# 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.
.

## 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.

V. A. KIREEV

## Gibbs free energy

[′gibz ¦frē ′en·ər·jē]
(thermodynamics)
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 ?
In the case of a singular loss function, the Bayesian estimation of the hidden component can be found as a maximum a posteriori probability (MAP) estimation, which leads us to the problem of minimization of the objective function, often called the Gibbs energy function [2].
5] Table 2: The values of the Gibbs energy for the synthesis reaction of the mineral [C.
In the present case, this treatment depends on the values of the standard molar Gibbs energies of transfer of the sulfonamides from neat water to the 1,4-dioxane + water solvent mixtures and the excess molar Gibbs energy of mixing for the co-solvent binary mixtures.
At equilibrium, the Gibbs energy of the reaction is equal to zero, and the activity of the condensed phase [M.
The liquid-water region 1, including saturated liquid line x = 0, is described by an equation for the reduced Gibbs energy as a function of reduced pressure and reciprocal reduced temperature.
Gibbs energy functions of all phases of the system and especially the excess Gibbs energy coefficients of the solution phases are necessary to calculate a temperature-composition phase diagram.
One of the explanations of noted peculiarities may be comparison of values of Gibbs energy, calculated in [5] for the reaction of nitrogen absorption by different molten metals in arc and plasma melting.
We may substantiate this fact by calculating the Gibbs energy of the photons within the human organism.
It consists of 12 calculation modules, displayed as options in the main menu: reaction equations; heat and material balances; equilibrium compositions; electrochemical cell equilibria; formula weights; phase stability diagrams; Eh - pH diagrams; enthalpy, entropy, heat capacity, Gibbs energy and Ellingham diagrams; mineralogy iterations; composition conversions; units conversions; and properties of elements.

Site: Follow: Share:
Open / Close