A convenient and common approach to calculate ΔrG° for reactions is by using standard state thermodynamic data. One method involves first determining the ΔrH° and ΔrS°, then using the equation:
It is also possible to use the standard Gibbs free energy of formation (ΔfG°) of the reactants and products involved in the reaction to calculate ΔrG°. ΔfG° is the Gibbs free energy change that accompanies the formation of one mole of a substance from its elements in their standard states. It is by definition zero for the most stable form of an elemental substance under standard state conditions.
For a generic reaction:
ΔrG° from ΔfG° would be: