Gibbs free energy


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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.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
The standard Gibbs free energy change when using hydrogen-producing acetogens for the metabolism of organic acids and alcohols is shown in Table 2 [20,22].
The general prediction of (9), however, is more subtle and includes the possibility of work being independent of peak cycle temperature for the case of reversible "combustion" cycle--which is consistent with the idea of Gibbs free energy [31].
At constant pressure, the force driving chemical reactions is normally quantified by the thermodynamic quantity Gibbs free energy or Gibbs energy ([DELTA]G).
Where [DELTA]G is the Gibbs free energy (J/mol), [DELTA]H is enthalpy (J/mol), T is the temperature in Kelvin (K), and [DELTA]S is the entropy (J/mol K).
In a third formulation, biological organization is the process generating a system of collective degrees of freedom, involving global degrees of freedom having significant free energy for governing the system to be able to actively maintain the vitality of the organism, where vitality V=G/T (G is the Gibbs free energy and T is the temperature) is its entropic distance from equilibrium (see Figure 1).
However, the driving force in any reaction is the desire to reduce the Gibbs free energy in a system (G).
Instead, biologists have used either the Helmholtz free energy function or the Gibbs free energy function, both of which are, in fact, special cases of the more general availability function.
To understand the chemical erosion of the refractory due to iron oxide attack, the various forms of iron oxide must be identified and classified according to Gibbs Free Energy of Formation at a given iron melting temperature (Table 1).