Equilibrium Constant, K
The equilibrium between reactants and products is described by an equilibrium constant. For the balanced reaction:
aA + bB cC + dD
The equilibrium constant, Keq is defined as:
Keq = ---------
where the  brackets indicate the concentration of the chemical species.
Rules for Writing K Expressions
- Products are always in the numerator.
- Reactants are always in the denominator.
- Express gas concentrations as partial pressure, P, and dissolved species in molar concentration, .
- The partial pressures or concentrations are raised to the power of the stoichiometric coefficient for the balanced reaction.
- 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.
Zn (s) + 2 H+(aq) Zn2+(aq) + H2 (g)
K = -----------
Specific Equilibrium Constants
The equilbrium constant has specific names for several classes of reactions:
- Gas-phase reactions that use units of partial pressure: Kp
- Dissociation of water: dissociation constant of water, Kw
- Dissociation of acids: acid dissociation constant, Ka
- Reaction of bases with water: base hydrolysis constant, Kb
- Solubility of precipitates: solubility product, Ksp
- Formation of complexes: formation constant, Kf
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.
Copyright © 2000 by Brian M. Tissue, all rights reserved.