There are several choices that could have been made; one example is the tabulation of the combustion reactions of all chemical compounds with O₂. The choice that was made and agreed upon by chemists is that of tabulating the heat of formation of every substance from its elements in standard reference states. The standard state of an element is the state in which it ordinarily is found - solid, liquid, or gas - at room temperature (298K) and one atmosphere pressure.

Therefore the heat of formation of any element in its standard state is zero by definition. If we have the heats of formation for all chemical compounds, all other reactions between compounds can be built up by the proper combination of formation reactions, and the heats of reaction can be built up in the same way.

Example. Calculate the heat of combustion of ethyl alcohol (ethanol) from heats of formation of reactants and products.

Solution. The balanced equation for combustion of ethanol is

C₂H₅OH(l) + 3O₂(g) � 2CO₂(g) + 3H₂O(l)