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.
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]bwhere the  brackets indicate the concentration of the chemical species.
For the example of water, H2O <==> H+ + OH-, the equilibrium constant is:
[H+] [OH-] Keq = ---------- [H2O]The concentration of water in a water solution is constant and this expression simplifies to:
Zn (s) + 2 H+(aq) <==> Zn2+(aq) + H2 (g)
PH2 [Zn2+] K = ----------- [H+]2
/chem-ed/equilibr/equilibr.htm, updated 2/9/97
Copyright © 1997 by Brian M. Tissue, all rights reserved.
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