Enthalpies of formation and reaction

The enthalpy of formation of a compound, ΔH_f, is the change in enthalpy when forming one mole of that compound from its constituent elements in their standard state. The enthalpy of formation of elements in their standard state is zero.

The enthalpy of reaction, ΔH_r is the change in enthalpy associated with a certain chemical reaction. It can be measured experimentally by using calorimetry or it can be calculated using Hess’ law. When calculating the enthalpy of a reaction, the sum of the enthalpies of formation of the reactants are subtracted from the sum of the enthalpies of formation of the products:

ΔH_r = ΣΔH_f(products) - ΣΔH_f(reactants)

The enthalpy of combustion is the enthalpy of reaction for a combustion reaction.