When concentrated hydrochloric acid is added to the pink colored cobalt(II) chloride solution, the solution gradually turns blue. However, the pink color is reestablished by diluting the solution with water. The overall cycle of color changing can be repeated many times. This observation can be explained in light of the Le Chatelier’s principle, if we consider the equilibrium that exists in this chemical system.
Aqueous cobalt(II) chloride solution has a pink color, because six water molecules surround one cobalt(II) ion, thus forming octahedral hexaaquacobalt(II) complex, [Co(H2O)6]2+:
When we increase the chloride ions concentration by adding concentrated hydrochloric acid into the pink [Co(H2O)6]2+ solution, water molecules from the inner complex sphere are gradually displaced. The exact formula of the complex compound depends on concentration of chloride ions. When chloride ions concentration is sufficiently high, all water molecules are displaced and blue tetrachlorocobaltate(II) complex is formed. This [CoCl4]2- complex is doubly negatively charged and four chloride ions are arranged around cobalt(II) ion in form of a regular tetrahedron:
These two complexes have different colors because they have different geometries, as well as different ligands (water vs. chloride ions) around cobalt ion.
There is a complex equilibrium between these two complexes, but it is simplified in the equation bellow:
According to the Le Chatelier’s principle, if a chemical dynamic equilibrium is disturbed by changing the conditions (concentration, temperature, volume or pressure), the position of equilibrium moves to counteract the imposed change. So if more reactant is added, the equilibrium shifts to the right in order to consume that extra reactant, which results in more product.
In the case of aqueous cobalt(II) chloride solution, the equilibrium can be disturbed by changing chloride ion concentration. When we add hydrochloric acid, the equilibrium shifts to the right in order to consume extra chloride ions, and blue tetrachlorocobaltate(II) is formed. Contrary, adding more water leads to the blue complex decomposition and formation of pink hexaaquacobalt(II) complex.