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
In 1878, Josiah Willard Gibbs created the free-energy function by com bining the first and second laws of thermodynamics. The basic equation is △G=△H-T△S (3) in which AG is the change in free energy of a system undergoing a trans- formation at constant pressure (P) and temperature (T),AH is the change in enthalpy(heat content) of this system, and AS is the change in entropy of this system
the Ag of a reaction depends both on the change in internal en ergy and on the change in entropy of the system. The change in free energy(AG)of a reaction, in contrast with the change in internal energy(AE) of a reaction, is a valuable criterion of whether it can occur spontaneously 1. A reaction can occur spontaneously only i/Ag ve 2.A system is at equilibrium and no net change can take place if Agis zero. 3. a reaction cannot occur spontaneously if AG is positive. An input of tree energy is required to drive such a reaction
Two additional points need to be emphasized here. First, the Ag of a reaction depends only on the free energy of the products(the final state minus that of the reactants(the initial state). The AG of a reaction is inde- pendent of the path(or molecular mechanism) of the transformation. The mecha- nism of a reaction has no effect on Ag Second, the Ag provides no informalion about the rale of a reaction. A negative A G indicates that a reaction can occur spontaneously, but it does not signify whether it will proceed at a perceptible rate. As will be dis cussed shortly(p. 188), the rate of a reaction depends on the free energy of activation(△G), which is unrelated to△G
STANDARD FREE-ENERGY CHANGE OF A REACTION AND ITS RELATION TO THE EQUILIBRIUM CONSTANT Consider the reaction A+B=C+D The AG of this reaction is given by [C][D] △G=△G°+ RTloge[A[B (6 in which AG is the standard free-energy change, R is the gas constant, Tis the absolute temperature, and [A], [B], [C], and [D] are the molar con centrations (more precisely, the activities) of the reactants. Ag is the free-energy change for this reaction under standard conditions-that is when each of the reactants A, B, C, and d is present at a concentration of 1.0 M(for a gas, the standard state is usually chosen to be I atmosphere Thus, the Ag of a reaction depends on the nature of the reactants(ex- pressed in the AG term of equation 6)and on their concentrations(ex- pressed in the logarithmic term of equation 6)