OCR A Jun 2016 Paper 2 Q3

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This question is about different ways of determining enthalpy changes. (a) A student carries out an experiment to determine directly the enthalpy change of reaction, Hr , shown below. Mg(s) + CuSO4(aq) MgSO4(aq) + Cu(s)The student measures the initial temperature of 25.0 cm3 of 0.500 mol dm3 CuSO4(aq). The student adds an excess of magnesium powder and stirs the mixture. The student measures the maximum temperature of the solution. Results = 21.5 C Initial temperature of solution Maximum temperature of solution = 63.0 C Density of the solution = 1.00 g cm3; specific heat capacity of the solution = 4.18 J g1 K1. (i) Calculate the enthalpy change of reaction, Hr , in kJ mol1. Give your answer to three significant figures. Hr =kJ mol1 [4] (ii) The student weighed out enough magnesium so that it would be in excess by at least 25%. The student had access to a two decimal-place balance. Calculate the minimum mass of magnesium that the student would need to weigh out on this balance.OCR 2016 mass =g [1] Turn over<br />
 (b) Enthalpy changes of formation can be determined indirectly from standard enthalpy changes of combustion, Hc o. Three enthalpy changes of combustion are shown below. Substance Hc o / kJ mol1 C(s) H2(g) C9H20(l) 394 286 6125 (i) Define standard enthalpy change of combustion. Include the standard conditions that are used.[3] (ii) The equation that represents the enthalpy change of formation, Hf , of nonane is shown below. Calculate the standard enthalpy change of formation of nonane. 9C(s) + 10H2(g) C9H20(l) Hf =kJ mol1 [2]OCR 2016<br />
 (c) The bond enthalpy for the bond in carbon monoxide can be calculated from the information below. CH4(g) + H2O(g) CO(g) + 3H2(g) H = +210 kJ mol1 Bond CH OH HH Average bond enthalpy / kJ mol1 413 464 436 (i) What is meant by the term average bond enthalpy?[2] (ii) Calculate the bond enthalpy for the bond in carbon monoxide. Show your working. bond enthalpy =kJ mol1 [3] [Total: 15]OCR 2016 Turn over<br />

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