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Chemistry LibreTexts

Equilibrium of Cobalt complexes

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Required Training

Required PPE

UC Lab Safety Fundamentals

Lab coat, safety glasses/goggles,

nitrile gloves

Equipment

Chemicals

Large test tube

~10 mL 0.2 M CoCl2 · 6H2O (MW: ~238)

DI water squirt bottle

Concentrated HCl in a drip top bottle

Hot water bath

Ice/salt bath

Procedure:

  1. Putting drops of conc. HCl will change the pink solution to blue
  2. Adding water changes the solution back to pink
  3. This reaction can have several color changes if given a large enough test tube.
  4. Heat will turn the solution blue, and the ice/salt bath will turn it pink.
  5. It is possible to turn only half of the test tube a color by carefully immersing it in a hot/ice bath with a steady hand.

Discussion:

This demonstration operates on the following reaction:

Co(H2O)62+aq + 4 Cl-aq (+ heat) → CoCl42-aq + 3 H2Ol

The blue form is anhydrous and tetrahedral coordinated, the pink form is hydrated and is octahedral coordinated. ΔH = ~42-54kJ/mol. Conc. HCl is not only a source of Cl- but also has a dehydrating effect. This demonstration projects well on an overhead, with a petri dish.

Hazards:

Be careful when using conc. HCl and make sure to wear all PPE. If you get any solution on your skin, be sure to wash for 15 minutes.

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Corrosive – Hydrochloric Acid

Disposal (by Storeroom)

Return the demonstration to the storeroom, where the cobalt waste will be submitted to EH&S.


Equilibrium of Cobalt complexes is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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