D20.2 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.

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

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Chemistry 109 Fall 2021 by John Moore, Jia Zhou, and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.