The equilibrium constant (denoted as Keq) is the constant given by the reaction quotient when a system has reached equilibrium. For an equilibrium given by

$ \alpha A +\beta B ... \rightleftharpoons \gamma C + \delta D ... $

the reaction quotient, and therefore the equilibrium constant when the reaction is at equilibrium, is equal to

$ \text{K}_\text{eq}=\frac{{[C]} ^\gamma {[D]}^\delta ... } {{[A]}^\alpha {[B]}^\beta ...} $

Given that Keq relies on concentration (given in mol/litre, it can only be used for measuring gases and solutions. Because of this, it is often used for measuring the strength of acids and bases (the acid dissociation constant) and solubility (the solubility product constant).

For any given equilibrium, Keq will only change with a change in temperature.

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