## Is the standard Gibbs freeenergy change for the reaction

If one starts with a mixture of A and B, they would begin to convert to C and D because if the latter have a low enough concentration, their Gibbs free energy will be less than that of the reactants. As the reaction proceeds, the combined Gibbs free energy of A and B will decrease, and the contribution from C and D will increase. Eventually, a point is reached where any change in the Gibbs free energy of the products exactly balances changes in the Gibbs free energy of the reactants. Any further reaction would cause an increase in the total Gibbs free energy. The total Gibbs free energy is then at a minimum, and if the reaction proceeded infinitesimally, the change in Gibbs free energy given by equation (5.6) would be zero. Thus, substituting equilibrium concentrations into (5.6) and setting AG

to zero, we have

[Ai:q[B]bq eq where Keq is defined as the equilibrium coefficient:

[Clc Dd

From equation (5.8) we also have that

The equilibrium constant is a unique thermodynamic constant for each reaction. We have already met one example of an equilibrium constant in the Ka for acid-base reactions as defined in equation (3.2).

By convention, when biochemists compute Keq and AG° for reactions that involve water or hydrogen ions, they assume that their concentrations are held constant at 1.0 and 10-7 M (pH 7.0), respectively.

Example 5.1 Compute AG° and Keq for the complete oxidation of oxalic acid. Answer First write the balanced equation for the oxidation: 