equilibrium, chemical: see chemical equilibrium.

equilibrium, state of balance. When a body or a system is in equilibrium, there is no net tendency to change. In mechanics, equilibrium has to do with the forces acting on a body. When no force is acting to make a body move in a line, the body is in translational equilibrium; when no force is acting to make the body turn, the body is in rotational equilibrium. A body in equilibrium at rest is said to be in static equilibrium. However, a state of equilibrium does not mean that no forces act on the body, but only that the forces are balanced. For example, when a lever is being used to hold up a raised object, forces are being exerted downward on each end of the lever and upward on its fulcrum, but the upward and downward forces balance to maintain translational equilibrium, and the clockwise and counterclockwise moments of the forces on either end balance to maintain rotational equilibrium. The stability of a body is a measure of its ability to return to a position of equilibrium after being disturbed. It depends on the shape of the body and the location of its center of gravity (see center of mass). A body with a large flat base and a low center of gravity will be very stable, returning quickly to its position of equilibrium after being tipped. However, a body with a small base and high center of gravity will tend to topple if tipped and is thus less stable than the first body. A body balanced precariously on a point is in unstable equilibrium. Some bodies, such as a ball or a cone lying on its side, do not return to their original position of equilibrium when pushed, assuming instead a new position of equilibrium; these are said to be in neutral equilibrium. In thermodynamics, two bodies placed in contact with each other are said to be in thermal equilibrium when, after a sufficient length of time, their temperatures are equal. Chemical equilibrium refers to reversible chemical reactions in which the reactions involved are occurring in opposite directions at equal rates, so that no net change is observed.

chemical equilibrium, state of balance in which two opposing reversible chemical reactions proceed at constant equal rates with no net change in the system. For example, when hydrogen gas, H₂, and iodine gas, I₂, are mixed, and gaseous hydrogen iodide, HI, is formed according to the equation H₂ + I₂ → 2HI, no matter how long the reaction is allowed to proceed some quantity of hydrogen and iodine will remain unreacted. The reason reactants in a reversible reaction are never completely converted to product is that an opposing reaction is taking place simultaneously, i.e., some of the newly formed HI is being converted back into hydrogen and iodine. For any particular temperature, a point of equilibrium is reached at which the rates of the two opposing reactions are equal and there is no further change in the system. This equilibrium point is characterized by specific relative concentrations of reactants and products and will also be reached from the opposite direction, i.e., if one starts with hydrogen iodide and allows it to decompose into hydrogen and iodine. The equilibrium point can be described by the mass action expression, which defines the equilibrium constant, K_eq, in terms of the ratio of the molar concentrations of the products to those of the reactants. For the reversible reaction used as an example, the equilibrium constant is K_eq=[HI]²/[H₂][I₂]; for the general reversible reaction nA + mB + · · · ⇌ pC + qD + · · · , the equilibrium constant is:where [A], [B], [C], [D], … are the molar concentrations of the substances and n, m, p, q, … are the coefficients of the balanced chemical equation. The larger the equilibrium constant for a given reaction, the more the reaction is favored, since a larger value of K_eq means larger concentrations of the products relative to the reactants. The equilibrium constant is related to the change in the standard free energy, G°, of the system by the equation ΔG° = -RT. ln K_eq, where R is a constant, T is the temperature in degrees Kelvin, and ln K_eq is the natural logarithm of the equilibrium constant. Chemical equilibrium can be defined for many types of chemical processes, such as dissociation of a weak acid in solution, solubility of slightly soluble salts, and oxidation-reduction reactions. In all of these cases, the equilibrium constant or its analogue is defined for certain conditions of temperature and other factors. If any of these factors change, the system will respond to establish a new equilibrium, in accordance with Le Châtelier's principle.

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.

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Condition in which the net force acting on a particle is zero. A body in equilibrium experiences no acceleration and, unless disturbed by an outside force, will remain in equilibrium indefinitely. A stable equilibrium is one in which small, externally induced displacements from that state produce forces that tend to oppose the displacement and return the body to equilibrium. An unstable equilibrium is one in which the least departures produce forces tending to increase the displacement. A brick lying on the floor is in stable equilibrium, while a ball bearing balanced on a knife-edge is in unstable equilibrium.

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