Equilibrium Constant (Keq)
Calculate the ratio of products to reactants at chemical equilibrium.
Predicts which direction a reaction favors.
The Formula
Keq = [C]ᶜ × [D]ᵈ / ([A]ᵃ × [B]ᵇ)
For the reaction aA + bB ⇌ cC + dD, the equilibrium constant is the ratio of product concentrations to reactant concentrations, each raised to their stoichiometric coefficient.
Variables
| Symbol | Meaning |
|---|---|
| Keq | Equilibrium constant (unitless for activities, or with units for concentrations) |
| [A], [B] | Equilibrium concentrations of reactants (mol/L) |
| [C], [D] | Equilibrium concentrations of products (mol/L) |
| a, b, c, d | Stoichiometric coefficients |
Example 1
For N₂ + 3H₂ ⇌ 2NH₃: [N₂]=0.5, [H₂]=0.3, [NH₃]=0.2. Find Keq.
Keq = [NH₃]² / ([N₂] × [H₂]³)
Keq = (0.2)² / (0.5 × (0.3)³)
Keq = 0.04 / (0.5 × 0.027) = 0.04 / 0.0135
Keq ≈ 2.96
Example 2
For H₂ + I₂ ⇌ 2HI at 448°C, Keq = 50. If [H₂]=[I₂]=0.1 M, find [HI].
50 = [HI]² / (0.1 × 0.1)
[HI]² = 50 × 0.01 = 0.5
[HI] = √0.5 ≈ 0.707 M
When to Use It
Use the equilibrium constant when:
- Predicting whether a reaction favors products (Keq > 1) or reactants (Keq < 1)
- Calculating equilibrium concentrations from initial conditions
- Determining how changes in conditions shift equilibrium (Le Chatelier's principle)
- Comparing the extent of different reactions