It is useful to be able to compare reaction enthalpies, such as standard combustion enthalpies, particularly as the enthalpies for many combustion reactions can be measured directly in a calorimeter.
However, standard reaction enthalpies are only the difference between the total enthalpies of reactants and the total enthalpies of products. An even more useful standard enthalpy change for comparison purposes is the standard enthalpy of formation (ΔfH°). This is the enthalpy change for the reaction in which one mole of substance in the standard state is formed from its elements in their standard state.
At 25°C the enthalpy changes for the reactions shown are equal to the enthalpies of formation.
Note:
The number of reactants in the defining equation equals the number of elements in the product.
The coefficient of the product substance MUST be 1. Fractional coefficients may be needed on reactants.
Note that many compounds cannot be formed directly from their elements (NH4Cl). The standard enthalpy of formation for such compounds is calculated by combining the enthalpy changes in an indirect pathway between the elements and the compound.
This definition implies that the standard enthalpy of formation of elements in their standard state is zero because ΔH° for the reaction in which an element in its standard state is formed from an element in its standard state is zero.
Enthalpies of formation defined in this way are very useful because if we know the enthalpies of the various substances involved in a reaction relative to their elements, then the enthalpy change accompanying the reaction can be calculated.
However, standard reaction enthalpies are only the difference between the total enthalpies of reactants and the total enthalpies of products. An even more useful standard enthalpy change for comparison purposes is the standard enthalpy of formation (ΔfH°). This is the enthalpy change for the reaction in which one mole of substance in the standard state is formed from its elements in their standard state.
At 25°C the enthalpy changes for the reactions shown are equal to the enthalpies of formation.
H2(g) + ½O2(g) 
H2O(l)
C(s) + O2(g) CO2(g)
H2O(l) ΔH° = ΔfH°(H2O,l)
C(s) + O2(g)
CO2(g) ΔH° = ΔfH°(CO2,g)
Note:
The number of reactants in the defining equation equals the number of elements in the product.
The coefficient of the product substance MUST be 1. Fractional coefficients may be needed on reactants.
Note that many compounds cannot be formed directly from their elements (NH4Cl). The standard enthalpy of formation for such compounds is calculated by combining the enthalpy changes in an indirect pathway between the elements and the compound.
This definition implies that the standard enthalpy of formation of elements in their standard state is zero because ΔH° for the reaction in which an element in its standard state is formed from an element in its standard state is zero.
O2(g) O2(g) ΔH° = ΔfH°(O2) = 0
O2(g) ΔH° = ΔfH°(O2) = 0Enthalpies of formation defined in this way are very useful because if we know the enthalpies of the various substances involved in a reaction relative to their elements, then the enthalpy change accompanying the reaction can be calculated.