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equilibrium, chemical
Condition in the course of a reversible chemical reaction in which no net change in the amounts of reactants and products occurs: Products are reverting to reactants at the same rate as reactants are forming products. For practical purposes, the reaction under those conditions is completed. Expressed in terms of the law of mass action, the reaction rate to form products is equal to the reaction rate to re-form reactants. The ratio of the reaction rate constants (i.e., of the amounts of reactants and products, each raised to the proper power), defines the equilibrium constant. Changing the conditions of temperature or pressure changes the reaction's equilibrium; a high temperature or pressure may be used to “push” a reaction that at ordinary conditions makes little product. See also H.-L. Le Châtelier.
Equilibrium, Chemical
the state of a system in which one or more reversible chemical reactions occur and in which the forward and reverse rates for each reaction are equal. The composition of the system therefore remains constant so long as the conditions of its existence are preserved. In the simplest case, given a homogeneous system in which a reversible chemical reaction
A + B ⇄ C + D
occurs, the rate of the forward reaction is proportional to the concentration of reactants:
v1 = k1[A][B]
The rate of the reverse reaction is proportional to the concentration of reaction products:
v2 = k2[C][D]
where k1 and k2 are the corresponding rate constants under the given conditions. At the initial moment, when [C] and [D] are equal to zero, v2 = 0 and v1 is determined by the initial concentrations of A and B. Rate vl decreases as A and B are consumed, while v2 increases as C and D are formed. Eventually, the rates become equal (v1 = v2); that is, chemical equilibrium is established. From the equality v1 = v2 it follows that
where [C], [D], [A], and [B] are the equilibrium concentrations of the substances and K is the equilibrium constant, whose value for each reversible reaction is dependent on external conditions. The relationship thus obtained expresses the law of mass action, which denotes the extent to which the original composition of a system can change through a spontaneous reaction under given conditions, that is, without any work being done on the system from without. Under conditions of chemical equilibrium, the concentrations (activity) of all the substances are interrelated; it is impossible to change one without changing all the others. The given expression for K holds for gaseous reactions at low pressures and for dilute solutions.
Thermodynamically, chemical equilibrium for both homogeneous and heterogeneous systems is defined as the most stable state under given conditions; that is, it is one in which, depending on the method of assigning external conditions, one or another thermodynamic function of the state reaches its minimum or maximum value. For isolated systems, those that exchange neither matter nor energy with the external medium, this function is called entropy. In the equilibrium state, the entropy is at its maximum. If heat exchange is possible with the surrounding medium but the temperature and pressure in the system are constant, then the isobaric-isothermal potential is at its minimum (see GIBBS FREE ENERGY). In the case of constant temperature and volume, the isochoric-isothermal potential will assume its minimum value (seeHELMHOLTZ FREE ENERGY).
The dependence of chemical equilibrium on external conditions is expressed qualitatively by the Le Châtelier-Braun principle and quantitatively by the corresponding thermodynamic equations. For example, the effect of temperature is expressed by the equations for the isobar or isochor of the reaction.
The study of chemical equilibrium has great theoretical and practical importance, expecially when applied to processes that occur in complex systems. The considerable difficulties encountered when experimental methods are used in the study of chemical reactions at elevated temperatures (high-temperature chemistry) have led to the development of calculations for equilibrium mixture compositions under given initial external conditions and given initial concentrations (or quantities) of components. In chemical technology, determining the position of chemical equilibrium at various pressures and temperatures and taking into account the reaction rates enable the selection of optimum process conditions, in particular, the conditions favoring the maximum yield of chemical products. The calculation of the initial composition of mixtures has become extremely important, as has that of the composition of quasi-equilibrium systems, in which one or more possible thermodynamic reactions practically do not occur or else proceed very slowly by virtue of their specific kinetic properties.
REFERENCES
Kurs fizicheskoi khimii, 2nd ed., vol. 1. Editor in chief, Ia. I. Gerasimov. Moscow, 1969.Termodinamicheskie i teplofizicheskie svoistva produktov sgoraniia. Spravochnik, vol. 1. Moscow, 1971.
M. E. ERLYKINA
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