equilibrium

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A stochastic process is said to be in equilibrium if, for every state of the process and for each time point, the probability of being in that state at the next time point is equal to the probability for the current time point.

A state in which a system has its energy distributed in the statistically most probable manner; a state of a system in which forces, influences, reactions, etc., balance each other out so that there is no net change. A body is said to be in thermal equilibrium if no net heat exchange is taking place within it or between it and its surroundings. A system is in chemical equilibrium when a reaction and its reverse are proceeding at equal rates (see also equilibrium constant). These are examples of dynamic equilibrium, in which activity in one sense or direction is in aggregate balanced by comparable reverse activity.

For more information on equilibrium, visit Britannica.com.

noun

A stable state characterized by the cancellation of all forces by equal opposing forces: balance, counterpoise, equipoise, stasis. See order/disorder.

Definition: balance; evenness
Antonyms: imbalance, unevenness

The state of being equally balanced; a state of a body in which the forces acting on it are equally balanced.

According to occultist Éliphas Lévi, magic harmony is said to depend upon equilibrium. In ceremonial magic operations, if the will of the operator is always at the same tension and directed along the same line, moral impotence will ensue. On the other hand, mediums who submit themselves passively to psychic forces are equally unbalanced. Lévi extols the all-powerful action of harmony in exalting the soul and giving it rule over the senses, guided by the will.

Sources:

Lévi, Éliphas. Transcendental Magic. New York: Samuel Weiser, 1972.

In economics, a state of the economy in which for every commodity or service (including labor), total supply and demand are exactly equal. Equilibrium is never actually attained; it is approximated by movements of the market.

The state in which market supply and demand balance each other and, as a result, prices become stable. Generally, when there is too much supply for goods or services, the price goes down, which results in higher demand. The balancing effect of supply and demand results in a state of equilibrium.

Investopedia Says:
The equilibrium price is where the supply of goods matches demand. When a major index experiences a period of consolidation or sideways momentum, it can be said that the forces of supply and demand are relatively equal and that the market is in a state of equilibrium. 

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A condition in which all influences acting cancel each other, so that a static or balanced situation results. In physics, equilibrium results from the cancellation of forces acting on an object. In chemistry, it occurs when chemical reactions are proceeding in such a way that the amount of each substance in a system remains the same. (See chemical equilibrium.)

1. a state of balance between or among opposing forces or processes that results in the absence of net change.
2. (in chemistry) a state of dynamic balance in a reversible chemical reaction when the reaction velocities in both directions are equal.

• Principles of Mechanics, Waves, and Measurement - equilibrium: condition in which net force on object is zero and there is no change in its motion
• Physical Properties and Changes - equilibrium: state in which forces acting on a system are balanced
• Chemical Properties and Reactions - equilibrium: state in which forces acting on a system are balanced
• Mystical States and Experiences - equilibrium: balance and harmony with nature, other men, or oneself

For the economic concept, see Dynamic equilibrium (economics)

A dynamic equilibrium exists once a reversible reaction ceases to change its ratio of reactants/products, but substances move between the chemicals at an equal rate, meaning there is no net change. It is a particular example of a system in a steady state. In thermodynamics a closed system is in thermodynamic equilibrium when reactions occur at such rates that the composition of the mixture does not change with time. Reactions do in fact occur, sometimes vigorously, but to such an extent that changes in composition cannot be observed. Equilibrium constants can be expressed in terms of the rate constants for elementary reactions.

Contents 1 Examples 2 Relationship between equilibrium and rate constants 3 See also 4 References

Examples

In a new bottle of cola the concentration of carbon dioxide in the liquid phase has a particular value. If half of 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 CO₂ may leave the liquid phase, but then another molecule of CO₂ will pass from the gas to the liquid. At equilibrium the rate of loss of CO₂ is equal to the rate of gain. In this case, the equilibrium concentration of CO₂ 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.^[1] 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 CO₂ 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.

