Chemistry 112

Complex Ions

Complex ions are the result of Lewis acid/base reactions. Metal cations act as Lewis acids and many anions and species containing lone pairs can act as Lewis bases.

Lewis acid/base complexes can react with a variety of stoichiometries that are not easily predicted. Further, the reactions are equilibria so the extent of reaction can vary considerably.

In general:

Mn+(aq) + q L(aq) [MLq]n+(aq)

The mass action expression is

Kf is called the formation constant or stability constant for the complex ion.

Fortunately, the Table of Complex Ion Formation Constants also gives the stoichiometry of the complex ion, so we can use this information to establish stoichiometry.

Common Lewis bases that give complex ions are NH3, CN, OH, Cl, Br, and I.

Complex ion formation can have a strong effect on solubility.



Example

Calculate the molar solubility of zinc carbonate in pure water and in 1.0 M ammonia at 25 °C.


The solubility of ZnCO3 in pure water is a simple solubility problem:

 

ZnCO3(s)

Zn2+(aq)

+

CO32–(aq)

 

Ksp = [Zn2+]e[CO32–]e = 1.4×10–11

Initial

 

 

0

 

0

Change

 

 

+ x

 

+ x

Equilibrium

 

 

x

 

x

1.4×10–11 = [x][x] = x2

x = 3.7×10–6

The molar solubility of ZnCO3 in water is 3.7×10–6 M



The solubility in 1.0 M NH3 is a little more involved.

 

 

 

ZnCO3(s)

Zn2+(aq)

+

CO32–(aq)

 

Zn2+(aq)

+

4 NH3(aq)

Zn(NH3)42+(aq)

 

 

 

We add the two reactions to find:

 

ZnCO3(s)

+

4 NH3(aq)

Zn(NH3)42+(aq)

+

CO32–(aq)

 

From the law of multiple equilibria

Kc = Kf×Ksp = (4.1×108)×(1.4×10–11) = 5.7×10–3

Initial

 

 

1.0

 

0

 

0

Change

 

 

– 4x

 

+ x

 

+ x

Equilibrium

 

 

1.0 – 4x

 

x

 

x

Solving the quadratic equation gives

x = 4.9×10–2 or x = 1.3

The molar solubility of ZnCO3 in 1.0 NH3 is 4.9×10–2 M.

The solubility increased by a factor of 104 in the presence of the ammonia!