We have seen earlier how enthalpy changes for chemical reactions (or physical changes such as vaporization) can be calculated with Hess's Law if we have measured enthalpy change values for thermochemical reactions that we can combine in a way to yield the desired chemical equation.  In order to reduce the number of enthalpies of reaction which must be tabulated for use in Hess's law calculations, chemists have  devised a clever way of reporting enthalpies of reaction.  They report the standard enthalpy of formation of each individual substance,  H^o_f.

The standard enthalpy of formation (also standard heat of formation), is the enthalpy change for the formation of one mole of the substance in its standard state from its constituent elements in their reference form and in their standard states.  The reference form just means the most stable form, for example for hydrogen, H₂ , the reference would be the gaseous form, for carbon, the reference form would be graphite instead of diamond etc.

The use of standard enthalpies of formation allows us to greatly simplify Hess's law calculations.  By using the following equation one can calculate the enthalpy for any reaction if the enthalpies of formation of the substances in the reaction are known.

H^o = n H^o_f (products) - m H^o_f (reactants)