I. Standard free energy change
The Gibbs-Helmholz equation for free energy change is
If the reaction starts at standard state conditions: temperature 25oC (T=273+25=298 K), pressure 1.105 Pa (1 atm), pH=7 and concentrations of reactants and products of 1 mol/L (1 mol/dm-3) then the free energy change is called standard free energy change (\Delta G^o).
The equation for standard free energy change is
When a reaction leaves the standard state because of a change in the ratio of the concentrations of the products to the reactants, the system has to be described in terms of non-standard-state free energies of reaction.
The difference between \Delta G^o and \Delta G for a reaction is important. There is only one value of \Delta G^o for a reaction at a given temperature, but there are an infinite number of possible values of \Delta G depending on the progress of the reaction, as is shown in the figure below. The dashed line shows the values of \Delta G for a reaction with \Delta G^o < 0. There is only a single value of \Delta G^o – at intercept, corresponding to the composition at which free energy does not change.
The points at which the straight line in the below figure cross the horizontal and vertical axes of this diagram are particularly important. The straight line crosses the vertical axis when the reaction quotient, Q, for the system is equal to 1. This point therefore describes the standard-state conditions, and the value of \Delta G at this point is equal to the standard-state free energy of reaction, \Delta G^o.
The point at which the straight line crosses the horizontal axis describes a system for which \Delta G^o is equal to zero. Because there is no driving force behind the reaction, the system must be at equilibrium. At this equilibrium point, there are more products than reactants.