Equilibrium in cobalt(II) chloride solution

Equilibrium of pink and blue solution

Scientific name: The equilibrium between two coloured cobalt species in aqueous solution

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Equilibrium in cobalt(II) chloride solution

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Safety

Wear eye protection goggles. Be careful with the concentrated acid! Don't drink or eat cobalt compounds and cobalt-containing solutions, they are very toxic!

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

(Co(H2O)6)2+(aq)(pink) + 4Cl-(aq) ↔ (CoCl4)2-(aq)(blue) + 6H2O

Step-by-step instruction

  1. Take 4 g of solid cobalt(II) chloride-6-water and dissolve it in 40 ml of water in a 250 ml heat resistant beaker. You have a light pink solution.
  2. Put the beaker on a magnetic mixer, dip a stir bar into the solution and turn on the mixing and heating. Effect of concentration:
  3. Add about 60 mL of concentrated hydrochloric acid to the beaker. You have a violet or blue solution. The color of the mixture depends on the ration between the acid and cobalt(II) chloride.
  4. Add a small amount of water to the beaker to make the solution pink.
  5. Add a small amount of hydrochloric acid to the beaker to make the solution blue. Effect of temperature:
  6. Make the solution pink. Place the beaker in the hot water. The solution became blue.
  7. Place the beaker in the ice. The solution became pink.

Scientific background

The two different colored Co(II) complex ions, (Co(H2O)6)2+ and (CoCl4)2-, exist together in equilibrium in the solution in the presence of chloride ions.

(Co(H2O)6)2+(aq)(pink) + 4Cl-(aq) ↔ (CoCl4)2-(aq)(blue) + 6H2O.

Chemical equilibrium behaves according to the Le Chatelier’s principle. It can be stated as "When a system at equilibrium is subjected to change in concentration, temperature, volume, or pressure, then the system readjusts itself to (partially) counteract the effect of the applied change and a new equilibrium is established."

In this experiment, effect of concentration is when we add hydrochloric acid to solution, the equilibrium shifts to the right to compensate the excess of chloride ions. The result is the formation of (CoCl4)2-. When we add water, the equilibrium shifts to the left and the result is (Co(H2O)6)2+. These two complexes have different colors because they have different geometries and different ligands around the cobalt ion.

Temperature effect present because the reaction of (CoCl4)2- formation is endothermic.

(Co(H2O)6)2+(aq)(pink) + 4Cl-(aq) ↔ (CoCl4)2-(aq)(blue) + 6H2O - Q.

When we heat the solution, the equilibrium shifts to the right to compensate the excess of heat. The result is the formation of (CoCl4)2-. When we place the beaker in the ice, the equilibrium shifts to the left and the result is excess of (Co(H2O)6)2+.

Danger:
Coolness:
Difficulty:

Published on 13 April 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst