(i) The Gibbs free energy is the maximum amount of non-expansion work that can be extracted from a closed system; this maximum can be attained only in a completely reversible process.

(ii) When a system changes from a well-defined initial state to a well-defined final state, the Gibbs free energy `DeltaG` equals the work exchanged by the system with its surroundings, minus the work of the pressure forces, during a reversible transformation of the system from the same initial state to the same final state.

(iii) Gibbs energy (`DeltaG`) is also the chemical potential that is minimized when a system reaches equilibrium at constant pressure and temperature. Its derivative with respect to the reaction coordinate of the system becomes zero at the equilibrium point.

(iv) As such, it is a convenient criterion of spontaneity for processes with constant pressure and temperature.

(v) The Gibbs free energy the greatest amount of mechanical work which can be obtained from a given quantity of a certain substance in a given initial state, without increasing its total volume or allowing heat to pass to or from external bodies, except such as at the close of the processes are left in their initial condition.

(vi) Every system seeks to achieve a minimum of free energy.

(vii) The energy released equals the maximum amount of work that can be performed as a result of the chemical reaction. ln contrast, if conditions indicated a positive `DeltaG`, then energy in the form of work would have to be added to the reacting system to make the reaction proceed.

(viii) Thus the direction of reaction will only be allowed if the total entropy change of the universe is equal to zero (an equilibrium process) or positive and correspondingly gibbs free energy criteria is :

`A. DeltaG > 0` the process is non-spontaneous.

`B. DeltaG = 0` system is at equilibrium

`C. DeltaG > 0` the process is spontaneous