Edexcel Jun 2012 Paper 1 Q23 (with explained solutions) |

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Edexcel Jun 2012 Paper 1 Q23

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23 (a) State Hesss Law. (1)(b) Methane burns in a limited supply of oxygen to give carbon monoxide and water. CH4(g) + 1O2(g) o CO(g) + 2H2O(l) The enthalpy change for this reaction cannot be determined directly, but can be found using the standard enthalpy changes of combustion of methane and carbon monoxide, together with Hesss Law. The standard enthalpy changes of combustion needed are for CH4 N-PRO1, DQGIRU&2N-PRO1. (i) Draw a Hesss Law diagram which would enable you to calculate the enthalpy change for the combustion of methane to carbon monoxide. CH4(g) + 1O2(g) o CO(g) + 2H2O(l) LL &DOFXODWHWKHHQWKDOSFKDQJHIRUWKLVUHDFWLRQLQN-PRO1. (2) (2) 12 *P39306A01224*<br />
(iii) Explain why the enthalpy change for this reaction cannot be determined directly. (1)(c) Explain why the calculation in part (b)(ii) would give an incorrect result for the enthalpy change for the reaction below. CH4(g) + 1O2(g) o CO(g) + 2H2O(g) (2)(Total for Question 23 = 8 marks) *P39306A01324* 13 Turn over<br />
BLANK PAGE 14 *P39306A01424*<br />

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Question Number 23(a) Question Number 23(b)(i) Acceptable Answers The heat/enthalpy/energy change (for a reaction) is independent of the path(way)/route IGNORE any extra detail referring to initial and final states Reject Mark Acceptable Answers Reject Mark Reject Mark CO2 + 2H2O (1) Both arrows in correct direction downwards (1) IGNORE state symbols, even if incorrect Mark the two points independently Acceptable Answers Question Number 23(b)(ii) H =890 ( 283) (1) =607 (kJ mol1) (1) Correct answer with no working scores (2) NOTE: +607 (kJ mol1) scores (1) only<br />
Mark Acceptable Answers Reject Cannot stop the reaction at CO OR the reaction produces CO2/complete combustion occurs OR may produce some carbon/soot OR cannot react exact amounts of methane to oxygen non-standard conditions Just incomplete combustion occurs Just forming other products /just a mixture of products Just methane is very reactive/ explosive Just heat loss Cannot measure the temperature change Question Number *23 (b) (iii)<br />
Question Number 23(c) Mark Acceptable Answers Reject First mark: State of the H2O Water is in the gas phase/water is (formed) as steam/water is not in its standard state/water is not (formed as a) liquid (1) Second mark: Idea of an energy change when there is a change of state Change of state involves an energy change /energy change (for the reaction given) is less exothermic (1) ALLOW more endothermic instead of less exothermic IGNORE references to non-standard conditions Energy change is more exothermic /less endothermic Heat loss Incomplete combustion<br />

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