Edexcel Jun 2011 Paper 5 Q17 (with explained solutions) |

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Edexcel Jun 2011 Paper 5 Q17

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PMT 17 SECTION C Answer ALL the questions. Write your answers in the spaces provided. Alcoholic drinks contain ethanol, CH3CH2OH, in aqueous solution. The percentage of alcohol in a drink can be determined by a redox titration, whilst the amount of alcohol present on the breath of someone who has consumed such a drink can be estimated using a breathalyser. The earliest breathalysers used the colour change that occurs when dichromate(VI) ions react with ethanol to measure the amount of alcohol. Later models measure the current from a fuel cell. Cheaper versions of these meters are available for drivers to buy for self-testing. Some police forces also use fuel cell breathalysers in conjunction with infrared breath analysers, which can determine the amounts of alcohol from an infrared spectrum. In an experiment to find out the concentration of ethanol in a drink, a small beaker containing 5.00 cm3 of a diluted sample of the drink is suspended above 10.0 cm3 of excess acidified sodium dichromate(VI) solution, of concentration 0.0800 mol dm3, and left for 24 hours in a warm place. 5 cm3 of diluted sample 10 cm3 of acidified sodium dichromate(VI) solution The ethanol vaporizes and reacts with some of the acidified sodium dichromate(VI) ions. Excess potassium iodide is then added to the unreacted acidified sodium dichromate(VI), forming iodine, I2(aq). The I2(aq) is then titrated with a solution of sodium thiosulfate, Na2S2O3, of concentration 0.0250 mol dm3. (a) Ethanol and dichromate(VI) ions in acidic solution react in the mole ratio 3:2. (i) Complete the two half-equations below. State symbols are not required. (2) Cr2O7 2 (cid:314) Cr3+ CH3CH2OH (cid:314) CH3COOH *P38480A01924* 19 Turn over<br />
(ii) Use either the half-equations in (i) or that the mole ratio of CH3CH2OH:Cr2O7 reaction between ethanol and acidified dichromate(VI) ions. State symbols are not required. 2 is 3:2 to construct the ionic equation for the *(iii) The iodine formed in the experiment reacted completely with 34.40 cm3 of the 0.0250 mol dm3 sodium thiosulfate solution. Use this information, the fact that the mole ratio of CH3CH2OH:Cr2O7 2 is 3:2 and the equations below, to calculate the concentration of the ethanol in the 5.00 cm3 of the diluted sample of the drink. PMT (1) (6) Cr2O7 2 + 14H+ + 6I(cid:237) (cid:314) 2Cr3+ + 3I2 + 7H2O 2S2O3 2(cid:237) + I2 (cid:314) S4O6 2(cid:237) + 2I(cid:237) 20 *P38480A02024*<br />
(iv) The solution used in the experiment was made by adding 10.0 cm3 of the drink to a 100 cm3 volumetric flask, making up the remainder of the volume using distilled water. Use the information and your final answer to (a)(iii) to calculate the concentration, in mol dm3, of ethanol in the undiluted drink. (1) PMT (v) Suggest why the sample was suspended above the acidified sodium dichromate(VI) solution, rather than simply being mixed with it. (1)(vi) Suggest two reasons why the apparatus was left in a warm place for 24 hours. What would be the effect on the final result if this procedure were not followed? (3)(vii) Do you think that this experiment gives a reliable result? Explain your answer. (1)*P38480A02124* 21 Turn over<br />
PMT *(b) (i) Explain how each type of breathalyser, mentioned in the passage, shows the amount of ethanol present. (3) Earliest typeFuel cellInfrared(ii) Suggest why infrared breathalysers do not use the OH absorption to detect the amount of alcohol on the breath. (1)22 *P38480A02224*<br />
PMT (iii) Suggest why some police forces use infrared breathalysers together with fuel cell breathalysers. (1)(iv) Suggest one advantage and one disadvantage of buying a personal breathalyser. (2)(Total for Question 17 = 22 marks) TOTAL FOR SECTION C = 22 MARKS TOTAL FOR PAPER = 90 MARKS *P38480A02324* 23<br />

