# D20.4 Calculating ΔG°

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:

ΔrG° = ΔrH° − TΔrS°

Exercise: Standard Gibbs Free Energy Change from Enthalpy and Entropy Changes

It is also possible to use the standard Gibbs free energy of formationfG°) 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. (Note that tabulated ΔfG° values are often specified at 298 K, therefore using such values to calculate ΔrG° is only valid at 298 K.)

For a generic reaction:

mA + nB ⟶ xC + yD

ΔrG° from ΔfG° would be:

ΔrG° = ∑ΔfG°(products) − ∑ΔfG°(reactants) = [xΔfG°(C) + yΔfG°(D)] – [mΔfG°(A) + nΔfG°(B)]

Exercise: Calculating Standard Gibbs Free Energy Change 