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Complexes
have an equilibrium
constant for their formation like any other chemical reaction. For example,
for the complex Cd(OH)4-2, the formation reaction is
- Cd+2(aq) + 4OH-(aq) ->
Cd(OH)4-2(aq)
and therefore the equilibrium
constant expression is
- K = [Cd(OH)4-2] /
[Cd+2][OH-]4
The equilibrium constant
is often called the formation constant for the reaction.
Example: What is the formation constant expression for the complex
Co(NH3)5NO2+2?
Solution: Just write out the formation reaction and then write the
equilibrium constant expression for that reaction. The formation reaction is
- Co+3(aq) + 5NH3(aq) + NO2-(aq)
->
Co(NH3)5NO2+2
Therefore,
the equilibrium constant expression (formation constant) is
- K = [Co(NH3)5NO2+2] /
([Co+3][NH3]5[NO2-])
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