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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.
The heat of combustion of ethanol is 1211 kJ mol -1. The experimentally determined value for the heat of combustion of ethanol is usually less than the accepted value of 1368 kJ mol -1 because some heat is always lost to the atmosphere and in heating the vessel.