Edexcel Jan 2012 Paper 2 Q20 (with explained solutions) |

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Edexcel Jan 2012 Paper 2 Q20

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PMT 20 Chlorine disinfectants are essentially solutions containing chlorine molecules and chlorate(I) ions in an equilibrium summarised by the equation Cl2(aq) + H2O(l) (cid:85) 2H+(aq) + ClO(cid:237)(aq) + Cl(cid:237)(aq) Equation 1 The chlorine content of a disinfectant was determined using the following procedure. 1. 10.0 cm3 of the disinfectant was transferred to a 250 cm3 volumetric flask. 2. Approximately 20 cm3 of nitric acid and 20 cm3 potassium iodide solution (both in excess) were added to the volumetric flask. 3. The solution in the volumetric flask was made up to the mark with distilled water and then mixed thoroughly. 4. 10.0 cm3 portions of the solution in the volumetric flask were titrated against a solution of sodium thiosulfate, concentration 0.109 mol dm(cid:237)(cid:22). Starch solution was (cid:68)(cid:71)(cid:71)(cid:72)(cid:71)(cid:3)(cid:81)(cid:72)(cid:68)(cid:85)(cid:3)(cid:87)(cid:75)(cid:72)(cid:3)(cid:72)(cid:81)(cid:71)(cid:16)(cid:83)(cid:82)(cid:76)(cid:81)(cid:87)(cid:3)(cid:82)(cid:73)(cid:3)(cid:87)(cid:75)(cid:72)(cid:3)(cid:87)(cid:76)(cid:87)(cid:85)(cid:68)(cid:87)(cid:76)(cid:82)(cid:81)(cid:3)(cid:68)(cid:81)(cid:71)(cid:3)(cid:87)(cid:75)(cid:72)(cid:3)(cid:80)(cid:72)(cid:68)(cid:81)(cid:3)(cid:11)(cid:68)(cid:89)(cid:72)(cid:85)(cid:68)(cid:74)(cid:72)(cid:12)(cid:3)(cid:87)(cid:76)(cid:87)(cid:85)(cid:72)(cid:3)(cid:90)(cid:68)(cid:86)(cid:3)(cid:21)(cid:26)(cid:17)(cid:22)(cid:24)(cid:3)(cid:70)(cid:80)3. The equations for the reactions involved in this procedure are Cl2(aq) + 2I(cid:237)(aq) (cid:111) I2(aq) + 2Cl(cid:237)(aq) I2(aq) + 2S2O3 (cid:21)(cid:237)(aq) (cid:111) 2I(cid:237)(aq) + S4O6 (cid:21)(cid:237)(aq) Equation 2 Equation 3 (a) (i) Calculate the number of moles of sodium thiosulfate used in the titration. (ii) Calculate the number of moles of iodine, I2, that reacted in the titration (step 4). (iii) Hence state the number of moles of chlorine, Cl2, in 10.0 cm3 of the solution in the volumetric flask. (2) (1) (1) 12 *P39303A01224*<br />
(iv) Calculate the concentration of chlorine, in mol dm(cid:237)(cid:22), in the original disinfectant. (2) PMT (b) Equation 1 is an example of a disproportionation reaction. Define the term disproportionation and explain, by considering the relevant oxidation numbers, why this reaction is a disproportionation. (3)(c) State the colours of the titration solution just before the starch solution is added, after the starch solution is added and the colour change at the end-point of the reaction. (2) Colour just before adding the starchColour after adding the starchColour at the end-point(Total for Question 20 = 11 marks) *P39303A01324* 13 Turn over<br />
PMT BLANK PAGE 14 *P39303A01424*<br />

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Acceptable Answers Reject Throughout 20 (a): IGNORE sf except 1 sf (penalise once) correct answer with no working scores full marks mark consequentially IGNORE units unless incorrect 0.109 x 27.35 x 103 (1) = 2.98115 x 103 (mol) = 2.98 x 103 / 0.00298(mol) 0.003 (1) cq only on some concentration x some volume Acceptable Answers Reject Moles I2 = 0.5 x moles thiosulfate = 0.5 x answer to (a)(i) = 1.490575 x 103 = 1.49 x 103 /0.00149(mol) Acceptable Answers Reject Moles of Cl2 = moles of I2 = answer to (a)(ii) = 1.49 x 103 /0.00149(mol) Mark Mark Mark Question Number 20 (a)(i) Question Number 20 (a)(ii) Question Number 20 (a)(iii) 6CH0201 1201<br />
Mark Mark Question Number 20 (a)(iv) Acceptable Answers Reject Mark consequentially on answer in (a)(iii) Amount in volumetric flask = 25 x answer to (a)(iii) (= 25 x 1.490575 x 103 = 3.72644 x 102) OR (25 x 1.49 x 103 = 3.725 x 102) (1) (= amount in 10 cm3 of disinfectant) Concentration = 100 x previous value (= 1000 x 3.73 x 102 /10 = 3.73 (mol dm3)) (1) Concentration = 100 x answer to (a)(iii) scores (1) Question Number Acceptable Answers Reject 20(b) (Atoms of) the same element Molecule/substance/ reactant /species Just Cl oxidized & reduced (in the same species) are oxidized and reduced (1) ALLOW chlorine for element Chlorine ON 0 oxidized to (+)1 in ClO (1) and reduced to 1 in Cl (1) Only penalise once if oxidized and reduced omitted Just Chlorine ON 0 oxidized to (+)1 and reduced to 1 or Chlorine oxidized to chlorate(I) and reduced to chloride(1 mark only) Only penalise once if oxidized and reduced reversed 6CH0201 1201<br />
Question Number Acceptable Answers Reject 20(c) Colour just before adding the Mark Just yellow, brown, gold purple starch: (very) pale yellow/straw coloured (1) Colour after adding the starch: Blue-black (ALLOW black or (dark) blue) Colour at the end point: colourless (1) Both colours required IGNORE Clear 6CH0201 1201<br />
Mark Question Number Acceptable Answers Reject 21(a) Names OR Formulae A = NaOH/KOH in ethanol /alcohol (1) B = NaOH/KOH in water/ aqueous (1) IGNORE any reference to ethanol /alcohol /dilute C = NaBr/KBr & (50% or moderately conc) H2SO4 / P & Br2 / PBr3 /PBr5 /NaBr /KBr & H3PO4 /HBr ALLOW phosphorus bromide (1) IGNORE red/white (phosphorus) D = NH3 (in alcohol /in a sealed tube /at high pressure) (1) IGNORE aqueous Water + ethanol /water + alcohol For A and B OH/alkali (penalise once) Dilute H2SO4 any mention of alkali any mention of acid Question Number 21(b)(i) Acceptable Answers Reject Mark A = elimination (1) IGNORE nucleophilic D = (nucleophilic) substitution (1) mention of dehydration in A mention of electrophilic in A or D 6CH0201 1201<br />

Question 19 20 21 22

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