Dynamic equilibrium

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Dynamic equilibrium

A system in dynamic equilibrium is a particular example of a system in a steady state

Steady state

A system in a steady state has numerous properties that are unchanging in time. The concept of steady state has relevance in many fields, in particular thermodynamics....
. 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 term dynamic equilibrium is used in thermodynamics

Thermodynamics

In physics, thermodynamics is the study of the conversion of heat energy into different forms of energy ; different energy conversions into heat energy; and its relation to macroscopic variables such as temperature, pressure, and volume....
for systems involving reversible reaction

Reversible reaction

A reversible reaction is a chemical reaction that results in an chemical equilibrium mixture of reactants and Product . For a reaction involving two reactants and two products this can be expressed symbolically asA and B can react to form C and D or, in the reverse reaction, C and D can react to form A and B....
s. It is said that the rate of the forward reaction is equal to the rate of the backward reaction.

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Encyclopedia

A system in dynamic equilibrium is a particular example of a system in a steady state

Steady state

Thermodynamics

Reversible reaction

Coca-Cola

Coca-Cola is a carbonation soft drink sold in stores, restaurants and vending machines worldwide . It is produced by The Coca-Cola Company in Atlanta, Georgia, and is often referred to simply as Coke or as Cola or Pop....
the concentration of carbon dioxide

Carbon dioxide

Carbon dioxide is a chemical compound composed of two oxygen atoms covalent bond to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state....
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

Partial pressure

In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
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

Henry's law

In chemistry, Henry's law is one of the gas laws, formulated by William Henry in 1803. It states that:An equivalent way of stating the law is that the solubility of a gas in a liquid is proportional to the pressure of that gas above the liquid....
, which states that the solubility of a gas in a liquid is directly proportional to the partial pressure

Partial pressure

In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
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 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 potential

Chemical potential

In thermodynamics, physics and chemistry, chemical potential, symbolized by ?, is a term introduced by the American engineer, chemist and mathematical physicist Willard Gibbs, which he defined as follows:...
s of carbon dioxide in the two phases to be equal to each other. Equality of chemical potential defines chemical equilibrium

Chemical equilibrium

In a chemical process, chemical equilibrium is the state in which the Activity or concentrations of the reactants and products have no net change over time....
. Other constants for dynamic equilibrium involving phase changes include partition coefficient

Partition coefficient

In the fields of organic chemistry and medicinal chemistry, a partition or distribution coefficient is the ratio of concentrations of a chemical compound in the two phases of a mixture of two immiscible solvents at equilibrium....
and solubility product. Raoult's law

Raoult's law

Established by Fran?ois-Marie Raoult, Raoult's law states:Once the components in the solution have reached chemical equilibrium, the total vapor pressure p of the solution is:...
defines the equilibrium vapor pressure

Vapor pressure

Vapor pressure , is the pressure of a vapor in Thermodynamic equilibrium with its non-vapor Phase s. All liquids and solids have a tendency to evaporate to a gaseous form, and all gases have a tendency to Condensation back into their original form ....
of an ideal solution

Ideal solution

In chemistry, an ideal solution or ideal mixture is a solution in which the enthalpy of solution is zero; the closer to zero the enthalpy of solution is, the more "ideal" the behavior of the solution becomes....
.

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

Acetic acid

Acetic acid, CH3COOH, also known as ethanoic acid, is an organic acid which gives vinegar its sour taste and pungent smell. Pure, water-free acetic acid is a colourless liquid that absorbs water from the environment , and freezes at 16.7 Celsius to a colourless crystalline solid....
, in aqueous solution.

CH₃CO₂H CH₃CO₂^- + H⁺

At equilibrium the concentration quotient, K, the acid dissociation constant

Acid dissociation constant

An acid dissociation constant, Ka, is a quantitative measure of the strong acid in solution. It is the equilibrium constant for a chemical reaction known as Dissociation in the context of acid-base reactions....
, is constant (subject to some conditions) In this case, the forward reaction involves the liberation of some proton

Proton

The proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H+....
s 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

Stability constants of complexes

A stability constant is an equilibrium constant for the formation of a complex in solution. It is a measure of the strength of the interaction between the reagents that come together to form the Complex ....
.

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

Nitrogen dioxide

Nitrogen dioxide is the chemical compound with the chemical formula NitrogenOxygen2. One of several nitrogen oxides, NO2 is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year....
dimerizes.

2NO₂ N₂O₄;

Dynamic equilibrium

See also