Equilibrium Constant, K

Introduction

The equilibrium between reactants and products is described by an equilibrium constant. For the balanced reaction:
aA + bB cC + dD
The equilibrium constant, K_eq is defined as:

      [C]c [D]d
Keq = ---------
      [A]a [B]b

Rules for Writing K Expressions

1. Products are always in the numerator.
2. Reactants are always in the denominator.
3. Express gas concentrations as partial pressure, P, and dissolved species in molar concentration, [].
4. The partial pressures or concentrations are raised to the power of the stoichiometric coefficient for the balanced reaction.
5. Leave out pure solids or liquids and any solvent. Only variables will be in a K expression: partial pressure of gases and concentrations of solutes in solution.

Example:

Zn _(s) + 2 H⁺_(aq) Zn²⁺_(aq) + H_{2 (g)}

     PH2 [Zn2+]
K = -----------
       [H+]2

Specific Equilibrium Constants

The equilbrium constant has specific names for several classes of reactions:

1. Gas-phase reactions that use units of partial pressure: K_p
2. Dissociation of water: dissociation constant of water, K_w
3. Dissociation of acids: acid dissociation constant, K_a
4. Reaction of bases with water: base hydrolysis constant, K_b
5. Solubility of precipitates: solubility product, K_sp
6. Formation of complexes: formation constant, K_f

Each of these classifications of reactions will have a convention for how to write the direction of the reaction. The different conventions are illustrated in the introduction to reactions document.