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)