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The Equilibrium Constant, Kc

Simple Explanation

The equilibrium constant, Kc, is a single number that quantifies exactly where an equilibrium sits — calculated from the equilibrium concentrations of products divided by reactants, each raised to the power of its coefficient in the balanced equation.

Why Do We Need It?

Kc turns a qualitative idea (equilibrium) into a precise, calculable quantity — letting chemists predict equilibrium concentrations, compare how far different reactions proceed, and design processes for maximum yield.

Formula

The Equilibrium Constant, Kc

For aA + bB ⇌ cC + dD: Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ

The ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of its coefficient in the balanced equation — a fixed value for a given reaction at a given temperature.

Kc: — the equilibrium constant (concentration-based), unitless in the way it is typically used here
[A], [B], [C], [D]: — equilibrium molar concentrations of each substance, in mol/L
a, b, c, d: — the balanced stoichiometric coefficients of each substance in the equation

When to use it: Whenever you need to quantify how far a reversible reaction proceeds toward products at equilibrium, or calculate an unknown equilibrium concentration.

Worked Example

Calculate Kc from equilibrium concentrations

For H₂(g) + I₂(g) ⇌ 2HI(g), at equilibrium [H₂] = 0.20 mol/L, [I₂] = 0.20 mol/L, and [HI] = 1.60 mol/L. Find Kc.

Why Does This Work?

At equilibrium, the ratio of product to reactant concentrations (in this specific form) settles to a fixed value for a given reaction at a given temperature — this happens because it is exactly the ratio at which the forward and reverse reaction rates become equal, so any system started from any initial concentrations will settle to concentrations satisfying this same ratio.

Real-Life Example

Predicting whether a reaction mixture is at equilibrium

A chemist mixes reactants and wants to know if the resulting mixture has reached equilibrium.

By measuring the actual concentrations and calculating the reaction quotient (using the same expression as Kc), the chemist can compare it to the known Kc value — if they match, the system is at equilibrium; if not, the reaction will continue shifting until they do.

Practice

For H₂(g) + I₂(g) ⇌ 2HI(g), equilibrium concentrations are [H₂] = 0.10 mol/L, [I₂] = 0.10 mol/L, [HI] = 0.80 mol/L. Find Kc.

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Common mistake

Forgetting to raise each concentration to the power of its coefficient in the balanced equation — for 2HI, the concentration of HI must be SQUARED, not just used as-is, in the Kc expression.

Quick Review

Kc = [products]^(coefficients) / [reactants]^(coefficients).
Kc has a fixed value for a given reaction at a given temperature.
Coefficients in the balanced equation become exponents in the Kc expression.