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E-3
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
their coefficients. The term, Q, can take on any positive value between zero (all reactants and no products)
and infinity (all products and no reactants). If a reaction is at equilibrium, Q has a specific value
characteristic for that reaction, called the equilibrium constant (K). To reach equilibrium, reaction occurs
until Q equals K.
It should be noted that the terms used in the equilibrium constant are actually dimensionless quantities,
known as “activities”, which represent the effective concentrations of the ions or molecules in solution. In
dilute solutions, the activities are numerically equal to the molar concentrations of the species in question
and to the partial pressures, in atmospheres, for gaseous reactants and products. Pure solids and pure
liquids are assigned an activity of 1. For the dissociation of acetic acid considered above, the ratio reduces
to:
[H3O
+
][CH3COO
-
]
[CH3COOH]
Any stress applied to a reaction at equilibrium (such as the addition of CH3COO
-
to an acetic acid solution)
causes the value of Q to change so that it is no longer equal to K. Le Châtelier's Principle is simply a
qualitative statement of the idea that the direction of spontaneous reaction in a system that is not at
equilibrium is the direction that causes the value of Q to regain the value of K. Thus, a new position of
equilibrium will be attained so that Q
=
K.
