Edexcel Jun 2013 Paper 3 Q2 (with explained solutions) |

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Edexcel Jun 2013 Paper 3 Q2

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2 An experiment to determine the enthalpy change of reaction between aqueous copper(II) sulfate and zinc was carried out as follows. 1. 50.0 cm3 of copper(II) sulfate solution, of concentration 1.00 mol dm3, was placed in a polystyrene cup. 2. The temperature of the solution was measured with a 0110 C thermometer and was found to be 23.0 C. 3. Zinc powder with a mass of 5 g (an excess) was added to the solution with vigorous stirring and the highest temperature recorded was 69.5 C. (a) (i) Write the ionic equation for the reaction between zinc and aqueous copper(II) ions, including state symbols. (2) (ii) Calculate the quantity of heat energy produced in the experiment above, giving your answer in J. (Assume that the heat capacity of the mixture is 4.18 J g1 C 1 and its density is 1.00 g cm3.) Use the expression energy transferred in joules = massspecific heat capacitytemperature change (2) (iii) Calculate the number of moles of copper(II) sulfate used in the experiment. (1) *P41652A0412*<br />
(iv) Use your answers from (a)(ii) and (a)(iii) to calculate the enthalpy change for the reaction in kJ mol1. Give your answer to three significant figures and include the appropriate sign. (b) The thermometer used in this experiment gave an uncertainty in each temperature reading of 0.5 C. (i) State the maximum temperature difference in this experiment that could have been obtained using this thermometer. (2) (1)(ii) What is the percentage error in the temperature change using this thermometer? (c) Using the same equipment, together with a stop clock, suggest a procedure that would improve the accuracy of this experiment by obtaining a more accurate temperature change. You must use the same mass of zinc powder and the same volume of 1.00 mol dm3 copper(II) sulfate solution. (1) (4)(Total for Question 2 = 13 marks) *P41652A0512* Turn over<br />

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Mark Mark Acceptable Answers Reject Zn(s) + Cu2+(aq)Zn2+(aq) + Cu(s) First mark for correct species in a balanced equation ALLOW hexaqua ions providing the equation is balanced IGNORE reversible arrows (1) Second mark for states Consequential on a reasonable attempt at the equation, including for a full equation or unbalanced equation E.g. ALLOW as reasonable Zn with one+ instead of 2+ Sulfate ions shown correctly but not cancelled out (1) Sulfate ions not cancelled out Reversed equation Zn and Cu metal both shown as ions Acceptable Answers Reject 9718 q = 50.0 x 4.18 x 46.5 (1) = 9718.5 (J) /9.7185 kJ (1) Correct answer with no working scores (2) IGNORE s.f. except 1 (ie allow 9719 / 9720 / 9700 or 9.719 / 9.72 / 9.7 kJ) IGNORE sign of q if given If mass used 55.0g and q = 55.0 x 4.18 x 46.5 = 10690.35 (J) scores (1) If mass used is 5g and q = 5 x 4.18 x 46.5 = 971.85(J) scores (1) Acceptable Answers 50.0 x 1 1000 = 0.05 (mol) Mark is for final answer Reject Mark Question Number 2(a)(i) Question Number 2(a)(ii) Question Number 2(a)(iii)<br />
Acceptable Answers H =9718.5(0.050 x 1000) = -194.370 (kJ mol 1) =194 (kJ mol 1) ALLOW =194000 J mol 1 First mark Value, ignore sign and sf Only penalise units if value is in J (mol -1) without stating this (1) TE (a)(ii)((a)(iii) x 1000) Using 10690.5 gives -2138810J = -214 kJ mol1 Second mark Sign and 3 s.f. (1) This mark depends on a correct calculation method Acceptable Answers 47.5 (oC) Acceptable Answers (1.0 x 100)46.5 = 2.1505376 = () 2.15 / 2.2 / 2 (%) IGNORE sf ALLOW answer with 47.5 in the denominator which gives 2.1052631 = () 2.11 / 2.1 / 2 (%) ALLOW TE on value (b)(i) Reject Mark Mark Mark Reject Reject 2.0 and 2.1 2.0 and 2.2 Question Number 2(a)(iv) Question Number 2(b)(i) Question Number 2(b)(ii)<br />
Acceptable Answers Question Number 2(c) Mark Reject Readings more often than every 15s First mark Measure the temperature (of copper(II) sulfate) every minute / at realistic time interval (15 s to 1 minute) for e.g. 24 minutes before adding zinc OR measure temperature (of copper(II) sulfate) before adding zinc (1) Second mark Measure temperature each minute / at realistic time intervals (after adding zinc) for several minutes (1) Note These readings may be started after the maximum temperature is reached / after reaction has stopped and taken until the mixture has cooled to room temperature. Intervals should be chosen to allow at least 4 readings on cooling section of curve. Third mark Plot a temperature - time graph / plot a graph using measurements (of temp and time) obtained (1) This mark can be awarded if first two marks are insufficient for credit Fourth mark Extrapolate to find T / maximum temperature (at the time of mixing ) OR Use properly described intersecting lines to find maximum temperature (1) Allow third and fourth marking points to be shown on annotated diagrams / graph If zinc is added in small portions or over a period of time only first and third marks can be awarded (Since measurements of cooling will be incorrect and there is no definite time when reaction starts) Total for Question 2 = 13 Marks<br />

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