Read in this section
On the previous page, we derived an expression which gives us a relationship between the Gibbs energy and temperature.
We start with the definition of the Gibbs energy, G = H – TS . When the system undergoes a change of state, we may write, for any general change.
The criterion for spontaneous change dG £ 0 may be very simply expressed in words as a tendency of the Gibbs energy to tend to a minimum value. i.e. reactions are only spontaneous in the direction of decreasing Gibbs energy.
Entropy is the fundamental basis for assigning the direction of spontaneous change, but to use it we must consider the entropy changes of both the system and the surroundings, which is somewhat inconvenient.