Equilibrium of blue and green solution
name: The equilibrium between two colored copper species in aqueous solution
The two different colored Cu(II) complex ions, (Cu(H2O)6)2+ and (CuCl4)2-, exist together in equilibrium in the solution in the presence of chloride ions.
(Cu(H2O)6)2+(aq)(blue) + 4Cl-(aq) ↔ (CuCl4)2-(aq)(green) + 6H2O.
Chemical equilibrium behaves according to the Le Chatelier’s principle. It can be stated as "When a system at equilibrium is subjected to change in concentration, temperature, volume, or pressure, then the system readjusts itself to (partially) counteract the effect of the applied change and a new equilibrium is established."
In this experiment, when we add hydrochloric acid to solution, the equilibrium shifts to the right to compensate for the excess of chloride ions. The result is the formation of (CuCl4)2-. When we add water, the equilibrium shifts to the left and the result is (Cu(H2O)6)2+. These two complexes have different colors because they have different geometries and different ligands around cobalt ion.