The Foundations of Chemistry

decrease in the concentration of the hydrated metal ion shifts the solubility equilibrium

to the right.

Many copper(II) compounds react with excess aqueous NH 3 to form the deep-blue

complex ion [Cu(NH 3 ) 4 ]^2 
.

Cu^2 
(aq)
4NH 3 (aq) 34 [Cu(NH 3 ) 4 ]^2 
(aq)

The dissociation of this complex ion is represented as

[Cu(NH 3 ) 4 ]^2 
(aq) 34 Cu^2 
(aq)
4NH 3 (aq)

Kd8.5 10 ^13

Recall that Cu^2 
(aq) is really a hydrated ion, [Cu(H 2 O) 6 ]^2 
. The preceding reaction and

its Kdexpression are represented more accurately as

[Cu(NH 3 ) 4 ]^2 

6H 2 O 34 [Cu(H 2 O) 6 ]^2 

4NH 3

Kd8.5 10 ^13

The more effectively a ligand competes with H 2 O for a coordination site on the metal

ions, the smaller Kdis. This tells us that in a comparison of complexes with the same

number of ligands, the smaller the Kdvalue, the more stable the complex ion. Some

complex ions and their dissociation constants, Kd, are listed in Appendix I.

Copper(II) hydroxide dissolves in an excess of aqueous NH 3 to form the deep-blue

complex ion [Cu(NH 3 ) 4 ]^2 
. This decreases the [Cu^2 
] so that [Cu^2 
][OH]^2 Ksp, and

so the Cu(OH) 2 dissolves.

Cu(OH) 2 (s) 34 Cu^2 
(aq) 
2OH(aq)

Cu^2 
(aq)
4NH 3 (aq) 34 [Cu(NH 3 ) 4 ]^2 
(aq)

overall rxn: Cu(OH) 2 (s)
4NH 3 (aq) 34 [Cu(NH 3 ) 4 ]^2 
(aq)
2OH(aq)

Similarly, Zn(OH) 2 dissolves in excess NH 3 to form [Zn(NH 3 ) 4 ]^2 
ions.

Zn(OH) 2 (s)
4NH 3 (aq) 34 [Zn(NH 3 ) 4 ]^2 
(aq)
2OH(aq)

Amphoteric hydroxides such as Zn(OH) 2 also dissolve in excess strong base by forming

complex ions (Section 10-6).

Zn(OH) 2 (s)
2OH(aq) 34 [Zn(OH) 4 ]^2 (aq)

We see that we are able to shift equilibria (in this case, dissolve Zn(OH) 2 ) by taking

advantage of complex ion formation.

[[Cu(H 2 O) 6 ]^2 
][NH 3 ]^4



[[Cu(NH 3 ) 4 ]^2 
]

[Cu^2
][NH 3 ]^4

As before, the outer brackets mean [[Cu(NH 3 ) 4 ] 2
]
molar concentrations. The inner
brackets are part of the formula of
the complex ion.

842 CHAPTER 20: Ionic Equilibria III: The Solubility Product Principle

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 19.12, Complex Ion Formation
and Solubility.

For brevity we shall omit H 2 O from
formulas of hydrated ions. For
example, we write [Cu(OH 2 ) 6 ]^2
as
Cu^2
.

Concentrated aqueous NH 3 was added slowlyto a solution of copper(II) sulfate, CuSO 4.

Unreacted blue copper(II) sulfate solution remains in the bottom part of the test tube.

The light-blue precipitate in the middle is copper(II) hydroxide, Cu(OH) 2. The top layer

contains deep-blue [Cu(NH 3 ) 4 ]^2 
ions that were formed as some Cu(OH) 2 dissolved in

excess aqueous NH 3.