Experiment: Le Chatelier’s Principle
Let's take the reaction
In an aqueous solution, the chromate ion, CrO42-(aq), can be converted to the dichromate ion, Cr2O72-(aq), and vice versa, the dichromate ion, Cr2O72-(aq)ion can be converted to the chromate ion, CrO42-(aq) ion.
2Na₂CrO₄ + H₂SO₄ = Na₂Cr₂O₇ + Na₂SO₄ + H₂O
chromate-dichromate (K2CrO4) equilibrium:
changes color from yellow to orange
2CrO42-+ 2H+↔H2O + Cr2O72-
dichromate-chromate (K2Cr2O7) equilibrium:
changes color from orange to yellow
Cr2O72-+ 2OH-↔H2O + 2CrO42-
Cr2O72-ions present with a small amount of CrO42-ions.Le Chatelier’s Principle states that if a stress is applied to a reaction at equilibrium, the reaction will shift to offset the stress applied. The addition of hydrogen ions or hydroxide ions constitutes a stress, as does the removal of either ion, and so it can be expected that the above equilibria will shift in response to concentration changes in these ions. For example, in eqn. 1, if [H+] is increased by adding HCl, there are more ions available for reaction and the reaction will favour the formation of products. Conversely, if [H+] is decreased (by adding OH-) there are fewer ions available for the forward reaction and the reaction shifts reverse to favor reactant formation.