12.10 The Effect of Temperature on the Co(H₂O)₆²⁺/CoCl₄^2- Equilibrium

Chemical Concepts Demonstrated: Equilibrium constants relative to temperature, color changes in transition metal complexes, LeChatelier's principle

Demonstration:

Co(H2O)62+ is formed by dissolving CoCl2*H2O in water. The first sample of the solution is heated to boiling. The first sample now contains a hot CoCl42- solution. AgNO3 is added to produce Co(H2O)62+ again. A second sample is used as a reference. HCl is added to a third sample at room temperature.

Observations:

    When the solution is heated to boiling, it turns from pink to blue.   This blue solution shifts back to pink as the AgNO₃ is added.  When HCl is added to a pink solution, it turns blue.

Explanations (including important chemical equation):

    The following equilibrium is observed:

    Co(H₂O)₆²⁺(aq) + 4 Cl^-(aq) <=> CoCl₄^2-(aq) + 6 H₂O(g)

    The Co(H₂O)₆²⁺ complex is pink, and the CoCl₄^2- complex is blue.

    This reaction is endothermic as written, so adding heat causes the equilibrium constant to shift to the right.  This, correspondingly, makes the solution blue.

    When the AgNO₃ is added, Cl^- is removed from solution.  This shifts the equation back to the left, and the solution turns pink again.

     When HCl is added, there is more Cl^- in solution, so the equilibrium is shifted to the right, and the solution turns blue.

    All of the above effects are variations of LeChatelier's principle.