Chemistry » Fundamental Equilibrium Concepts » Equilibrium Constants

Summarizing Equilibrium Constants

Key Concepts and Summary

For any reaction that is at equilibrium, the reaction quotient Q is equal to the equilibrium constant K for the reaction. If a reactant or product is a pure solid, a pure liquid, or the solvent in a dilute solution, the concentration of this component does not appear in the expression for the equilibrium constant. At equilibrium, the values of the concentrations of the reactants and products are constant. Their particular values may vary depending on conditions, but the value of the reaction quotient will always equal K (Kc when using concentrations or KP when using partial pressures).

A homogeneous equilibrium is an equilibrium in which all components are in the same phase. A heterogeneous equilibrium is an equilibrium in which components are in two or more phases. We can decide whether a reaction is at equilibrium by comparing the reaction quotient with the equilibrium constant for the reaction.

Key Equations

  • \(Q=\phantom{\rule{0.2em}{0ex}}\cfrac{\left[\text{C}\right]^{x}{\left[\text{D}\right]}^{y}}{\left[\text{A}\right]^{m}{\left[\text{B}\right]}^{n}}\phantom{\rule{5em}{0ex}}\text{where}\phantom{\rule{0.2em}{0ex}}m\text{A}+n\text{B}⇌x\text{C}+y\text{D}\)

  • \({Q}_{P}=\phantom{\rule{0.2em}{0ex}}\cfrac{{\left({P}_{C}\right)}^{x}{\left({P}_{D}\right)}^{y}}{{\left({P}_{A}\right)}^{m}{\left({P}_{B}\right)}^{n}}\phantom{\rule{5em}{0ex}}\text{where}\phantom{\rule{0.2em}{0ex}}m\text{A}+n\text{B}⇌x\text{C}+y\text{D}\)

  • P = MRT

  • KP = Kc (RT)Δn


equilibrium constant (K)

value of the reaction quotient for a system at equilibrium

heterogeneous equilibria

equilibria between reactants and products in different phases

homogeneous equilibria

equilibria within a single phase


equilibrium constant for reactions based on concentrations of reactants and products


equilibrium constant for gas-phase reactions based on partial pressures of reactants and products

law of mass action

when a reversible reaction has attained equilibrium at a given temperature, the reaction quotient remains constant

reaction quotient (Q)

ratio of the product of molar concentrations (or pressures) of the products to that of the reactants, each concentration (or pressure) being raised to the power equal to the coefficient in the equation

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