Free energy, `G`, denotes the self intrinsic electrostatic potential energy of a system. This means that in any molecule if we calculate the total electrostatic potential energy of all the charges due to all the other charges, we get what is called the free energy of the molecule. It tells about the stability of a molecule with respect to another molecule. Lesser the free energy of a molecule more stable it is.

Every reaction proceeds with a decrease in free energy. The free energy change in a process is expressed by `Delta G`. If it is negative, it means that products have lesser `G` than reactants, so the reaction goes forward. If it is positive the reaction goes reverse and if it is zero the reaction is at equilibrium.

`Delta G` is the free energy change at any given concentration of reactants and products. If all the reactants and products are taken at a concentration of `1` mole per liter, the free energy change of the reaction is called `Delta G^o` (standard free energy change).

One must understand that `Delta G^o` is not the free energy change at equilibrium. It is the free energy change when all the reactants and products are at a concentration of `1` mole/L. `Delta G^o` is related to `K` (equilibrium constant) by the relation, `Delta G^o = - RT ln K`.

The units of `Delta G^o` depends only on `RT`. `T` is always in Kelvin, and if `R` is in Joules, `Delta G^o` will be in joules, and if `R` is in calories then `Delta G^o` will be in calories.