NCSU – Dept. of Chemistry – Lecture Demonstrations
Equilibrium
Le Chatelier’s Principle
Description: Le Chatelier’s principle is demonstrated by either invoking a color
change of anhydrous (blue) CoCl2 or hexahydrate (pink) CoCl2•6H2O OR by
observing the color change involved with the conversion of the chromate ion to
the dichromate ion by addition of acid or base.
Materials:
CoCl2 (anhydrous to hexahydrate)
0.05 M CoCl2
Conc. HCl
1 M AgNO3
Acetone
Evaporating dish or beaker
Glass stirring rod
Cr2O72- to CrO420.1 M K2Cr2O7
0.1 M K2CrO4
1 M NaOH
1 M HCl
Petri dishes
Glass stirring rod
Procedure:
For large lecture halls, project demonstration using a document camera.
CoCl2 (anhydrous to hexahydrate)
1. In an evaporating dish, add 50 mL of CoCl2 solution (pink). To this, add
HCl until the solution turns purple and then blue. The solution can be
converted back to the pink hexahydrate species with the addition of water.
At this point, the blue anhydrous species can again be regenerated by the
addition of acetone. In each case, an equal volume of reagent will be
required to shift the equilibrium.
2. In a separate evaporating dish, generate the blue anhydrous species with
the addition of HCl as described in step 1. Slowly add 1 M AgNO3 with
stirring. AgCl will precipitate out and the solution will turn pink.
NCSU – Dept. of Chemistry – Lecture Demonstrations
Equilibrium
Cr2O72- to CrO42Similar to the above demonstration, color changes are observed upon either
addition of NaOH to K2Cr2O7 (orange to yellow) or the addition of HCl to
K2CrO4 (yellow to orange).
Discussion: While the nature of the species in solution during the cobalt
demonstration is not fully known, this experiment serves as an excellent
demonstration of Le Chatelier’s principle. In dilute aqueous solutions the
dominant cobalt species is Co(H2O)62+. Replacement of the water molecules
with Cl1- ions eventually produces a cobalt species with at least two chloride ions
within the inner coordination sphere. It is believed that the transition from pink to
blue occurs upon changing from the hydrated CoCl1+ species to a hydrated CoCl2
species. The shift from hexahydrate to anhydrous is favored by an increase in
temperature (hence the use of concentrated HCl) as well as the presence of
organic solvents (acetone, 2-propanol, 1-butanol). If a 1:1 mixture of water and
2-propanol is used, less HCl is required to observe a change in color. The
addition of AgNO3 to the anhydrous species results in the formation of the Co2+
hexahydrate species. This is due to the abstraction of Cl1- ions in CoCl42- by Ag1+
and the subsequent precipitation of AgCl. The chemical equation used to
describe this equilibrium is commonly written as:
CoCl42-(aq) + 6 H2O (l)
[Co(H2O)6]2+(aq) + 4 Cl1-(aq)
For the chromate demonstration, CrO42- is in equilibrium with the
dichromate ion (Cr2O72-) as described by the following chemical equation:
2 CrO42- (aq) + 2 H1+ (aq)
Cr2O72- (aq) + H2O (l)
According to the above chemical equation, the equilibrium will shift to the
right with addition of acid to generate the orange dichromate ion in solution.
Likewise, addition of base to a dichromate solution will shift the equilibrium to the
left, producing the yellow color indicative of a high chromate concentration in
solution.
NCSU – Dept. of Chemistry – Lecture Demonstrations
Equilibrium
Safety: Wear proper protective equipment including gloves and safety glasses
when preparing and performing this demonstration. Concentrated solutions of
acids and bases (>2 M) can irritate the skin and cause burns. When diluting
concentrated acids, add acid to water to avoid spattering. Silver nitrate solutions
are irritating to the skin and eyes and may cause staining of the skin or clothing.
Both chromium and cobalt are toxic metals and should be handled with care.
Disposal: Individual samples can be neutralized slowly (i.e. add an acidic
sample slowly to a basic sample) and disposed in an appropriate aqueous waste
container.
References:
Shakhashiri, B. Z. In Chemical Demonstrations: A Handbook for Teachers of
Chemistry; The University of Wisconsin Press: 1983; Vol. 1, p 280-285; 250-270.
Summerlin, L. R.; Ealy, J. L. In Chemical Demonstrations: A Sourcebook for
Teachers; American Chemical Society: 1985; Vol. 1, p 32-33; 56-59.
Grant, A. W. J. Chem. Educ. 1984, 61, 466.
Martins, L. J. A. ; Barbosa da Costa, J. J. Chem. Educ. 1986, 63, 989.
Video:
http://www.youtube.com/watch?v=GS9kIj9n-BU (CoCl2)
http://www.youtube.com/watch?v=zP9qEiaL4kQ (chromate)