The sign and magnitude of Gibbs energy change of a chemical process tells about its spontaneity and useful work that could be extracted from it. Explain.

Explanation

The process which takes place on its own accord without the aid of an external agency is called spontaneous process. To sum up, the criteria for spontaneous of process in terms of `DeltaG` can be given as
i) If `DeltaG` is -ve the process is spontaneous
ii) If `DeltaG` is zero, the process does not occur (or) the system is in equilibrium
iii) If `DeltaG` is +ve, the process does not occur in the forward direction The spontaneity of a reaction and the algebraic signs of `DeltaH, DeltaS,DeltaG` and the magnitude of T are related to each others as given in the below table.

For a spontaneous process in an isolated system the change in entropy should be positive. But many reactions with `DeltaS` in negative are known to occur spontaneously. These reactions takes place on account of coupling of these reactions with reactions of large negative `DeltaG` values.