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E-2
If we now add more acetate ions (CH3COO
-
(aq)
) to a solution of acetic acid, Le Châtelier's Principle predicts
that the system will respond in a way that counteracts the disturbance or stress (the increased [CH3COO
-
(aq)
]), hence, some CH3COO
-
(aq)
and H3O
+
(aq)
will react to form more CH3COOH
(aq)
and H2O
(l)
. The result is
that a new position of equilibrium is attained that lies further to the left than in the initial solution of acetic
acid. The concentrations of all species adjust in such a way, that the ratio:
[H3O
+
][CH3COO
-
]
[CH3COOH][H2O]
has the same value it had before addition of the acetate ion. As usual, the square brackets are used to
indicate “the molar concentration of”.
If we add more acetic acid to the aqueous solution of acetic acid described in equation (1), Le Châtelier's
Principle predicts that some CH3COOH
(aq)
will react with H2O
(l)
to form more CH3COO
-
(aq)
and H3O
+
(aq)
.
A new position of equilibrium, to the right of the original equilibrium, will be reached as the system
counteracts the stress of the added acetic acid. The net changes in concentrations of all species are such
that the ratio:
[H3O
+
][CH3COO
-
]
[CH3COOH][H2O]
has the same value it had before addition of the CH3COOH.
This simplified ratio can be used to characterize the state of the reaction described in equation (1) above.
The following generalizations relating Le Châtelier's Principle to equilibrium can be derived from this
example. (Assume temperature and volume remain constant.)
1.
The position of equilibrium shifts to the right if the concentration or partial pressure of the reactants
is increased or if the concentration or partial pressure of the products is decreased.
2.
The position of equilibrium shifts to the left if the concentration or partial pressure of the products is
increased or if the concentration or partial pressure of the reactants is decreased.
3.
The equilibrium position shifts, by changing the concentrations or partial pressures of all species,
until the ratio of products to reactants is restored to its original equilibrium value.
