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Calculating the Equilibrium Constant

The equilibrium constant (K) quantifies the ratio of product concentrations to reactant concentrations at equilibrium for a reversible reaction.

Key Concepts​

  • For a reaction: aA + bB ⇌ cC + dD
  • Equilibrium constant: K = [C]^c [D]^d / [A]^a [B]^b
  • Units of K depend on the reaction stoichiometry, but K is often treated as unitless
  • K >> 1: products favored
  • K << 1: reactants favored

Steps to Calculate K​

  1. Write the balanced chemical equation
  2. Determine equilibrium concentrations of all species
  3. Substitute concentrations into the K expression
  4. Solve for K

Example​

For N₂ + 3 H₂ ⇌ 2 NH₃, if [NH₃] = 0.50 M, [N₂] = 0.25 M, [H₂] = 0.75 M:
K = [NH₃]^2 / ([N₂][H₂]^3)
K = (0.50)^2 / (0.25 × 0.75^3) ≈ 1.78

Importance​

  • Predicts the extent of a reaction
  • Helps in reaction optimization and industrial chemical processes
  • Essential for understanding chemical equilibria

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