Chemistry Reference
In-Depth Information
then the resulting salt solution will be acidic by hydrolysis and the pH of an
aqueous solution of the salt will be less than 7.
This can be demonstrated by considering the reactions that occur
when ammonium chloride is hydrolysed. The salt dissociates completely to
give hydrated ammonium ions and hydrated chloride ions. The Cl
ion is
not very reactive towards water, but the ammonium ions react with water to
give ammonium hydroxide. This is because NH
4
is the conjugate acid of
the weak base NH
3
, and must therefore be quite strong. The NH
4
reacts
with water as follows to produce H
3
O
ions:
NH
4
Cl
3
NH
4
Cl
H
3
O
An increase in the concentration of H
3
O
ions results in a fall in pH, and
an acidic solution.
The pH of this solution can be calculated by using the equation
derived for a weak acid, equation (1.2) above:
NH
4
H
2
O
3
NH
3
5
p
K
a
5
pH
log
c
If the salt results from the reaction of a strong base and weak acid (e.g.
sodium acetate from reaction of sodium hydroxide and acetic acid), then
the solution formed on hydrolysis will be basic, i.e.
NaOH
CH
3
COOH
3
CH
3
COO
Na
H
2
O
CH
3
COO
Na
H
2
O
3
CH
3
COOH
OH
Na
Na
does not react with water to any great extent, but CH
3
COO
is the
conjugate base of the weak acid CH
3
COOH and is therefore strong enough
to react with water to produce OH
ions.
The increase in concentration of OH
gives a basic solution, the pH
of which can be calculated from the equation for the pH of weak bases,
equation (1.3).
p
K
w
5
p
K
b
5
pH
log
c
or, if p
K
b
is replaced by the expression p
K
w
p
K
a
,
5
p
K
w
5
p
K
a
5
pH
log
c
which is probably the easiest form to remember.
The final scenario involves a salt formed between a weak acid and a
weak base (e.g. ammonium acetate, NH
4
CH
3
COO
). The H
and OH
ions formed by hydrolysis of ammonium acetate occur in roughly equal
concentrations, which will yield a neutral salt.
