Depending on the nature of the process, the free energy is determined differently.
For example, the Gibbs free energy change is used when considering processes that occur under constant pressure and temperature conditions, whereas the Helmholtz free energy change is used when considering processes that occur under constant volume and temperature conditions.
The value and even the sign of both free energy changes can depend upon the temperature and pressure or volume.
For cases involving an isolated system where no energy is exchanged with the surroundings, spontaneous processes are characterized by an increase in entropy.
As an example, the conversion of a diamond into graphite is a spontaneous process at room temperature and pressure.
Another way to explain this would be that even though the conversion of diamond into graphite is thermodynamically feasible and spontaneous even at room temperature, the high activation energy of this reaction renders it too slow to observe.
For a process that occurs at constant temperature and pressure, spontaneity can be determined using the change in Gibbs free energy, which is given by:
For the latter two cases, the temperature at which the spontaneity changes will be determined by the relative magnitudes of ΔS and ΔH.
When using the entropy change of a process to assess spontaneity, it is important to carefully consider the definition of the system and surroundings.
This criterion can then be used to explain how it is possible for the entropy of an open or closed system to decrease during a spontaneous process.