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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.
1 Heat of Combustion of a substance is the heat liberated when 1 mole of the substance undergoes complete combustion with oxygen at constant pressure.