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E-4
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.
As an example, consider solubility of the metal salt, MX2 in water:
MX
2(s)
M
2+
(aq)
+ 2X
-
(aq)
(3)
The reaction quotient of this reaction is Q =
[M
2+
][X
-
]²
[MX2]
. The concentration of a solid is also known as the
density. Since the activity of a solid is assigned a value of 1 and does not change, it does not appear in the
reaction quotient expression. It too is incorporated into this expression. This simplifies the reaction
quotient for this reaction to Q = [M
2+
][X
-
]².
The equilibrium constant for this type of reaction is called the solubility product constant and for
equation (3) is defined by following equation:
K = K
sp
= [M
2+
][X
-
]²
(4)
