Question
Sat February 11, 2012 By:
 

An aq. pink colored solution of cobalt(II) chloride changes to deep blue on addition of excess of HCl. Why?

Expert Reply
Sun February 12, 2012
The element cobalt can form compounds in two different oxidation states, +2 and +3. The +2 state is more common. The ion Co2+ (aq) is pink. Other compounds of cobalt(II), which include both anhydrous Co2+ and complex ions, are commonly blue. 

If an aqueous solution contains both cobalt(II) and chloride ions, the blue ion CoCl42- forms, in equilibrium with the pink Co2+ (aq) ion.

               
               
    CoCl42- (aq) <===========> Co2+ (aq) + 4Cl1-(aq)  
      Blue   Pink    

At relatively low concentrations of chloride, the equilibrium lies far to the right, and the solution is pink. If there is a large concentration of excess chloride, the equilibrium tends to the left, and the solution tends to be blue.

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