Chemistry Reference
In-Depth Information
It was not long after this that the 'hydro-halic' acids
(HCl, HBr, and so on) were discovered and they
had no oxygen present in them. This produced a
modified theory in 1810 when the English chemist
Sir Humphry Davy (1778-1829) suggested that all
acids contain hydrogen as the important element;
however, it was pointed out that there were many
hydrogen-containing substances that were not acids.
The German chemist Justus von Liebig (Figure 8.7)
then made the next useful proposal about acids
(1838) when he suggested that acids were substances
that can react with metals to produce hydrogen gas.
These ideas were rather limiting since they only
applied to aqueous solutions. There were situations
where acid-base reactions were taking place in
solvents other than water, or even in no solvent
at all. This problem was addressed in 1923 by the
Danish chemist Johannes Brønsted (1879-1947)
and the English chemist Thomas Lowry (1874-
1936) when they independently proposed a more
general definition of acids and bases, and the study
of acids and bases took a great step forward. This
theory became known as the
Brønsted-Lowry
theory of acids and bases.
The Brønsted-Lowry theory
This theory defined:
•
an acid as an H
+
ion (or proton) donor
•
a base as an H
+
ion (or proton) acceptor.
The theory explains why a pure acid behaves
differently from its aqueous solution, since for an
acid to behave as an H
+
ion donor it must have
another substance present to accept the H
+
ion. So
the water, in the aqueous acid solution, is behaving
as a Brønsted-Lowry base and accepting an H
+
ion.
Generally:
HA(
aq
)
+ H
2
O(
l
) → H
3
O
+
(
aq
)
+ A
−
(
aq
)
acid
base
If a substance can behave both as a Brønsted-
Lowry acid and as a Brønsted-Lowry base then it
is called
amphoteric
. Water has this ability. As well
as reacting with acids (above) it can also react with
Brønsted-Lowry bases such as ammonia in the
following way to form the base OH
−
:
NH
3
(
aq
) + H
2
O(
l
) → NH
4
+
(
aq
)
Figure 8.7
Justus von Liebig (1803-1873), who defined acids more closely.
This theory was followed in 1884 by the first really
comprehensive theory of acids and bases, produced by
the Swedish chemist Svante Arrhenius (1859-1927).
He suggested that since these acid solutions were
electrolytes (see Chapter 5) their solutions contained
many ions. According to Arrhenius' theory, acids
produce hydrogen ions (H
+
) when they dissolve in
water, whereas bases produce hydroxide ions (OH
-
).
It was thus recognised that water plays an
important part in the acidity of a substance. This
led to the suggestion that the hydrogen ion cannot
exist alone in aqueous solution. This was prompted
by the fact that gaseous hydrogen chloride, HCl(
g
),
is not acidic but when it dissolves in water an acidic
solution is produced.
+ OH
−
(
aq
)
base
acid
The reaction between hydrogen chloride gas and
ammonia can be described as an acid-base reaction
under this theory. The hydrogen chloride molecule
acts as a proton donor and the ammonia molecule
acts as the proton acceptor (Figure 8.8).
HCl(
g
) + NH
3
(
g
) → NH
4
+
Cl
−
(
s
)
acid
base
