9.3 Gibbs Free Energy and Thermodynamic Favorability

Gibbs Free Energy

The |Gibbs Free Energy is essentially the thermodynamic potential for a chemical reaction. The change in Gibbs free energy for a reaction is denoted by ${\Delta G}$ and is measured in kilojoules per mole. Note that this is the standard Gibbs free energy, meaning that all substances must be solids, liquids, solutions at 1.0 M or gasses at 1.0 atm.

The value for the change in Gibbs free energy of a reaction helps determine whether a reaction is thermodynamically favored or not. If ${\Delta G<0}$, then a reaction is thermodynamically favored, meaning that it will occur on its own. If ${\Delta G>0}$, then it is thermodynamically unfavored, meaning it won’t occur. Note that sometimes, the word spontaneous is used to describe thermodynamically favored reactions. However, it is important to know that this does not mean that it happens quickly or without cause, but instead that it will happen given enough time.