Definition of the standard Gibbs free energy

The Phase Rule describes the possible number of degrees of freedom in a (closed) system at equilibrium, intermsofthenumberof separate phases and the number of chemical constituents in the system. It was deduced from thermodynamic principles by J. W. Gibbs in the 1870s. (http //jwgibbs.cchem.berkeley.edu/phase rule.html) 

Measurement of the internal energy of a system requires that we have already defined a reference or standard state. This is the state where a component is most stable at normal temperature and pressure. When A G is measured at the standard conditions, it is denoted 

However, standard conditions in biochemistry and in biology are different, since chemical reactions in cells occur at around pH 7. Therefore, standard conditions in biochemistry differ from those in chemistry, which implies that the standard Gibbs free energy within a biological system is denoted as A G0/. Standard conditions in biochemistry and biology are a pH equal to 7 and a constant water concentration that does not appear in the mathematical definition of the equilibrium constant. 

The pressure ratio gives the equilibrium constant Kcq, and so we obtain  

If AG0 0, the exponential will be positive, and Keq higher than 1. Thus, the lower the AG0/, the higher Keq will be. 

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Yet another approach to the determination of AG is apparent when considering another definition of the standard Gibbs free-energy change as the difference between the energies of formation of products and reactants ... 

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