number 2 (d) Question Number 3 (a)(i) Acceptable Answers Reject Fe2+ (1) + (1) NH4 2- (1) SO4 ALLOW: FeSO4.(NH4)2SO4 (xH2O) Fe(NH4)2 (SO4)2 (2) +/ NH4 NH3 / NH3 If the formula of D is given rather than the ions, the presence of NH3 scores (0). Mark (2) ALLOW a formula containing all three ions without charges and unbalanced (1) Acceptable Answers Reject Mark 50 x 0.02 = 0.001 mol I2 1000 25 x 2 = 0.05 mol (CH3)2CO 1000 Both quantities correct and propanone in excess Both quantities correct but incorrect or missing excess (1) One quantity correct (2) (1) Question Number 3 (a)(ii) Acceptable Answers Reject (Graduated) Pipette ( + filler) Accept recognisable spelling e.g pipet Burette Measuring cylinder Mark Question Number 3 (a)(iii) Measuring cylinder Acceptable Answers Ignore volumes e.g. 25cm3 measuring cylinder Reject Mark Burette/pipette 6CH0801 1201<br />
Acceptable Answers Question Number 3 (a)(iv) To quench /slow /stop the reaction Reject (1) By reacting with the sulfuric acid / neutralize (the acid) / remove the H+ ions (1) Mark independently Question Number 3 (b)(i) Acceptable Answers Reject From yellow/orange/red-brown /brown (1) To colourless / pale yellow (1) Second mark is dependent on the firstto clear /yellow Mark Mark Acceptable Answers Question Number 3 (b)(ii) Starch (1) From blue / blue-black / black To colourless Mark independently (1) Reject From Purple. to clear Mark Question Number 3 (b)(iii) As the colour of the iodine solution Acceptable Answers begins to fade/turns pale yellow/straw coloured/just before the end point (1) Reject Mark When colourless / yellow/brown Question Number 3 (c)(i) Acceptable Answers Reject Iodine concentration is proportional to the volume of sodium thiosulfate solution Mark 6CH0801 1201<br />
Acceptable Answers Question Number 3 (c)(ii) Axes correctly labelled including units with sensible scales (more than half graph paper used in each direction) (1) Points correctly plotted and best fitting straight line drawn (1) Absence of units Dots joined with straight lines Reject Mark Allow - completely correct but axes wrong way round (1) Acceptable Answers Reject Mark (1) reaction is order zero rate is constant/gradient is constant/ [iodine] does not affect rate (1) Mark independently Reject Mark Acceptable Answers Iodine is not involved in the rate determining step, (because order is zero with respect to iodine) If 3ciii is not zero order, the mark is only awarded if the candidate states that the order is inconsistent. 6CH0801 1201 Question Number 3 (c)(iii) Question Number 3 (d)<br />

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