DG^o = -RT ln K_c

Among other things this relationship reveals what K_c alone cannot. For example, larger equilibrium constants generally mean more products and fewer reactants at equilibrium. But it is difficult to compare equilibrium constants which have different exponents.

Standard free energy values, on the other hand, can be easily compared. Reactions with negative standard free energies "favor" products at least a little at equilibrium. The more negative, the more products. Negative standard free energies correspond to K_c values which are greater than 1. Positive standard free energies correspond to K_c values which are less than 1.