Equilibrium Constant Definition

The equilibrium constant is a ratio based on equilibrium concentrations.
The equilibrium constant is a ratio based on equilibrium concentrations. Rafe Swan, Getty Images
by Anne Marie Helmenstine, Ph.D.
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
Updated March 08, 2017

Equilibrium Constant Definition: The equilibrium constant is the ratio of the equilibrium concentrations of the products raised to the power of their stoichiometric coefficients to the equilibrium concentrations of the reactants raised to the power of their stoichiometric coefficients.

For a reversible reaction:

aA + bB → cC + dD

The equilibrium constant, K, is equal to:

K = [C]c·[D]d/[A]a·[B]b

where
[A] = equilibrium concentration of A
[B] = equilibrium concentration of B
[C] = equilibrium concentration of C
[D] = equilibrium concentration of D