Question
Prove that ∆G = –T∆Stotal.
Solution
The change in entropy in a process carried out in a non-isolated system is given by
∆Stotal = ∆Ssystem + ∆Ssurroundings ...(1)
Consider an isothermal process carried out at constant pressure in which heat q is transferred by the surroundings to the system.
Substituting this value in (1), we have,
It has been shown that 
must be positive for a process to be spontaneous. Therefore, equation (2) becomes useful in predicting the spontaneity of a process in terms of ∆G.
