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A-Level Enthalpy, entropy and the feasibility of reactions: worked solution
2 marks. Full working, one step per line.
Question
Calcium carbide reacts violently, even explosively, with water. Take the standard enthalpies of formation (kJ mol-1) as +226.6 for C2H2(g), -59.0 for CaC2(s), -285.8 for H2O(l) and -985.2 for Ca(OH)2(s), and calculate the standard enthalpy change of reaction 1.
Worked answer
Reaction: CaC2(s) + 2H2O(l) -> Ca(OH)2(s) + C2H2(g). Hr = sum(Hf products) - sum(Hf reactants) = [Hf(Ca(OH)2) + Hf(C2H2)] - [Hf(CaC2) + 2*Hf(H2O)] = [(-985.2) + (+226.6)] - [(-59.0) + 2*(-285.8)] = (-758.6) - (-630.6) = -128.0 kJ mol⁻¹.
Practise this topic
This question is part of A-Level Enthalpy, entropy and the feasibility of reactions, in A-Level H2 Chemistry.
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