Chemistry Reference
In-Depth Information
Considering the reaction
A
B
3
C
D
the equilibrium constant (
K
) for the reaction is given by the product of the
concentrations of the reaction products divided by the product of the
concentrations of the reactants, or
[C]
[D]
K
----——
[A]
[B]
Clearly, if the equilibrium lies to the right-hand (or products) side, the
numerator in the above expression will be greater than the denominator,
and
K
will be greater than 1. Conversely, if the reaction does not proceed
very far and the equilibrium lies closer to the left-hand side, [A]
[B] will
be larger than [C]
[D] and
K
will be less than 1.
Strictly speaking, the law of mass action states that 'the rate of a
chemical reaction is proportional to the active masses of the reacting
substances', but for dilute solutions active mass may be replaced by
concentration, which is much easier to measure.
The law of mass action can be applied to the dissociation of water, a
weak electrolyte widely used as a solvent in biological and pharmaceutical
systems:
H
2
O
3
H
OH
The equilibrium constant for this reaction is given by
[H
]
[OH
]
———------
[H
2
O]
In pure water, and in dilute aqueous solutions, the concentration of
molecular water, [H
2
O] is so large as to be considered constant
(approximately 55.5
M
), so the above expression simplifies to
K
K
w
[H
]
[OH
]
(1.1)
and
K
w
is called the
ionic product
or
autoprotolysis constant
of water. The
value of this equilibrium varies with temperature but is usually quoted as
1
mole litre
1
, or
mole
2
litre
2
(also sometimes written as mole
2
dm
6
where 1 dm
3
10
14
at 25
C. The units of
K
w
are mole litre
1
1 litre).
[OH
], the hydrogen ion concentration
in water is given by the square root of
K
w
, which is 1
Since, in pure water, [H
]
10
7
mole litre
1
.
Solutions in which the hydrogen ion concentration is greater than
10
7
mole litre
1
are called acidic, while solutions with a concentration of
hydrogen ions less than 10
7
mole litre
1
are referred to as alkaline.
