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E-3
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.
The generalizations, derived from the acetic acid example, can now be re-written in terms of Q:
1.
If a disturbance (stress) makes the value of Q less than the value of K (more reactants or less
products than required for equilibrium), the position of equilibrium shifts to the right,
i.e., the reaction proceeds to form more products until Q = K.
2.
If a disturbance (stress) makes the value of Q greater than the value of K (less reactants or more
products than required for equilibrium), the position of equilibrium shifts to the left; i.e., the reaction
proceeds to form more reactants until Q = K.
3.
If an equilibrium has been disturbed, the value of Q will change until it again equals K.
Some equilibrium constants are so widely used, that they have specific names. For example, the
equilibrium constant for the dissociation of an acid in water is called the acid dissociation constant (K
a
).
For acetic acid its value is 1.76
x 10
-5
:
K
a
=
[H3O
+
][CH3COO
-
]
[CH3COOH]
= 1.76 x 10
-5
(2)
(Note that [H2O] is incorporated into the value of K
a
for dilute solutions as explained earlier.)
Another kind of equilibrium constant is used to describe the solubility of slightly soluble compounds. The
solubility of a compound is the amount of solid that dissolves in a given volume of solution. The solubility
is temperature dependent.
