Equilibrium of pink and blue solution
name: The equilibrium between two coloured cobalt species in aqueous solution
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The two different colored Co(II) complex ions, (Co(H2O)6)2+ and (CoCl4)2-, exist together in equilibrium in the solution in the presence of chloride ions.
(Co(H2O)6)2+(aq)(pink) + 4Cl-(aq) ↔ (CoCl4)2-(aq)(blue) + 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, effect of concentration is when we add hydrochloric acid to solution, the equilibrium shifts to the right to compensate the excess of chloride ions. The result is the formation of (CoCl4)2-. When we add water, the equilibrium shifts to the left and the result is (Co(H2O)6)2+. These two complexes have different colors because they have different geometries and different ligands around the cobalt ion.
Temperature effect present because the reaction of (CoCl4)2- formation is endothermic.
(Co(H2O)6)2+(aq)(pink) + 4Cl-(aq) ↔ (CoCl4)2-(aq)(blue) + 6H2O - Q.
When we heat the solution, the equilibrium shifts to the right to compensate the excess of heat. The result is the formation of (CoCl4)2-. When we place the beaker in the ice, the equilibrium shifts to the left and the result is excess of (Co(H2O)6)2+.