Hess's law states that enthalpy changes are additive. Thus the value of the standard enthalpy of reaction can be calculated from standard enthalpies of formation of products and reactants as follows: 
Here, the first sum is over all products and the second over all reactants,  and  are the stoichiometric coefficients of products and reactants respectively,  and are the standard enthalpies of formation of products and reactants respectively, and the </sup> superscript indicates standard state values. This may be considered as the sum of two (real or fictitious) reactions:
:Reactants → Elements (in their standard states)
and Elements → Products <br>