Equilibrium Constant (K)
Compute K from the equilibrium concentrations and coefficients of a reversible reaction.
aA + bB ⇌ cC + dD
Leave a concentration or coefficient at 0 to drop that slot (it counts as a factor of 1).
Concentration-based Kc. K is dimensionless by the standard-state convention.
For the reaction aA + bB ⇌ cC + dD, the equilibrium constant is K = ([C]ᶜ · [D]ᵈ) ÷ ([A]ᵃ · [B]ᵇ), using equilibrium concentrations in mol/L. With [A] = [B] = 0.5 mol/L, [C] = [D] = 1.0 mol/L and all coefficients 1, K = (1 · 1) ÷ (0.5 · 0.5) = 4.
What the equilibrium constant tells you
At equilibrium a reversible reaction stops changing on the macroscopic scale: the forward and reverse rates are equal and the concentrations hold steady. The equilibrium constant K captures that steady balance as a single number — the ratio of product concentrations to reactant concentrations, each raised to its stoichiometric coefficient. A large K means the position of equilibrium lies far toward the products; a small K means reactants dominate.
Concentrations in mol/L at equilibrium; exponents are the balanced coefficients
Worked example
A reaction A + B ⇌ C + D reaches equilibrium with [A] = [B] = 0.5 mol/L and [C] = [D] = 1.0 mol/L. All coefficients are 1.
- 1 Write the balanced equation. A + B ⇌ C + D, so every coefficient (a, b, c, d) is 1.
- 2 Build the K expression. Products over reactants, each raised to its coefficient: K = ([C]¹ · [D]¹) ÷ ([A]¹ · [B]¹).
- 3 Substitute equilibrium concentrations. K = (1.0 · 1.0) ÷ (0.5 · 0.5) = 1 ÷ 0.25.
- 4 Evaluate. K = 4. Because K > 1, the equilibrium favors the products.
Reading the value of K
K compares products to reactants at equilibrium; the thresholds are guidelines, not hard cut-offs.
| Value of K | Position of equilibrium | Interpretation |
|---|---|---|
| K ≫ 1 | Far toward products | Reaction goes nearly to completion |
| K ≈ 1 | Balanced | Comparable amounts of reactants and products |
| K ≪ 1 | Far toward reactants | Little product forms |
Kc, Kp, and what gets left out
Kc vs Kp. This calculator computes Kc from molar concentrations. For gas-phase reactions you can instead use Kp, built from partial pressures; the two are related by Kp = Kc(RT)Δn, where Δn is the change in moles of gas.
Pure solids and liquids are omitted. Their “concentration” doesn’t change as the reaction proceeds, so by convention they’re assigned an activity of 1 and dropped from the expression — only gases and dissolved (aqueous) species appear.
Units convention. Strictly, K is built from dimensionless activities (each concentration divided by the 1 mol/L standard state), so K itself carries no units. The calculator follows this convention and reports K as a pure number.