Chemistry Reference
In-Depth Information
4.2
Acidity and p
K
a
values
Or, put another way:
For the ionization of the acid HA in water
•
the smaller the value of p
K
a
, the stronger is the
acid;
K
•
the larger the value of p
K
a
, the weaker is the
acid.
H
2
O
+
A
H
3
O
+
A
the equilibrium constant
K
is given by the formula
12 to
52, but it must be appreciated right from the start that
a difference of one p
K
a
unit actually represents a 10-
fold difference in
K
a
and, thus, a 10-fold difference in
H
3
O
+
concentration. A twofold difference in acidity
would be indicated by a p
K
a
difference of just 0.3
units (log 2
We find that p
K
a
values range from about
−
[A
−
][H
3
O
+
]
[HA][H
2
O]
K
=
where [HA] signifies the concentration of HA, etc.
However, because the concentration of water is
essentially
0
.
3). Accordingly, a difference of
n
p
K
a
units indicates a 10
n
-fold difference in acidity,
so that the range
−
12 to 52 actually represents a
huge factor of 10
64
. A compound with p
K
a
<
5
is regarded as a reasonably strong acid, and those
with p
K
a
<
0 are very strong acids. At first glance,
negative p
K
a
values seem rather strange, but this
only means that the equilibrium lies heavily towards
ionization;
K
a
is large and, therefore, p
K
a
=−
=
constant
in
aqueous
solution,
a
new
equilibrium constant
K
a
is defined as
[A
−
][H
3
O
+
]
[HA]
K
a
=
K
a
is termed the
acidity constant
, and its magnitude
allows us to classify acids as
strong acids
(a
large value for
K
a
and, consequently, a high H
3
O
+
concentration) or
weak acids
(a small value for
K
a
and, thus, a low H
3
O
+
concentration). For example,
the strong acid HCl has
K
a
=
log
K
a
becomes negative.
10
7
. However, for
weak acids, the amount of ionization is much less
and, consequently, the value of
K
a
is rather small.
Thus, acetic acid CH
3
CO
2
Hhas
K
a
=
H
2
O A
H
3
O
A
+
+
10
−
5
.
To avoid using such small numbers as these,
K
a
is
usually expressed in the logarithmic form
p
K
a
where
1
.
76
×
p
K
a
= −log
10
K
a
K
a
= 0.01
K
a
= 0.1
K
a
= 1
K
a
= 10
K
a
= 100
p
K
a
=−
log
10
K
a
increasing acid strength
Accordingly, the p
K
a
for acetic acid is 4.75:
p
K
a
= 2
p
K
a
= 1
p
K
a
= 0
p
K
a
= −1p
K
a
= −2
10
−
5
)
=−
(
−
p
K
a
=−
log
(
1
.
76
×
4
.
75
)
=
4
.
75
As we use p
K
a
values, we shall find that, in most
cases, relative, rather than specific, values are all
we need to consider to help us predict chemical
behaviour and reactivity. Thus, from p
K
a
values, we
can see that acetic acid (p
K
a
4.75) is a weaker acid
than hydrochloric acid (p
K
a
−
The
p
K
a
for
hydrochloric
acid
can
similarly
be
calculated to be
−
7:
log
(
10
7
)
pK
a
=−
=−
7
7).
p
K
a
values for a wide variety of different com-
pounds are given in Tables 4.1 - 4.6. Compounds are
listed in order of increasing acidity. Although p
K
a
values included extend from about 52 to
This means there is an inverse relationship between
the strength of an acid and p
K
a
:
10, values
in the middle of the range are known most accurately.
This is because they can be measured readily in aque-
ous solution. Outside of the range from about 2 to 12,
p
K
a
values have to be determined in other solvents,
−
•
a strong acid has a large
K
a
and, thus, a small p
K
a
,
i.e. A
−
is favoured over HA;
•
a weak acid has a small
K
a
and, thus, a large p
K
a
,
i.e. HA is favoured over A
−
.
