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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 By definition, the heat of combustion is minus the enthalpy change for the combustion reaction, ie, -Î´ reaction H. 2  By definition, the heat of combustion is a positive value. 3  Heat of Combustion can be measured experimentally.