# equilibrium constant

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## e·qui·lib·ri·um con·stant (*K*_{eq}),

in the reaction A + B ⇄ C + D at equilibrium (that is, no net change in concentrations of A, B, C, or D), the concentrations of the four components are related by the equation

See also: Henderson-Hasselbalch equation, mass:action ratio.

*K*_{eq}= [C][D]/[A][B];*K*_{eq}is the equilibrium constant If any component in the reaction has a multiplier (for example, H_{2}⇄ 2H), that multiplier appears as an exponent in the calculation of*K*(for example,*K*_{eq}= [H]^{2}/[H_{2}]). When this equation is applied to the ionization of a substance in solution,*K*_{eq}is called the dissociation constant (*K*_{d}) and its negative logarithm (base 10) is the p*K*_{d}.See also: Henderson-Hasselbalch equation, mass:action ratio.

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