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
+ 3O2(g) ®
2CO2(g) + 3H2O(l)