Equilibrium Constant (Keq) Calculator

The equilibrium constant (Keq) describes the ratio of product concentrations to reactant concentrations at equilibrium.

For the reaction: aA + bB ⇌ cC + dD

Keq = Kc = [C]^c × [D]^d / ([A]^a × [B]^b)

All concentrations are in mol/L, measured at equilibrium.

Rules:

• Pure solids and pure liquids are excluded from the expression (their “concentration” is constant)
• Keq is temperature-dependent but pressure/concentration independent (at equilibrium)
• Large Keq (» 1): equilibrium favors products
• Small Keq (« 1): equilibrium favors reactants
• Keq ≈ 1: significant amounts of both reactants and products present

Kp for gas-phase reactions:

Kp = Kc × (RT)^Δn

Where Δn = (moles of gaseous products) − (moles of gaseous reactants). R = 0.08206 L·atm/mol·K.

Relationship to ΔG°:

ΔG° = −RT ln Keq

Temperature effect on Keq: Using van’t Hoff equation:

d(ln Keq)/dT = ΔH° / RT²

For exothermic reactions (ΔH° < 0): Keq decreases with temperature. For endothermic reactions (ΔH° > 0): Keq increases with temperature. (This is Le Chatelier’s principle in mathematical form.)

Examples:

• H₂(g) + I₂(g) ⇌ 2HI(g): Kc = 794 at 25°C (strongly favors HI)
• N₂(g) + 3H₂(g) ⇌ 2NH₃(g): Kc = 6 × 10⁵ at 25°C, but only 977 at 500°C (Haber process runs at 400-500°C for kinetics)