Edexcel Jun 2017 (IAL) Paper 3 Q3 (with explained solutions) |

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Edexcel Jun 2017 (IAL) Paper 3 Q3

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3 A student carried out an experiment to determine the enthalpy change of combustion of methanol, CH3OH. Diagram thermometer beaker water spirit burner methanol Results and data Name and formula of alcohol = methanol, CH3OH Molar mass of CH3OH = 32.0 g mol1 Volume of water in beaker = 200 cm3 Mass readings Spirit burner + methanol before combustion = 173.75 g Spirit burner + methanol after combustion = 172.66 g Temperatures Water before heating = 21.0 C Water after heating = 45.5 C Specific heat capacity of water = 4.18 J g1 C 1 (a) What assumption about the water in the beaker allows the student to be able to state that the mass of water is 200 g ? (1)*P48384A01120* 11 Turn over<br />
(b) (i) Calculate the heat energy, in joules, gained by the water. (1) Use the expression energy transferred (J) = mass of waterspecific heat capacity of watertemperature rise (ii) Calculate the number of moles of methanol burned in the experiment. (iii) Hence calculate the enthalpy change of combustion of methanol. Give your answer to a number of significant figures consistent with the data and readings in the table. Include a sign and units in your answer. (1) (3) 12 *P48384A01220*<br />
(c) (i) Each reading of the thermometer used in the experiment has an uncertainty of 0.5 C. Calculate the overall percentage uncertainty in the value of the temperature change in this experiment. (1) (ii) Calculate the maximum temperature change that could have been measured during this experiment, using this thermometer, which has an uncertainty of 0.5 C for each temperature reading. (1) (d) The students evaluation of the experiment included the following points:My calculated value for the enthalpy change of combustion was less exothermic than the Data booklet value, mainly due to heat losses to the surroundings I noted that it had continued to lose mass, even when it was not being usedWhen I rechecked the mass of the spirit burner plus methanol after combustion,At the end of the experiment, I noticed the formation of a black solid on the base of the beaker. (i) Explain why the spirit burner continued to lose mass, even when not in use. (1)(ii) Suggest the identity of the black solid. (1)*P48384A01320* 13 Turn over<br />
(iii) Explain how the formation of the black solid identified in (d)(ii) will lead to a less exothermic value for the enthalpy change of combustion. Do not refer to heat losses to the surroundings. (1)(Total for Question 3 = 11 marks) 14 *P48384A01420*<br />

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Mark (1) Mark (1) Mark (1) Acceptable Answers Reject Incorrect units 200 cm3 weighs 200 g Mass = volume Density of water = 1(.0) g cm3 OR 1(.0) g/cm3 OR 1(.0) g per cm3 ALLOW 1(.0) g ml-1 OR 1 cm3/ml of water has a mass of 1g OR ALLOW 1cm3 equals 1g Reject Reject Acceptable Answers (Energy released = 200 x 4.18 x 24.5) = 20482 (J) IGNORE SF except 1 SF Sign (+ or -) in front of value ALLOW 20.482 kJ Acceptable Answers (Moles CH3OH = 1.0932.0) = 0.0340625 (mol) = 0.0341 (mol) IGNORE SF except 1 SF NOTE 1.09 X 32.0 = 34.88 Question Number 3(a) Question Number 3(b)(i) Question Number 3(b)(ii)<br />
Acceptable Answers Reject Correct final answer with correct sign, units and SF with or without working scores (3) 20.482 0.0341 = 601.306422 (kJ mol1) = 601 kJ mol1(3SF) 600 kJ mol1 (2SF) Value will depend on number of sig figs used in answers for (b)(i) and (b)(ii) E.g. 602 comes from using 0.034 TE on (b)(i) and b(ii) 1st mark Answer to ((b)(i)(b)(ii)) / 1000 2nd mark Minus sign and answer in kJ mol1 / kJ/mol ALLOW Minus sign and answer in J mol1 / J/mol 3rd mark Final answer given to 2 SF or 3 SF Acceptable Answers Reject ()1.0 x 100% 24.5 = 4.08163 = ()4.08(%) IGNORE SF oC Question Number 3(b)(iii) Question Number 3(c)(i) Mark (3) Mark (1)<br />
Question Number 3(c)(ii) uestion Number 3(d)(i) Question Number 3(d)(ii) Question Number 3(d)(iii) Acceptable Answers Reject (24.5 + 1.0 =) 25.5(OC) OR (46.020.5 =) 25.5(OC) OR (24.5 x 104.08 =) 25.5(OC) 100 Acceptable Answers Reject Evaporation/vaporisation (of methanol / alcohol) ALLOW a description of the liquid turning into a gas Just "methanol is volatile" Acceptable Answers Reject Carbon / soot Any other additional substances with the correct answer. e.g. soot and CO Acceptable Answers Reject Incomplete reaction Incomplete combustion/oxidation Insufficient oxygen so that the reaction does not go to completion less CO2 produced so fewer (v. strong) C=O bonds formed ALLOW layer of soot insulates the water from the flame / layer of soot absorbs heat Mark (1) Mark (1) Mark (1) Mark (1) (Total for Question 3 = 11 marks)<br />

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