Free Energy

Table of Contents
Thermodynamics Gateway Page
In this module:
DG and Reactions
DG and Temperature
Coupling Reactions
Free Energy in Industry
Free Energy in Living Things
Free Energy and Equilibrium
GOALS: When you have completed this module, you should be able to:
  • Use DGº values to predict whether a reaction is product- or reactant-favored.
  • Show how reactant-favored reactions can be coupled to product-favored reactions to make both proceed toward products.
  • Use DGº values to calculate equilibrium constants.

In the Entropy Module you learned how to predict whether a reaction is product- or reactant-favored. The process involved evaluating the entropy change of the universe and applying the Second Law of Thermodynamics: if the entropy of the universe increases, the reaction is product-favored. If it decreases, the reaction is reactant-favored.

DSuniv = DSsyst - DHsyst/T    (1)

Wouldn't it be nice if there were a single number you could calculate that would give you the same information? It turns out that there is such a number, called the Gibbs Free Energy , named after Josiah Willard Gibbs, the scientist who first described it, and defined as DG = -TDSuniv. If equation (1) is manipulated a little, you can see how DG relates to DH and DS:

DSuniv = DSsyst - DHsyst/T
-TDSuniv = DHsyst - TDSsyst
DGsyst = DHsyst - TDSsyst

In this module you will learn how to use the Gibbs Free Energy to analyze reactions and why conserving Gibbs Free Energy is so important.

Free Energy