Department of Chemistry and Biochemistry

Chloro Complexes of Cobalt(II)

Materials

For Procedure 1:
    0.1M  solution of cobalt(II) chloride hexahydrate (CoCl₂ ^. 6H₂O)
                                12M   hydrochloric acid (HCl)
                                rinse bottle of deionized water
                                disposable pipettes
                                crystallizing dish

For Procedure 2:    large test tube, boiling chip, gas burner & ice bath

Procedures (choose one or both)

1. Pour enough cobalt solution to cover the bottom of a crystallizing dish. Add conc. HCl dropwise until solution turns blue. After solution turns blue, add distilled water and mix until the solution turns pink. Repeat cycle.
2. Before class starts, add 12M HCl to cobalt solution until it just turns blue. Add just enough water until the cobalt solution becomes pink again. Pour solution into a big fat test tube, add a boiling chip, and heat the tube with a torch or burner near the top of the tube. Heating at the bottom makes the solution jump out of the tube. When solution starts boiling, it will turn blue. Submerge the bottom half of the tube in an ice water solution, bottom half will be pink, top half will be blue.

Reactions

The reaction responsible for the color change is:

CoCl(H2O)5+(aq) + Cl-	<--->	CoCl2(H2O)2 (aq) + 3H2O
Octahedral (pink)		Tetrahedral (blue)

Evidence exists that the shift to tetrahedral coordination and the blue color occurs when the second chloride ion enters the coordination sphere.

In the totally aqueous system where the color change is induced by the addition of concentrated HCl, the relationship has been reported to be:

Octahedral (pink)		Tetrahedral(blue)
Co(H2O)62+		CoCl2(H2O)2
CoCl(H2O)5+		CoCl3(H2O)-
		CoCl42-

Waste Management:

The waste solutions from this demonstration should be labeled for pickup by EH&S (Toxic, Corrosive).

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Last Updated: 5/23/02