At points where the state of the substance changes, the change in entropy of the system is large, although the change of entropy of the universe is zero. Reactions tend to proceed in the direction of greatest entropy. If a reaction as written gives an increase in entropy, the reaction proceeds without external influence and is therefore spontaneous.
Another form of the second law of thermodynamics when DH=0 is
If the reaction is reversible such that a small change in one of the external conditions is sufficient to drive it in the opposite direction, equation (3) takes the form
When the reaction is a change of state, entropy also obeys the following relationship:
