Gibbs Free Energy is a measure of the favourability of a reaction. If Gibbs Free Energy is negative the reaction is spontaneous if Gibbs Free Energy is positive the reaction is non-spontaneous and if Gibbs Free Energy is 0 the reaction is reversible.

ΔG = ΔH – TΔS

Where:

• ΔG = Gibbs Free Energy
• ΔH = Enthalpy Change: The energy released or absorbed during a reaction.
• ΔS = Entropy Change: The change in the disorder of a system. The more disordered the higher the entropy. This increases when the number of particles increases and when there is a change of solid to a liquid or to a gas. Gases are the most disordered with solids being the least disordered.
• T: Temperature

Gibbs Free Energy is always negative when the reaction is exothermic and the entropy is positive whilst Gibbs Free Energy is always positive when the reaction is endothermic with negative entropy. In both these cases, the reactions are either always spontaneous or non-spontaneous.

When the reaction is exothermic the reaction only happens when the temperature is small so that TΔS is smaller than ΔH, leaving ΔG negative.

When the reaction is endothermic the reaction only happens when the temperature is high so that TΔS is bigger than ΔH, making ΔG negative.

Spontaneity vs Reactivity

A spontaneous reaction means that it is thermodynamically possible for that reaction to occur, and does not mean that the reaction will definitely happen. Two such reactions are the rusting of iron, a reaction that takes months if not years to proceed and the change of diamonds into graphite. Both these reactions are spontaneous, but both of these reactions happen very slowly.