Copper (II) sulfate solution (c=0.1 mol l) (Xn), ammonia solution (c=2 mol/l) (Xi), concentrated sulfuric acid (C)
a) Add ammonia solution drop by drop to the copper (II) sulfate solution in a test tube until a light blue precipitate forms. Then add more ammonia solution until a clear solution forms.
b) Now add concentrated sulfuric acid (caution!).
c) Add more ammonia solution (caution!).
a) A deep purple-blue, clear solution forms from the originally light blue solution.
b) The solution turns light blue again.
c) A deep blue color appears again, as in a), but without precipitation
Many of the ligand exchange reactions are reversible processes. In many exchange reactions, an equilibrium is established, the position of which, depends strongly on the concentration of the ligands or the temperature, based on Le Chatelier’s principle.
Recall again the equilibrium described above between the light blue hexaaqua copper(II)complex and the purple tetraammine copper (II) complex:
[Cu(H2O)6]2+ + 4 NH3 <-> [Cu(NH3)4]2+ + 6 H2O
The solution with the ammine complex can be converted into the light blue aqua complex solution by adding acid because ammonia is continuously protonated. Ultimately the following reaction takes place:
[Cu(NH3)4]2+ + 4 H3O+ + 2 H2O <-> [Cu(H2O)6]2+ + 4 NH4+
This shifts the ligand exchange reaction to the side of the aqua complex.