OCR A Jan 2012 Paper 2 Q3

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3 Enthalpy changes can be determined directly or indirectly. (a) A student investigates the reaction between magnesium and dilute hydrochloric acid. Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g) The student determines the enthalpy change for this reaction. In her experiment, she reacts 0.486 g of magnesium with 50.0 cm3 of 2.00 mol dm3 HCl(aq). The HCl(aq) is in excess. The temperature of the solution changes from 19.2 C to 32.0 C. (i) Calculate the energy released, in kJ, during this reaction. The specific heat capacity of the solution = 4.18 J g1 K1. The density of the solution is 1.00 g cm3. (ii) Calculate the amount, in moles, of magnesium used by the student. energy = kJ [2] amount =mol [1] (iii) Calculate the enthalpy change of reaction. Give your answer to three significant figures. enthalpy change of reaction = kJ mol 1 [3]OCR 2012<br />
 (b) The student wants to determine the enthalpy change of formation of calcium carbonate, CaCO3(s). Ca(s) + C(s) + 1O2(g) CaCO3(s) (i) What is meant by the term standard enthalpy change of formation? You should state the standard conditions in your answer.[3] (ii) Using the following data and enthalpy cycle,fill in the boxes on the enthalpy cycle with the correct enthalpy change values calculate the enthalpy change of formation, Hf, of CaCO3(s). reaction enthalpy change, H / kJ mol1 CO2(g) C(s) + O2(g) H2(g) + O2(g) CaCO3(s) + 2HCl(aq) Ca(s) + 2HCl(aq) H2O(l) CaCl2(aq) + H2O(l) + CO2(g) CaCl2(aq) + H2(g) 393 285 54 168 Ca(s) + 2HCl(aq) + C(s) + 1O2(g) Hf CaCO3(s) + 2HCl(aq)CaCl2(aq) + H2(g) + C(s) + 1O2(g)CaCl2(aq) + H2O(I) + C(s) + O2(g) CaCl2(aq) + H2O(I) + CO2(g)Hf =kJ mol1 [3] [Total: 12] Turn overOCR 2012<br />

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