Gibbs Free Energy

KEY POINTS:

• A spontaneous chemical reaction is one which proceeds of it’s own accord without any external help (e.g. input of energy).
• Overall spontaneity is determined by the enthalpy and entropy change associated with a reaction, linked in the following equation:
  ΔG  =  ΔH  –  TΔS
• Where ΔG is the Gibbs Free Energy change. This is a measure of the stored energy in a reaction, and for the reaction to be spontaneous the ΔG must be negative.
• By considering the signs of the enthalpy and entropy changes the following is apparent for whether a reaction will be spontaneous or not:
  

  	ΔH positive	ΔH negative
  ΔS positive	Spontaneous at high T	Spontaneous at all T
  ΔS negative	Never spontaneous	Spontaneous at low T

• Therefore you cannot have a chemical reaction with a decrease in the surrounding temperature and a decrease in entropy. And overall spontaneity is very dependent on temperature in many cases. You must state whether or not a reaction is spontaneous at a specific temperature – and to determine the temperature at which a reaction becomes feasible just allow ΔG = 0.
• Don’t forget, some reactions may be spontaneous but require an initial spark to get things going – just think about your bunsen burners!