Entropy (S) is a molecular 'disorder', or more precisely 'the number of microscopic arrangements of energy possible in a macroscopic sample'. Entropy depends strongly on the state and increases with rise in temperature.

Entropy changes (ΔS) are positive for reactions that generate gas molecules. The Second Law of Thermodynamics defines that the total entropy always increases in a spontaneous process, and reaches a maximum value at equilibrium. To apply this to chemical reactions it is necessary to include entropy changes in the surroundings caused by heat output or input. Both external and internal changes are taken account of by defining the Gibbs free energy change (ΔG): for a reaction performing at constant temperature.