What are the differences between standard enthalpy change of formation and standard enthalpy change of combustion? With examples pls (such as the diffences when drawing Hess's Law cycle)



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Standard enthalpy change of formation is the energy change accompanied with the formation of one mole of a compound from the constituent elements, with all substances in their standard states. For example, standard enthalpy of formation of methane, C(s) + 2H2(g) → CH4(g); ΔH0 = -74.78 kJ mol–1. On the other hand, standard enthalpy change of combustion is the enthalpy change when one mole of a reactant completely burns in oxygen under standard thermodynamic conditions. E.g. ΔH0(C) Methane = -890.3 kJ mol−1

CH4(g) + 2O2(g) → CO2(g) + 2H2O; ΔH0 = -890.3 kJ mol–1


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