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E-2
a new position of equilibrium is attained that lies further to the left(i.e. conversion of products to reactants)
than in the initial (nonequilibrium or stressed) 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(i.e. conversion of reactants to products) than in the initial
(nonequilibrium or stressed) 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 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.
Part II. Reaction Quotient and Equilibrium
We can extend Le Châtelier's Principle to characterize a general reaction in terms of the reaction quotient,
Q. A general equation may be written as follows: a A
(aq)
+ b B
(aq)
+ …
c C
(aq)
+ d D
(aq)
+ …
The reaction quotient, Q, for dilute solutions is defined as the ratio:
Q =
[C]© [D]
d
...
[A]
a
[B]
b
...
where a, b, c, d, ... are stoichiometric coefficients. The numerator is obtained by multiplying together the
concentrations of the products, each raised to a power equal to its stoichiometric coefficient in the balanced
equation. The denominator is obtained in the same manner using the concentrations of the reactants and
