Dynamic Equilibrium

A system in dynamic equilibrium is a particular example of a system in a steady state. In a steady state the rate of inputs is equal to the rate of outputs so that the composition of the system is unchanging in time. For example, a lake is in a steady state when water flows in at the same rate as water flows out.

The very specialized term 'thermodynamic equilibrium' is used in thermodynamics for an isolated system in which the rate of every internal forward reaction is equal to the rate of its backward reaction, and there is no actual transport of matter or energy. This is the principle of detailed balance. Both reactions do in fact occur, sometimes vigorously, but to such a closely balanced extent that changes in composition cannot be observed.

For example, in a new bottle of cola the concentration of carbon dioxide in the liquid phase has a particular value. If half the liquid is poured out and the bottle is sealed, carbon dioxide will leave the liquid phase at an ever decreasing rate and the partial pressure of carbon dioxide in the gas phase will increase until equilibrium is reached. At that point a molecule of CO ₂ may leave the liquid phase, but then another molecule of CO ₂ will pass from the gas to the liquid. At equilibrium the rate of loss of CO ₂ is equal to the rate of gain. In this case, the equilibrium concentration of CO ₂ in the liquid is given by Henry's law, which states that the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid. This relationship is written as

where k is a temperature-dependent constant, p is the partial pressure and c is the concentration of the dissolved gas in the liquid. Thus, the partial pressure of CO ₂ in the gas has increased until Henry's law is obeyed. The concentration of carbon dioxide in the liquid has decreased and the drink has lost some of its fizz.

Henry's law may be derived by setting the chemical potentials of carbon dioxide in the two phases to be equal to each other. Equality of chemical potential defines chemical equilibrium. Other constants for dynamic equilibrium involving phase changes include partition coefficient and solubility product. Raoult's law defines the equilibrium vapor pressure of an ideal solution.

Dynamic equilibria can also exist in a homogeneous system. A simple example occurs with acid-base equilibria such as the "dissociation" of acetic acid, in aqueous solution.

At equilibrium the concentration quotient, K , the acid dissociation constant, is constant (subject to some conditions)

In this case, the forward reaction involves the liberation of some protons from acetic acid molecules and the backward reaction involves the formation of acetic acid molecules when an acetate ion accepts a proton. Equilibrium is attained when the sum of chemical potentials of the species on the left-hand side of the equilibrium expression is equal to the sum of chemical potentials of the species on the right-hand side. At the same time the rates of forward and backward reactions are equal to each other. Equilibria involving the formation of chemical complexes are also dynamic equilibria and concentrations are governed by the stability constants of complexes.

Dynamic equilibria can also occur in the gas phase as, for example, when nitrogen dioxide dimerizes.

It should be noted that in the gas phase, square brackets are not used as these indicate a concentration, instead a capitalised P is used to indicate partial pressure.

Dynamic Equilibrium

Dynamic Equilibrium. The previous pages have provided you with a lot of information about solutions, but only a little about the process by which they are made.

...

Dynamic Equilibrium

...

Exploratory Activity: Dynamic Equilibrium

Exploratory Activity: Dynamic Equilibrium. Introduction: The concept of equilibrium is a very important one to scientists in all fields. Static equilibrium refers to a condition in ...

...

Chemical equilibrium - Wikipedia, the free encyclopedia

If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. For example, adding more S from the outside will cause ...

...

dynamic equilibrium - Medical Definition

Do you have more to add? Share your linguistic knowledge or observation.

...

Dynamic Equilibrium Home Page

Register right now! Contact Information: Dynamic Equilibrium. Nancy Toner Weinberger, Owner & Director. Michelle Sebren, Administrative Assistant

...

DYNAMIC EQUILIBRIUM

DYNAMIC EQUILIBRIUM (Updated Oct. 5, 2004) Lonergan’s notion of dynamic equilibrium partly evolved from the General Equilibrium theory in Walras’ studies of 1898 that was ...

...

Dynamic equilibrium - Wikipedia, the free encyclopedia

A system in dynamic equilibrium is a particular example of a system in a steady state. In a steady state the rate of inputs is equal to the rate of outputs so that the composition ...

...

dynamic equilibrium - definition of dynamic equilibrium by the Free ...

e·qui·lib·ri·um (kw-l b r-m, k w-) n. pl. e·qui·lib·ri·ums or e·qui·lib·ri·a (-r-) 1. A condition in which all acting influences are canceled by others, resulting in a ...

...

equilibrium: Definition, Synonyms from Answers.com

equilibrium n. , pl. , -riums , or -ria . A condition in which all acting influences are canceled by others, resulting in a stable, balanced, or

...