Introduction to Chemical Equilibrium


A chemical specie will always exist in equilibrium with other forms of itself. The other forms may exist in undetectable amounts but they are always present. These other forms arise due to the natural disorder of nature that we call entropy (it's impossible to be perfect). As an example, pure water consists of the molecular compound and dissociated ions that exist together in equilibrium:
H2O(l) <=> H+(aq) + OH-(aq)
The (l) subscript refers to the liquid state, and the (aq) subscript refers to ions in aqueous solution.

Equilibrium Constant

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:

      [C]c [D]d
Keq = ---------
      [A]a [B]b
where the [] brackets indicate the concentration of the chemical species.

For the example of water, H2O <==> H+ + OH-, the equilibrium constant is:

      [H+] [OH-]
Keq = ----------
The concentration of water in a water solution is constant and this expression simplifies to:
Kw = (55.56 M)*Keq = [H+] [OH-]
where Kw is called the dissociation constant of water and equals 1.00x10-14 at room temperature. The concentrations of [H+] and [OH-] therefore equal 1.00x10-7 M.

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.


Zn (s) + 2 H+(aq) <==> Zn2+(aq) + H2 (g)

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

Examples of Equilibria

Further Information

/chem-ed/equilibr/equilibr.htm, updated 2/9/97

Copyright © 1997 by Brian M. Tissue, all rights reserved.

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