Gibbs free energy

Also found in: Dictionary, Thesaurus, Medical, Legal, Financial, Acronyms, Wikipedia.

Gibbs free energy:

see free energyfree energy
or Gibbs free energy,
quantity derived from the relationships between heat and work studied in thermodynamics and used as a measure of the relative stability of a physical or chemical system, i.e., the tendency of the system to react or change.
..... Click the link for more information.

Gibbs Free Energy


(also Gibbs function), one of the characteristic functions of a thermodynamic system, denoted by G and determined by enthalpy H, entropy S, and temperature T by the equality

(1)G = H - TS

The Gibbs free energy is a thermodynamic potential. In an isothermic equilibrium process occurring at constant pressure, the decrease in the Gibbs free energy of a given system equals the total work done by the system in this process minus the work against external pressure (that is, the Gibbs free energy equals the maximum useful work). It is usually expressed in kilojoules per mole or kilocalories per mole. With the aid of the Gibbs free energy and its derivatives, simple expressions can be found for other thermodynamic functions and properties of the system (including internal energy, enthalpy, and chemical potential) under conditions of constant temperature and pressure. Under these conditions, any thermodynamic process can occur without work expenditure externally only in the direction that corresponds to a decrease in G (dG < 0). The limit of its occurrence without work expenditure, that is the equilibrium condition, is the minimum value of G (dG = 0, d2G > 0). The Gibbs free energy is widely used in examining various thermodynamic processes occurring at constant temperature and pressure. It is used to determine the work of the reverse magnetization of magnets and the polarization of a dielectric under these conditions. Knowledge of the Gibbs free energy is important for a thermodynamic study of phase changes. The equilibrium constant Ka of a chemical reaction at some temperature T is determined by the standard change in the Gibbs free energy ΔGT° according to the relation

(2) ΔGT° = -RT ln Ka,T

Also widely used is the Gibbs free energy ΔGfor° of the formation of a chemical compound, equal to the change in the Gibbs free energy in the reaction of forming a given compound (or simple substance) from the standard state of the corresponding simple substances. For any chemical reaction, ΔGfor° equals the algebraic sum of the products ΔGT° of the substances participating in the reaction and their coefficients in the reaction equation. For 298.15° K the ΔGfor° are already known for several thousand substances, which makes it possible to calculate the corresponding ΔG° and Ka for a large number of reactions.

Along with equation (1), the Gibbs free energy can also be determined by internal energy U, the Helmholtz free energy A, and the product of volume V and pressure p, from the equalities

(3)G = U - TS + pV

(4)G = A + pV

For a long time various authors called the characteristic function of the Gibbs free energy by various names—for example, free energy, free energy at constant pressure, thermodynamic potential, Gibbs thermodynamic potential, isobaric-isothermic potential, and free enthalpy. Different symbols were used to denote this function (Z, F, Φ). The term “Gibbs free energy” and the symbol G used here correspond with a decision of the 18th congress of the International Union of Pure and Applied Chemistry held in 1961.


Gibbs free energy

[′gibz ¦frē ′en·ər·jē]
The thermodynamic function G = H-TS, where H is enthalpy, T absolute temperature, and S entropy. Also known as free energy; free enthalpy; Gibbs function.
References in periodicals archive ?
Isokinetic temperature and Gibbs free energy at the isokinetic temperature of the damaged beans and the control Adsorption Desorption Parameters damaged control damaged control [T.
Table 4 presents fitted equations of Gibbs free energy of roasted and ground coffee in different particle sizes.
Gibbs free Energy (AGR) 192,97 KJ/mol Reaction Entropy (AGR) -3,43KJ/mol*[degrees]C Temperature 300 K
Since living organisms utilize not thermal energies, but the Gibbs free energy to govern their chemical reactions, and since they utilize their vitality to activate collective degrees of freedom in a level that is autonomous, which does not depend directly on thermal energies distributed on a different level, namely, on the level of individual degrees of freedom, therefore, biological organization does not use energy that could be distributed in entropic processes.
When we write either the Helmholtz or the Gibbs free energy as a function of two variables, we imply a list of 11 restrictions, and these restrictions rule out inclusion of many forms of energy that are relevant to thermodynamics.
When comparing the Gibbs Free Energy of Formation for mullite, fayalite and tephroite, the reactions between the iron- and manganese oxides and the silica will have the following values:
Gibbs free energy was assessed using the following Equation: 9
The change of Gibbs free energy (G) and entropy (S) of binding reaction also can be calculated.
The compatibility of two polymers is governed by at least two important factors: (1) the Gibbs free energy of mixing, [DELTA][G.
The photons would recharge all the electric and concentration gradients, all the proteins would become regenerated and re-polarized, the Gibbs free energy would be increased to a large enough value that it may easily decrease and so all the important chemical reactions are available again for the biological coupling processes.