Chemistry Reference
In-Depth Information
'million times as strong'. The answer, obvious to anyone who is familiar
with logarithms, is that chloroacetic is 100 times stronger than acetic acid.
This is because the difference in p
K
a
is two units on a log scale, and the
antilog of 2 to the base 10 is 100. It is important for students (and gradu-
ates!) to appreciate that pH and p
K
a
are
logarithmic
relationships and that
a
K
value corresponding to a p
K
a
of 2.7 is not really close to a K value
corresponding to a p
K
a
of 4.7.
Equation (1.4) can be rewritten in a logarithmic form by taking the
negative logarithm of both sides, to give
p
K
a
p
K
b
p
K
w
14
(1.5)
In addition, since p
K
b
may be rewritten as p
K
w
p
K
a
, this allows equation
(1.3) to be rewritten omitting any reference to p
K
b
as
pH
5
p
K
w
5
p
K
a
5
log
c
or alternatively
5
pH
(p
K
w
p
K
a
log
c
)
Hydrolysis of salts
When a salt is dissolved in water, the compound dissociates completely to
give solvated anions and cations. This breaking of bonds by the action of
water is called
hydrolysis
and the salt is said to be
hydrolysed
.
The pH of the resulting solution depends on whether the salt was
formed from reaction of strong or weak acids and bases and there are four
possible combinations.
For example, if the salt results from reaction between a strong acid
and a strong base (e.g. NaCl), then the resulting solution will be neutral,
and NaCl is termed a neutral salt. Of the two ions produced, Na
and Cl
,
only the Cl
reacts with water:
Cl
H
2
O
3
HCl
OH
This reaction does not occur to any great extent since the Cl
is the conju-
gate base of a strong acid, namely HCl. The Cl
is therefore a very weak
conjugate base and its reaction with water can be neglected.
If the salt results from reaction between a strong acid and a weak base
(e.g. the reaction of ammonia and hydrogen chloride to give ammonium
chloride),
HCl
NH
3
3
NH
4
Cl
