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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.
Examples of combustion reactions include: 2 H2 + O2 â 2H2O + heat. CH4 + 2 O2 â CO2 + 2 H2O + heat. Other examples include lighting a match or a burning campfire. To recognize a combustion reaction, look for oxygen in the reactant side of the equation and the release of heat on the product side.