One difficulty in using entropy as a criterion of whether a biochemical process can occur spontaneously is that the entropy changes of chemical reactions are not readily measured. Furthermore the criterion of sponta neity given in equation 2 requires that both the entropy change of the surroundings and that of the system of interest be known. These difficu ties are obviated by using a different thermodynamic function called the free energy, which is denoted by the symbol G(or F, in the older literature