Gibbs free energy

(redirected from Gibbs energy)
Also found in: Dictionary.

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 ?
5] Table 2: The values of the Gibbs energy for the synthesis reaction of the mineral [C.
At equilibrium, the Gibbs energy of the reaction is equal to zero, and the activity of the condensed phase [M.
This study calculated the real equilibrium oxygen partial pressure and Gibbs energy values of each oxidation reaction over two steels according to equations (4) and (2).
All thermodynamic properties for water can be derived from Equation 3 by using the appropriate combinations of the dimensionless Gibbs energy [[gamma].
The steam region 2, including the saturated vapor line, is described by an equation for the reduced Gibbs energy as a function of pressure and temperature.
The Gibbs energy is then obtained from the GibbsHelmholtz equation
exs] is the excess Gibbs energy, and expressed by the Redlich-Kister polynomial [14, 18], where L is the binary interaction parameter that has been optimized by several workers [14, 18, 19].
In practice, most of the part of the negentropy of the radiation is transformed into internal work and cell maintenance functions recharging the living organism, pushing it to a high level of Gibbs energy.
In a binary system, n defines the composition where the Gibbs energy of mixing is a maximum or minimum or the composition where the temperature of a miscibility gap is maximum or minimum.
3] Plotting the Gibbs energy of oxidation reactions against temperature was first proposed by Ellingham[4] and is particularly valuable in dealing with the extraction of metals.
Several of the tables have been expanded and updated, including: bond dissociation energies, NIST atomic transition probability tables, properties of semiconductors, atomic masses and abundances, threshold limits for airborne contaminants, and the standard transfomed Gibbs energy of formation for important biochemical species.