Edexcel Jan 2014 (IAL) Paper 1 Q19 (with explained solutions) |

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Edexcel Jan 2014 (IAL) Paper 1 Q19

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19 This question is about the flammable liquid, methanol, CH3OH. (a) Methanol starts to have toxic effects when it is present in blood at levels of above 200 mg in 1000 g. Express this concentration in parts per million. (b) The enthalpy change of combustion of methanol was measured using a spirit burner to heat a known mass of water in a calorimeter. The temperature increase of the water in the calorimeter was measured when a known mass of methanol was burned. (i) Write an equation for the complete combustion of methanol, CH3OH, under standard conditions. Include state symbols in the equation. (ii) Identify two other products that could form if the combustion was incomplete. (1) (2) (1) 12*P42986A01724* 17 Turn over<br />
(c) The results of the experiment are summarised in the table below. Mass of water in the calorimeter Mass of spirit burner + contents (initial) Mass of spirit burner + contents (final) Temperature of water (initial) Temperature of water (final) 150.0 g 52.24 g 51.60 g 21.4(cid:113)C 37.2(cid:113)C (i) Calculate the heat energy produced in this experiment using the equation Heat energy produced (J) = mass of water4.18temperature change (ii) Calculate the number of moles of methanol burned in this experiment. (iii) Calculate the enthalpy change of combustion of methanol in kJ mol1. Give your answer to three significant figures. (1) (1) (2) 18 *P42986A01824*<br />
(iv) The experimental result differs from the true value for the enthalpy change of combustion of methanol. State one factor in the experimental method, other than heat losses or incomplete combustion, which causes the result to differ from the true value. Explain the effect this factor has on the magnitude of the experimental value compared to the true value. (2) FactorExplanation(d) The value of the enthalpy change for the combustion of methanol can be calculated from the mean bond enthalpies of the substances in the reaction. Give two reasons why this value differs from the value obtained in the experiment, even after corrections are made for experimental error. (2) Reason 1Reason 2(Total for Question 19 = 12 marks) *P42986A01924* 19 Turn over<br />

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Acceptable Answers 200 / 2 x 102 (ppm) Question Number 19(a) Question Number 19(b)(i) CH3OH(l) + 3/2O2 (g)CO2(g) + 2H2O(l) Acceptable Answers Formulae (1) Balancing and state symbols (1) Allow multiples 2nd mark dependent on 1st Question Number 19(b)(ii) Carbon / C / soot AND carbon monoxide / CO Acceptable Answers Both needed Acceptable Answers Question Number 19(c)(i) (150 x 4.18 x 15.8) = 9906.6 / 9907 / 9910 (J) / 9.9066 kJ Ignore sf except 1 sf / Ignore signs here Acceptable Answers Question Number 19(c)(ii) Question Number 19(c)(iii) (9.9066/0.0200) = 495.33 (0.64/32) = 0.02(00) (mol) Acceptable Answers H = -495 (kJ mol1) Value (1) Sign and 3sf (1) Allow TE from (c)(i) and / or (c)(ii) (answer to (c)(i) in kJ/ answer to (c)(ii)) No 2nd mark if units given are incorrect e.g. kJ mol or kJ/mol1 Reject Reject CH3OH(aq) / (g) / 2H2O(g) Reject Graphite Reject kJ mol1 Reject Reject Mark Mark Mark Mark Mark Mark<br />
Acceptable Answers Question Number 19(c)(iv) Mark the two points independently 1st mark: Evaporation of alcohol (from burner) / alcohol is volatile /CH3OH is volatile ALLOW H2O forms as steam, not water IGNORE Water evaporates (from apparatus) (1) 2nd mark: (Actual) mass/moles (methanol) burned is less and (so) enthalpy change will be less negative/less exothermic / less / smaller OR Estimate of mass/moles (methanol) burned is too high and (so) enthalpy change will be less negative/less exothermic / less / smaller OR Temperature rise will be less than it should be and (so) enthalpy change will be less negative/less exothermic / less / smaller (1) IGNORE Any mention of specific heat capacity Reject Mark Weighing errors / Other equipment errors (eg distance between calorimeter and spirit burner) Any answers that suggest lab value more exothermic / greater value of enthalpy change<br />
Acceptable Answers Mark each point independently 1st mark: ANY ONE OF: Bond enthalpies vary with environment Mean bond enthalpies do not equal actual bond enthalpies (for these reactants) / mean bond enthalpies are not exact values Bond enthalpies used are average values (from a range of compounds) 2nd mark: ANY ONE OF: Bond enthalpies refer to gases OR Bond enthalpies refer to gaseous bonds OR Methanol is a liquid OR Water is a liquid (under standard conditions) (1) IGNORE References to non-standard conditions / incomplete combustion / not in same state (1) Question Number 19(d) Total for Question 19 = 12 marks Reject Mark<br />

Question 17 18 19 20

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