What does all this discussion of physical properties and entropies have to do with chemistry? Entropy is the missing factor that we needed when we saw that energy, or enthalpy, alone is not enough to determine whether a chemical reaction will be spontaneous. A drop in enthalpy (9H negative) will help to make a process spontaneous, but is not enough by itself to guarantee that it will be so. Spontaneity is favoured by simultaneously minimizing H and maximizing S, or minimizing H and -S. We can define a new function whose minimization combines both of the above requirements. This is the free energy, G:

G = H - TS

The units of H (cal mole-l) and S (cal deg 1 mole-l) require that S be multiplied by the absolute temperature, T.