Gibbs free energy is a useful concept which lets us determine how likely a reaction is to take place spontaneously. It is defined using the following equation:

ΔG = ΔH – TΔS

1. A reaction will take place if it creates a reduction in Gibbs free energy, ie ΔG < 0.
2. Gibbs energy is reduced if H is reduced. H is the heat energy locked up in the system.
3. Gibbs energy is reduced if S is increased. S is a measure of Entropy or Disorder.

A reaction is most likely to happen if ΔH < 0 and ΔS > 0, ie if the system enters a lower energy state and becomes more disordered.

Exothermic and Endothermic reactions

1. In an exothermic reaction heat is given out so heat energy in the system is reduced. ΔH <0. This is favourable.
2. In an endothermic reaction heat is taken in so heat energy in the system is increased. ΔH >0. This is unfavourable

An Endothermic reaction can still take place if it results in a large enough increase in entropy, ie ΔS > 0.

Water can melt even though this is an endothermic reaction since liquid water is less organised and therefore has a higher entropy than solid crystalline ice.

Polythene will shrink in an oven even though this is an endothermic reaction (it requires heat to take place). This is because the shrunken polythene is less ordered and has higher entropy.

For more information on answering these sorts of questions see the Bodner Research Web