Nonequilibrium state

In thermodynamics, a nonequilibrium state refers to the state of existence of a given system or body in which the variation of the thermodynamic potential quantifying the system is not equal to zero. [1] A system or body in a nonequilibrium state means that unbalanced potentials (or driving forces) exist within the system. [2]


A system not in equilibrium or in a nonequilibrium state generally equates to the conclusion that the variation in the entropy of the system is positive and has not reached a maximum value; the specifics of which, however, depending on the type of system, i.e. isolated, closed, open, etc., as well as what variables are being held constant, e.g. isothermal, isobaric, isochoric, etc. Systems not found in states not meeting what is called the criterion of equilibrium are said to be out of equilibrium or in a non-equilibrium. A nonequilibrium state, in simple terms, refers to the state of a system that is not in equilibrium.

There are, however, four general types or states of nonequilibrium: (a) non mechanical equilibrium, (b) non thermal equilibrium, (c) non phase equilibrium, and (d) non chemical equilibrium; each of which requires detailed discussion. Chemical equilibrium, for instance, refers to systems in which the rate of the forward reaction equals the rate of the reverse reaction, and therefore the equilibrium constant is zero, which is point assigned to a free energy variation of zero.

The details of the description of systems found in what is called a “state of equilibrium”, as contrasted to systems not in a state of equilibrium or nonequilibrium for short was worked out in American engineer Willard Gibbs’ 1876 On the Equilibrium of Heterogeneous Substances, in what he described as a ‘general theory of thermodynamic equilibrium’, the core statement of which is a as follows:

“It is an inference naturally suggested by the general increase of entropy which accompanies the changes occurring in any isolated material system that when the entropy of the system has reached a maximum, the system will be in a state of equilibrium.”

Conversely, a system of this type is in a state wherein the value of entropy has not reached a maximum value, the system can be said to be in a nonequilibrium state.

The degree to which a system is out of thermodynamic equilibrium is sometimes quantified in terms of what are called near-equilibrium variables, justified on what is called the local equilibrium approximation, using terms such as quasi-equilibrium or near-equilibrium state (near-equilibrium thermodynamics).

Another variant, developed generally by Belgian chemist Ilya Prigogine, is what is called the far-from-equilibrium state.

It is often said that so-called living systems exist continuously under nonequilibrium conditions. [3]

See also
‚óŹ Equilibrium state

1. Gibbs, Willard. (1876). "On the Equilibrium of Heterogeneous Substances", Transactions of the Connecticut Academy (pg. 55), III. pp. 108-248, Oct., 1875-May, 1876, and pp. 343-524, may, 1877-July, 1878.
2. Cengel, Yunus A. and Boles, Michael A. (2002). Thermodynamics: an Engineering Approach (section: State and Equilibrium, pg. 12). McGraw-Hill.
3. Voet, Donald, Voet Judith G. (2004). Biochemistry: Biomolecules, mechanism of Enzyme Action, and Metabolism (pg. 575). Wiley & Sons.

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