Consider the reaction

 

A → B

A mathematical relation for the free energy change can be derived.

ΔG = ΔG° + RT ln 

Where

 

ΔG° = Standard free energy change
R = Gas constant
T = Absolute temperature
ln = Natural logarithm
B = Concentration of product
A = Concentration of reactant
ΔG° is related to equilibrium constant (K_eq)

When a reaction A ↔ B is at equilibrium (eq), the free energy change is zero. The above equation may be written as,

ΔG = 0 = ΔG° + RT In 

Hence ΔG° = −RT ln K_eq.

When ΔG° is negative (the products contain less free energy than the reactants), the reaction will proceed to form the products under standard conditions.

When ΔG° is positive (the products of the reaction contain more free energy than the reactants), the reaction will tend to go in the reverse direction.

Relationship among K′_eq and ΔG° and direction of the chemical reaction:

When K′eq is	ΔG° is	Starting with 1M concentration of substrate
> 1.0	negative	proceeds forward
1	zero	remains at equilibrium
< 1.0	positive	proceeds backward