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Gibbs free energy is a thermodynamic quantity equal to the enthalpy (of a system or process) minus the product of the entropy and the absolute temperature.

Gibbs free energy can be defined as the thermodynamic potential signifying the maximum amount of reversible work that can be performed by a thermodynamic system when the temperature and the pressure are constant. Gibbs free energy is also known as Gibs energy or Gibbs function or also as free enthalpy.

The second law of the thermodynamics state that the entropy of universe increases for a spontaneous process.

⩟Suniverse = ⩟Ssystem + ⩟Ssurroundings > 0

The change in Gibbs free energy at a constant pressure and temperature is said to be

⩟G = ⩟H – T⩟S

When the value of ⩟G is negative, then the process is known as exergonic and will proceed spontaneously. The spontaneity of the process can be temperature dependent.  If the value is positive, then the reaction is non-spontaneous and is known as energonic. If the value is equal to 0, then the system is said to be at equilibrium with no net change in either of the direction.

Gibbs free energy combines the entropy and enthalpy into one single value. The change in the free energy can predict the direction of the chemical reaction when the temperature and the pressure are constant.

The concept of Gibbs free energy was developed by Josiah Willard Gibbs in 1870. Originally, he coined the term as “available energy” and published a paper in 1873 to outline the use of his equation in predicting the behaviour of the systems when combined.

The decrease in the Gibbs free energy gets the maximum amount of the non expansion work extracted from a thermodynamically closed system (i.e. a system in which the exchange of heat and work takes place with its surroundings but not with the matter) but the maximum work is attained from a completely reversible reaction only. With the transformation of the substance from the initial to the final state, there is a decrease in the Gibbs free energy which is equals to the work done by the system to its surroundings with exclusion of pressure forces.

According to Gibbs, the initial state of the body is means to be in such a way that the body can pass from it to the states of dissipated energy by the reversible processes.


  • Gibbs free energy depends on the difference in the free energy of the products and the reactants (in the final or the initial state).

Gibbs free energy is independent of the transformation pathway and remains