Enthalpy is dependent on the chemical energy. Because there is no zero point for energy, the absolute enthalpy of a substance cannot be determined, and it is only possible to determine changes in enthalpy.
Chemists find it convenient to tabulate enthalpy changes for various defined processes such as vaporisation. The equation for which the enthalpy change is equal to the enthalpy of vaporisation is shown for water below.
Another example of a widely used defined enthalpy change is the standard enthalpy of formation (ΔfH°).
The standard enthalpy of formation of a compound is the enthalpy change for the reaction in which one mole of the compound in its standard state is formed from its elements in their standard state.
At 25°C the enthalpy changes for the reactions shown equal enthalpies of formation.
The standard enthalpy of formation of elements in their standard state is zero because ΔH° for the reaction in which these are formed from elements in their standard state is zero.
The standard reaction enthalpy for a reaction (ΔrH°) can be calculated if the standard enthalpies of formation of all of the reactants and products are known.
ΔrH° =
z is the coefficient of each of the reactants and products in the balanced equation for the reaction.
Chemists find it convenient to tabulate enthalpy changes for various defined processes such as vaporisation. The equation for which the enthalpy change is equal to the enthalpy of vaporisation is shown for water below.
H2O(l) 
H2O(g)
ΔH° = ΔvapH°(H2O)
H2O(g) ΔH° = ΔvapH°(H2O)
Another example of a widely used defined enthalpy change is the standard enthalpy of formation (ΔfH°).
The standard enthalpy of formation of a compound is the enthalpy change for the reaction in which one mole of the compound in its standard state is formed from its elements in their standard state.
At 25°C the enthalpy changes for the reactions shown equal enthalpies of formation.
H2(g) + ½O2(g) H2O(l)
ΔH° = ΔfH°(H2O,l)
C(s) + ½O2(g) CO(g)
ΔH° = ΔfH°(CO,g)
H2O(l) ΔH° = ΔfH°(H2O,l)
C(s) + ½O2(g)
CO(g) ΔH° = ΔfH°(CO,g)
Note that the number of reactants in these equations is the same as the number of elements in the product.
Fractional coefficients are used on the reactants because this equation must have ONE mole of product.
Fractional coefficients are used on the reactants because this equation must have ONE mole of product.
The standard enthalpy of formation of elements in their standard state is zero because ΔH° for the reaction in which these are formed from elements in their standard state is zero.
O2(g) O2(g)
ΔH° = ΔfH°(O2) = 0
O2(g) ΔH° = ΔfH°(O2) = 0
The standard reaction enthalpy for a reaction (ΔrH°) can be calculated if the standard enthalpies of formation of all of the reactants and products are known.
ΔrH° =
Σ (zΔfH°(products)) - ∑(zΔfH°(reactants))
z is the coefficient of each of the reactants and products in the balanced equation for the reaction.