E720: Effect of temperature - [Co(H2O)6]^2+/[CoCl4]^2-

Two different colored cobalt(II) complexes exist in equilibrium, [Co(H2O)6]2+ (pink) and [CoCl4]2- (blue).  This equilibrium may be disturbed by changing temperature - when placed in a cold bath, the solution will turn pink, on a hot plate, the solution will turn blue.

TO CONDUCT DEMONSTRATION:

Two erlenmeyer flasks are provided containing aqueous solution of cobalt(II) and chloride ion, solution should be violet (approximately equal amounts of [Co(H2O)6]2+ (pink) and [CoCl4]2- (blue).  Place one flask into a cold bath, and the other in a hot bath.  The flask in the cold bath will turn pink, the flask in the hot bath will turn blue.  You can show that this is reversible by swapping the flasks.

EQUATIONS:

[Co(H2O)6]2+ (aq) (pink) + 4 Cl- (aq) <--> [CoCl4]2- (aq) (blue) +6 H2O (l)

As written this reaction is endothermic.

SAFETY AND DISPOSAL:

Care should be taken when handling a hot flask or hot plate.  The flasks will be reused, so the solution should not be disposed of.

PRIOR TO LECTURE:

Supply the following:

  1. Cold bath - add ice to cold water in a large beaker or crystallization dish
  2. Hot bath - place a large beaker or crystallization dish partially full of water on a hot plate.
  3. Two flasks of cobalt solution.
  4. Tongs or heat-protective gloves to handle the hot flask.
  5. Goggles.

REFERENCE:

Royal Chemical Society - LearnChemistry

ACKNOWLEDGEMENTS:

Susan Hendrickson, Laurel Hyde Boni, and Kristin Boles, Spring 2017