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Section C Acceptable Answers 2 + 14H+ + 6e2Cr3+ + 7H2O (1) Cr2O7 CH3CH2OH + H2OCH3COOH + 4H+ + 4e (1) Allow multiples Ignore state symbols Reject Acceptable Answers Reject Mark Mark 2 + 16H+Equations with electrons 3CH3CH2OH + 2 Cr2O7 3CH3COOH + 4Cr3+ + 11H2O Allow hydrogen ions and water molecules not cancelled e.g. 3CH3CH2OH + 2 Cr2O7 3CH3COOH + 4Cr3+ + 11H2O + 12H+ 3CH3CH2OH + 2 Cr2O7 3CH3COOH + 4Cr3+ + 14H2O 3CH3CH2OH + 2 Cr2O7 3CH3COOH + 4Cr3+ + 14H2O + 12H+ 2 + 16H+ + 3H2O2 + 28H+ + 3H2O2 + 28H+Question Number 17 (a)(i) Question Number 17 (a)(ii)<br />
Reject Mark Acceptable Answers Moles of thiosulfate = 34.40/1000 x 0.025 = 8.6 x 10-4 (mol) (1) So moles I2 = 4.3 x 10-4 (mol) (1) So moles of dichromate unreacted = 4.3 x 10-4 / 3 = 1.433 x 10-4 (mol) (1) So moles of dichromate reacted with ethanol = 8 x 10-41.43 x 10-4 = 6.567 x 10-4 (mol) (1) So moles of ethanol = 6.567 x 10-4 x 3/2 mol = 9.85 x 10-4 (mol) (1) Concentration = 9.85 x 10-4 /0.005 = 0.197 (mol dm3) (1) Correct answer (0.197) with no working 6 marks Allow 4 max. for missing subtraction in step 4 and gaining answer of 0.043(0) Acceptable Answers 0.197 x 10 = 1.97 (mol dm3) Allow answer to (a) (iii) x 10 Acceptable Answers Reject Reject To prevent other (non volatile) substances (in the drink) from reacting with the dichromate ions Acceptable Answers because it allows the ethanol to evaporate (allowing it to mix with the dichromate) (1) make sure all ethanol reacts (1) Concentration / results would have been lower than the actual value (1) Reject Mark Mark Mark Question Number *17 (a)(iii) Question Number 17 (a)(iv) Question Number 17 (a)(v) Question Number 17 (a)(vi)<br />
Acceptable Answers Reject Mark No, as only one sample titrated so no evidence that results are repeatable / no, as not all the ethanol has evaporated/no, as the dichromate may have reacted with something else/ no, as not all the ethanol has reacted Allow only 1 titration carried out Acceptable Answers Reject Early breathalysers: (the extent to which) dichromate turns green (1) Fuel cells: (more alcohol means larger) current / quantity of electricity (1) Infrared breathalysers: (more alcohol means greater) absorbance (1) Ignore reference to specific bonds provided they are present in ethanol Acceptable Answers Water (in the breath) also has an OH bond Allow other named molecules on the breath provided they have an OH bond Mark Just colour change Just potential difference measured Just voltage measured Just gives a peak Reject Mark Acceptable Answers Reject Mark Answers only related to accuracy Additional evidence is more reliable Or Police often use IR as well as fuel cell breath test to provide sufficient evidence to prosecute (without need for blood test) Or Fuel cell breathalysers are portable and determine whether or not to check with IR at the police station Question Number 17 (a)(vii) Question Number *17 (b)(i) Question Number 17 (b)(ii) Question Number 17 (b)(iii)<br />
Reject Mark Question Number 17 (b)(iv) Acceptable Answers Advantage It could check if you are below the legal limit / safe to drive (1) Disadvantage It may not be sensitive enough to give an accurate reading / may give a value that does not closely match police value OR It encourages people to drink and drive (1) TOTAL FOR SECTION C = 22 MARKS<br />

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