As a single equation with three variables, equation 6.26 does not have a unique solution
for the concentrations of CH3COOH, CH3COO–, and H3O+. At constant
temperature, different solutions of acetic acid may have different values for [H3O+],
[CH3COO–] and [CH3COOH], but will always have the same value of Ka.
If a solution of acetic acid at equilibrium is disturbed by adding sodium acetate,
the [CH3COO–] increases, suggesting an apparent increase in the value of Ka. Since
Ka must remain constant, however, the concentration of all three species in equation
6.26 must change in a fashion that restores Ka to its original value. In this case,
equilibrium is reestablished by the partial reaction of CH3COO– and H3O+ to produce
additional CH3COOH.
The observation that a system at equilibrium responds to a stress by reequilibrating
in a manner that diminishes the stress, is formalized as Le Châtelier’s principle.
One of the most common stresses that we can apply to a reaction at equilibrium
is to change the concentration of a reactant or product. We already have seen,
in the case of sodium acetate and acetic acid, that adding a product to a reaction
mixture at equilibrium converts a portion of the products to reactants. In this instance,
we disturb the equilibrium by adding a product, and the stress is diminished
by partially reacting the excess product. Adding acetic acid has the opposite effect,
partially converting the excess acetic acid to acetate.
In our first example, the stress to the equilibrium was applied directly. It is also
possible to apply a concentration stress indirectly. Consider, for example, the following
solubility equilibrium involving AgCl