Edexcel Jun 2012 Paper 3 Q4 (with explained solutions) |

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Edexcel Jun 2012 Paper 3 Q4

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In two similar, separate experiments, the enthalpy changes for the reactions of potassium hydrogencarbonate, KHCO3, and potassium carbonate, K2CO3, with excess dilute hydrochloric acid were determined. D 7KHILUVWH[SHULPHQWZDVWRILQGWKHHQWKDOSFKDQJHH1, for the reaction KHCO3(s) + HCl(aq) o KCl(aq) + H 2O(l) + CO2(g) Measurement Mass of solid potassium hydrogencarbonate added to hydrochloric acid Volume of hydrochloric acid Temperature of hydrochloric acid before addition of solid potassium hydrogencarbonate Final temperature of solution Reading 3.48 g 25.0 cm3 22.0 C 12.0 C (i) Calculate the heat energy absorbed, in joules, by the reaction of KHCO3(s) with the solution of dilute hydrochloric acid. Use the expression energy absorbed (J) = 25.04.18temperature change (ii) Calculate the number of moles of KHCO3(s) used. Molar mass of KHCO3(s) is 100 g mol1. (1) (1) *P39310A0916* Turn over<br />
(iii) Use your answers to (a)(i) and (ii) to calculate, in kJ mol1, the enthalpy change, H1, when one mole of KHCO3(s) reacts completely with the acid. Include the VLJQIRUH1. (2) (b) In the second experiment, the enthalpy change for the reaction between potassium carbonate and dilute hydrochloric acid was calculated from the results. K2CO3(s) + 2HCl(aq) o 2KCl(aq) + H2O(l) + CO2(g) 7KHPRODUHQWKDOSFKDQJHH2, for this reaction was calculated to be 34.0 kJ mol1. (i) State one way in which the temperature change is different when equal numbers of moles of potassium hydrogencarbonate and potassium carbonate react separately with the same volume of excess dilute hydrochloric acid. (1)(ii) Give oneDVVXPSWLRQPDGHZKHQFDOFXODWLQJWKHYDOXHVRIH1DQGH2 from experimental results. (1)10 *P39310A01016*<br />
F 7KHYDOXHVRIH1DQGH2 may be used to determine the enthalpy change for the IROORZLQJUHDFWLRQ Hr). 2KHCO3(s) o K2CO3(s) + H2O(l) + CO2(g) A Hess cycle based on these reactions is shown below. 2KHCO3(s) + 2HCl(aq) Hr K2CO3(s) + H2O(l) + CO2(g) + 2HCl(aq) 2KCl(aq) + 2H2O(l) + 2CO2(g) L 8VH+HVVV/DZWRFRPSOHWHDQH[SUHVVLRQIRUHrLQWHUPVRIH1DQGH2. Hr = LL &DOFXODWHWKHYDOXHRIHr in kJ mol1. Include a sign in your answer. (1) (2) (Total for Question 4 = 9 marks) *P39310A01116* 11 Turn over<br />
BLANK PAGE 12 *P39310A01216*<br />

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Correct Answer (25.0 x 4.18 x 10.0 =) 1045/1050/1000 (J) ALLOW 1.045/1.05/1.0 kJ IGNORE signs IGNORE sf except 1 Correct Answer 3.48 = 0.0348/0.035 (mol) 100 IGNORE sf except 1 Reject Mark Reject Mark Correct Answer Reject Mark Answer to (a)(i) in kJ = 1.045 Answer to (a)(ii) 0.0348 = +30.03/30.0/30 (kJ mol-1) ALLOW any answer which rounds to 30 to 2 sf. Mark CQ on (a)(i) and (ii) which should be checked. Value must round to the correct answer to 2 sf. Sign value Mark independently (1) (1) Correct Answer Decrease (in temperature) for KHCO3 and an increase (in temperature) for K2CO3 OR Endothermic for KHCO3 and exothermic for K2CO3 (1) (1) 1.0/0.035=29 2 Mark Reject Just Temperature change different Question Number 4(a)(i) Question Number 4(a)(ii) Question Number 4(a)(iii) Question Number 4(b)(i)<br />
Reject Thermometer is accurate Mark Reject Reject Mark Mark Question Number 4(b)(ii) No heat lost/no heat gained/ Correct Answer reaction is complete/ specific heat capacity of the solution is the same as water/molar mass of KHCO3 is 100 (not 100.1)/density of solution is 1.00 gcm-3 Correct Answer (Hr =) 2 x H1H2 Correct Answer (1) (1) (Hr = (2 x +30)(34) = ) + 94 (kJ mol-1) value sign The sign mark is only available if the value is correct or almost correct. ALLOW (Hr = (+30)(34) = ) = +64 (kJ mol-1) IGNORE sf except 1 (1) Question Number 4(c)(i) Question Number 4(c)(ii)<br />

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