CH₃CO₂H CH₃CO₂^- + H⁺

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.

2NO₂ N₂O₄;

In the gas phase, square brackets are not used as these indicate a concentration, instead a capitalised P is used to indicate partial pressure.^[2]

Relationship between equilibrium and rate constants

In a simple reaction such as the isomerization:

there are two reactions to consider, the forward reaction in which the species A is converted into B and the backward reaction in which B is converted into A. If both reactions are elementary reactions, then the rate of reaction is given by^[3]

where k_f is the rate constant for the forward reaction and k_b is the rate constant for the backward reaction and the square brackets, [..] denote concentration. If only A is present at the beginning, time t=0, with a concentration [A]₀, the sum of the two concentrations, [A]_t and [B]_t, at time t, will be equal to [A]₀.

% concentrations of species in isomerization reaction. k_f = 2 s^-1, k_r = 1 s^-1

The solution to this differential equation is

and is illustrated at the right. As time tends towards infinity, the concentrations [A]_t and [B]_t tend towards constant values. Let t approach infinity, that is, t→∞, in the expression above:

In practice, concentration changes will not be measurable after . Since the concentrations do not change thereafter, they are, by definition, equilibrium concentrations. Now, the equilibrium constant for the reaction is defined as

It follows that the equilibrium constant is numerically equal to the quotient of the rate constants.

In general they may be more than one forward reaction and more than one backward reaction. Atkins states^[4] that, for a general reaction, the overall equilibrium constant is related to the rate constants of the elementary reactions by

See also

• Equilibrium chemistry
• Static equilibrium
• Chemical equilibrium
• Radiative equilibrium

References

Atkins, P.W.; de Paula, J. (2006). Physical Chemistry (8th. ed.). Oxford University Press. ISBN 0-19-870072-5. 

1. ^ Atkins, Section 5.3
2. ^ Denbeigh, K (1981). The principles of chemical equilibrium (4th. ed.). Cambridge, U.K.: Cambridge University Press. ISBN 0-521-28150-4. 
3. ^ Atkins, Section 22.4
4. ^ Atkins, Section 22.4

v t e Chemical equilibria Concepts Acid dissociation constant · Binding constant · Binding selectivity · Buffer solution · Chemical equilibrium · Chemical stability  · Chelation  · Determination of equilibrium constants · Dissociation constant · Distribution coefficient · Distribution ratio · Dynamic equilibrium · Equilibrium chemistry · Equilibrium constant · Equilibrium unfolding · Equilibrium stage · Henry's law · Liquid-liquid extraction · Macrocycle effect  · Phase diagram · Predominance diagram · Phase rule · Reaction quotient · Self-ionization of water  · Solubility equilibrium · Stability constants of complexes · Thermodynamic equilibrium · Vapor-liquid equilibrium

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Common misspelling(s) of equilibrium

• equilibrum
• equilibium

Dansk (Danish)
n. - balance, ligevægt, sindsro, balancetilstand

Nederlands (Dutch)
evenwicht, evenwichtigheid

Français (French)
n. - équilibre, aplomb, stabilité

Deutsch (German)
n. - Gleichgewicht

Ελληνική (Greek)
n. - ισορροπία

Italiano (Italian)
equilibrio

Português (Portuguese)
n. - equilíbrio (m)

Русский (Russian)
равновесие, уравновешенность, самообладание

Español (Spanish)
n. - balance, equilibrio

Svenska (Swedish)
n. - jämvikt, jämviktsläge, avvägande

中文（简体）(Chinese (Simplified))
平衡, 均衡, 平静

中文（繁體）(Chinese (Traditional))
n. - 平衡, 均衡, 平靜

한국어 (Korean)
n. - 균형, 평온

日本語 (Japanese)
n. - 釣合い, 平衡, 安定

العربيه (Arabic)
‏(الاسم) توازن‏

עברית (Hebrew)
n. - ‮שיווי משקל, איזון נפשי או שכלי‬

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