Equilibrium Constant (Kc) Calculator
Calculate the equilibrium constant Kc for a reaction aA + bB ⇌ cC + dD from equilibrium concentrations and stoichiometric coefficients.
How it works
Kc = ([C]^c × [D]^d) ÷ ([A]^a × [B]^b) for aA + bB ⇌ cC + dD
The equilibrium constant Kc is the ratio of product concentrations to reactant concentrations at equilibrium, each raised to its stoichiometric coefficient. A large Kc means the position of equilibrium lies toward the products; a small Kc means reactants dominate. Kc depends only on temperature, not on the starting amounts.
Worked example
For A + B ⇌ C + D with equilibrium concentrations [A] = [B] = 0.5 M and [C] = [D] = 1.0 M: Kc = (1.0 × 1.0) ÷ (0.5 × 0.5) = 1.0 ÷ 0.25 = 4.0, so products are favoured.
Sources: Equilibrium constant — Wikipedia
Frequently asked questions
What does a large or small Kc tell me?
Kc ≫ 1 means products dominate at equilibrium; Kc ≪ 1 means reactants dominate; Kc near 1 means comparable amounts of both. It measures the position of equilibrium, not how fast it is reached.
Are pure solids and liquids included?
No. Pure solids and pure liquids have activities of 1 and are left out of the Kc expression. Enter only aqueous or gaseous species; set the coefficient of any omitted term to 0.
What is the difference between Kc and Kp?
Kc uses molar concentrations while Kp uses partial pressures for gaseous reactions. They are related by Kp = Kc(RT)^Δn, where Δn is the change in moles of gas.
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