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So O2 has zero of heat of formation (or enthalpy of formation). The enthalpy of a reaction is defined like this: H = Hproducts-H reactants. The enthalpy of reaction in this case, is the same as the heat of combustion. Hproducts is obtained summing the heats of formation.
= 1211 kJ mol-1. Therefore the molar heat of combustion of ethanol is 1211 kJ mol-1 (The molar enthalpy of combustion of ethanol is 1211 kJ mol-1). (Note that the enthalpy change for the reaction is negative because the reaction is exothermic, so the enthalpy change for the reaction is -1211 kJ mol-1, ÎH = - 1211 kJ mol-1)