Question

The questions below refer to the following system: Co(H2O)6 2+ + 4 Cl– CoCl4 2– + 6H2O pink blue

When cobalt(II) chloride is added to pure water, the Co2+ ions hydrate.

The hydrated form then reacts with the Cl– ions to set up the equilibrium shown here. 27.) What change will the system undergo if hydrochloric acid is added? (It should become more blue and the equilibrium will shift to the right.)

Answer 1

[Co(H₂O)₆] ²⁺ + 4 Cl^–   <---> [CoCl₄] ^2– + 6H₂O

    Pink                                           Blue

When Cl^- is added to a solution of [Co(H₂O)₆] ²⁺ , Cl^- ligands replace the H₂O ligands to form a tetra-coordinated [CoCl₄] ^2–. In this reaction entropy increases as total no of species is greater at the right side. This favors the reaction to proceed to the right side. But in the reaction 6 coordinate bonds are broken and 4 coordinate bonds are formed, thus the energy released cannot totally compensate the energy required to break the bonds. Moreover chloride ligands do not form very strong coordinate bonds. The reaction is thus endothermic.

In the presence of excess water the reaction thus proceed to the left to form [Co(H₂O)₆] ²⁺.

But in presence of concentrated Cl^- (when little water is present) the reaction proceed to the right to form [CoCl₄] ^2-, this is because of the Le Chatelier's principle. So when excess HCl is added to the system the reaction proceeds to the right to give a blue solution.