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

Asked by  | 11th Feb, 2012, 12:44: PM

Expert Answer:

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.

Answered by  | 12th Feb, 2012, 09:48: AM

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