Edexcel Jan 2013 Paper 2 Q21 (with explained solutions) |

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Edexcel Jan 2013 Paper 2 Q21

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SECTION B Answer ALL the questions. Write your answers in the spaces provided. 21 Chlorine is used to prevent the growth of bacteria in swimming pool water. It reacts as shown below. Cl2(aq) + H2O(l) U HCl(aq) + HClO(aq) (a) (i) By giving appropriate oxidation numbers, explain why this is a disproportionation reaction. (3)(ii) State and explain the effect on the position of equilibrium if concentrated hydrochloric acid is added to a sample of chlorinated swimming pool water. (2)*P41213A0824*<br />
(b) In a similar reaction, chlorine reacts with sodium hydroxide to make household bleach. Cl2(aq) + 2NaOH(aq) o NaCl(aq) + NaClO(aq) + H2O(l) The concentration of NaClO in diluted bleach was measured by titration. A 25.0 cm3 sample of bleach was pipetted into a conical flask. Approximately 1.5 g of solid potassium iodide and 10 cm3 of hydrochloric acid with concentration 2.00 mol dm3 were added. Each mole of ClO, from the NaClO in the solution of bleach, produced one mole of iodine, I2, which was titrated with sodium thiosulfate solution. (i) Complete the ionic half-equations below for the reaction of ClO with acidified potassium iodide by balancing them and adding electrons where required. ClO + H+ o Cl + H2O I o I2 (ii) Use your answer to (a)(i) to write the overall ionic equation for the reaction between ClO and I ions in acidic conditions. (2) (1) (iii) The iodine in the sample required a mean (average) titre of 24.20 cm3 of 0.0500 mol dm3 sodium thiosulfate solution. Thiosulfate ions react with iodine as shown below. 2S2O3 2 + I2 o S4O6 2 + 2I Calculate the number of moles of iodine in the solution. (iv) What is the number of moles of ClO ions in the sample of diluted bleach? (2) (1) *P41213A0924* Turn over<br />
(v) Hence calculate the concentration, in mol dm3, of ClO in the diluted bleach. (vi) 1.5 g of potassium iodide, KI, contains 9.04103 mol of I. Use your answers to (b)(ii) and (b)(iv) to show by calculation why this amount was suitable. (1) (2)(vii) A student carrying out this titration measured the mean (average) titre as 24.50 cm3. What is the percentage difference in this students titre, compared with the accurate value of 24.20 cm3? (1) 10 *P41213A01024*<br />
(viii) The difference between the students mean titre and the accurate value was not due to the limitations in the accuracy of the measuring instruments. Suggest one possible reason for this difference. (1)(c) Suggest one damaging effect to the upper atmosphere which could be caused by the presence of chlorine compounds. (1)(Total for Question 21 = 17 marks) *P41213A01124* 11 Turn over<br />

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Section B Question Number 21(a)(i) First two marks Acceptable Answers Cl in Cl2 is 0 Goes to +1 in HClO Goes to 1 in HCl (2) All three correct for two marks Any two correct for one mark Ignore correct oxidation numbers for other elements If three correct numbers given without saying what species they are in max 1 for these two marks Third mark Cl/Cl2/the same element is both oxidized and reduced Allow same molecule/species/ type of atom is both oxidized and reduced if answer elsewhere has been in terms of chlorine OR Cl/Cl2/the same element both increases and decreases in oxidation number OR Chlorine both loses and gains electrons (1) Question Number 21(a)(ii) Equilibrium moves to the left / moves in Acceptable Answers reverse direction / moves to increase concentration of reactants (1) To use up (some of) added HCl/ to react with added HCl/ to stop formation of HCl/ restores equilibrium by producing more chlorine and water (1) Second mark depends on first Allow moves to decrease concentration of products/HCl for both marks Reject Mark Mark Only Cl+ for oxidation number +1 Only Cl- for oxidation number -1 (treat each separately) For each incorrect oxidation number change for O and H, lose one mark. 0 to +1 described as reduction and/or 0 to -1 described as oxidation (for third mark) Reject Just reverse reaction is favoured Just to counteract the change in the system To minimise effect of HCl<br />
Reject Mark Equations without electrons Question Number 21(b)(i) ClO + 2H+ + 2e(- ) Cl + H2O Acceptable Answers ALLOW ClO + 2H+ Cl + H2O2e(-) (1) 2II2 + 2e() ALLOW 2I Allow multiples Ignore state symbols even if incorrect2e(-)I2 (1) Question Number 21(b)(ii) ClO + 2H+ + 2ICl + H2O + I2 Acceptable Answers Mark independently. No TE on 21(b)(i) Question Number 21(b)(iii) Moles thiosulfate = (24.20 x 0.0500 / Acceptable Answers 1000) = 1.21 x 10-3 /1.2 x 10-3 /0.00121/ 0.0012 (mol) (1) Moles iodine = half moles of thiosulfate = 6.05 x 10-4 / 6.1 x 10-4 / 0.000605 / 0.00061 (mol) (1) Correct answer without working (2) Reject Mark Equations including electrons Mark Reject 1.20 x 10-3 (mol) 1x 10-3/ 0.001 6.0 x 10-4 (mol) 6 x 10-4 (mol) Question Number 21(b)(iv) Moles ClO = 6.05 x 104 (mol) Acceptable Answers Reject Mark TE on (b)(ii) and (b)(iii): If ratio ClO :I2 = 2:1 answer is 2 x answer to (b)(iii) If ratio ClO :I2 = 1:2 answer is half of answer to (b)(iii)<br />
Reject Answers to 1 significant figure Mark Mark Reject KI is in excess if no calculation has been done. Acceptable Answers Question Number 21(b)(v) Concentration = (6.05 x 104 x 1000/25) = 2.42 x 102 / 0.0242/ 0.024/ 2.4 x 102 (mol dm3) TE. Answer to (b)(iv) x 1000 25 Question Number 21(b)(vi) (Minimum) amount of I to react with OCl- Acceptable Answers =2 x answer to (b)(iv) = 2 x 6.05 x 104 = 1.21 x 103 (mol) (1) Allow TE for 2 x answer to (b)(iv) Ignore s.f. Moles of I (9.04 x 103) is more than this number of moles of ClO / I is in excess / KI is in excess / so that all the ClO can react (1) OR 9.04 x 103 mol I can react with 4.52 x 10-3 mol OCl- (1) Ignore s.f. TE from incorrect equation in (b)(ii) Moles OCl- (6.05 x 104 ) is less than this/ I is in excess / KI is in excess / so that all the ClO can react (1) Question Number 21(b)(vii) 0.30 x 100 / 24.2 Acceptable Answers (=1.2396694) = 1.24/ 1.2 % Reject Mark<br />
Acceptable Answers Question Number 21(b)(viii) Judgement (of colour change) at end point / adding starch too early in the titration / jet of burette not filled Errors must cause an increase in titre. Ignore Just Human error Just overshot endpoint Transfer errors / spillage Errors due to misreading burette / pipette Reject Some potassium iodide did not dissolve Leaving funnel in burette Errors which affect both the students titre and an accurate titre using the same solutions e.g. impure solutions Question Number 21(c) Acceptable Answers Reject (Cl radicals) break down ozone (layer)/ ozone depletion / ozone (layer) thinning Allow damage ozone (layer)/ react with ozone Global warming Causes acid rain Mark Mark Total for Q21 = 17 marks<br />

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