Equilibrium Constant

The equilibrium constant (Keq) is a numerical value that describes the ratio of product concentrations to reactant concentrations once a reversible reaction has reached chemical equilibrium — the point where the forward and reverse reaction rates are equal and concentrations stop changing. Keq is calculated by raising each product and reactant concentration to the power of its coefficient in the balanced equation, then dividing the product terms by the reactant terms.

A Keq value much greater than 1 indicates the reaction strongly favors products at equilibrium, while a value much less than 1 indicates the reactants are favored and the reaction barely proceeds forward. Crucially, Keq is constant for a given reaction at a fixed temperature — it does not change no matter what starting concentrations you use, only the position at which equilibrium is reached changes.

For gas-phase reactions, chemists often use Kp, the equilibrium constant expressed in terms of partial pressures rather than concentrations. The Equilibrium Constant Calculator and Kp Calculator both compute these values, and can convert between Kc and Kp when the change in moles of gas is known.

For a general reaction aA + bB ⇌ cC + dD:

Keq = [C]^c × [D]^d / ([A]^a × [B]^b)

Where square brackets denote equilibrium molar concentrations, and a, b, c, d are the stoichiometric coefficients from the balanced equation.

Keq is also related to Gibbs free energy by:

ΔG° = -RT × ln(Keq)

Consider the reaction: N2(g) + 3H2(g) ⇌ 2NH3(g)

At equilibrium, suppose [N2] = 0.5 M, [H2] = 1.0 M, and [NH3] = 0.2 M.

Keq = [NH3]^2 / ([N2] × [H2]^3) = (0.2)^2 / (0.5 × 1.0^3) = 0.04 / 0.5 = 0.08

Since Keq is less than 1, this particular equilibrium mixture favors the reactants, meaning relatively little ammonia has formed compared to the nitrogen and hydrogen present. Check similar setups with the Equilibrium Constant Calculator.

• Keq only depends on temperature: Unlike concentrations, which shift with the amounts of reactants added, Keq itself stays fixed for a reaction unless the temperature changes.
• It connects directly to Gibbs Free Energy: A large Keq corresponds to a strongly negative standard Gibbs free energy change, meaning the reaction is thermodynamically favorable toward products.
• Kc and Kp serve different reaction types: Kc is used for concentration-based equilibria in solution, while Kp is used for gas-phase equilibria expressed in partial pressures, and the two can be converted using the ideal gas law.
• Equilibrium constants underlie acid-base chemistry: The acid dissociation constant (Ka) and base dissociation constant (Kb) are specialized equilibrium constants that directly determine a solution's pH.
• A Keq of 1 means neither side is favored: When Keq equals 1, products and reactants are present in comparable proportions at equilibrium, with neither direction strongly dominant.