Question

1. Write the chemical equation when excess chloride ion is added to aqueous solution of cobalt chloride. Note observed color of each complex ion. 2. What is the likely composition of the solution when the intermediate or transition color is reached? How does this provide visual proof of the idea that not all reaction go to completion?
3. Use LeChatelier's principle to explain the color changes observed upon addition of water and calcium chloride to an equilibrium mixture of the two complex ions in this reaction. 4. What was the effect of adding AgNO3 on the position of equilibrium for these two complex ions? Is this effect consistent with LeChatelier's Principle.

Answer 1

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)