Chemistry Reference
In-Depth Information
H
2
(g)
þ
1
=
2
O
2
(g)
!
H
2
O (l)
; D
H
¼
285 kJ/mol
(7.3)
2H
2
(g)
þ
O
2
(g)
!
2H
2
O (l)
; D
H
¼
570 kJ
(7.4)
Acids and bases.
There are at least three different levels for the definition of the
term “acid” (and also three levels for the term “base”), according to the historical
development:
1. The oldest definition was
structure-related
: “An acid is a substance that colors
the indicator litmus red or that dissolves limestone by the generation of a gas”
(Boyle, 1663).
2. The next definitions were
structure related
after the composition of different
acid molecules were analyzed: “acids are hydrogen compounds (i.e.
CH
3
COOH), the H atom can be replaced by a metal atom” (i.e. CH
3
COONa,
Liebig, 1838), or “acids dissociate in water, in an aqueous solution there are
H
+
(aq) ions and acid-rest ions.” (Arrhenius, 1884)
3. At last there is a
function-related
definition: “particles that can donate protons
are called acids” (Brønsted, 1923): particles like the H
2
O molecule or the HSO
4
ion can be an acid or a base particle depending on the reaction partner, the acid
particle in hydrochloric acid is not the HCl molecule but the H
3
O
+
(aq) ion.
The Arrhenius definition was a big improvement to all earlier definitions: it
stated that H
+
(aq) ions in acidic solutions are responsible for the acidic properties;
in alkaline solutions the OH
(aq) ions are responsible for the alkaline properties.
Therefore, this definition still appears in many of today's curricula in school.
But there are open questions. It was entirely unclear why some acid molecules
were ionized completely, whereas some acids only ionized to a certain degree and
had to be given a “degree of ionization.” The substance ammonia and its alkaline
solution in water could not be explained with this definition: OH
ions do not split
off from NH
3
molecules. Therefore, ammonium hydroxide NH
4
OH was invented
and the aqueous solution was explained with the corresponding ions. However, this
substance does not exist as a solid and cannot donate hydroxide ions by dissociation
in water, like other solid hydroxides do. Even today one can find old containers with
the label “NH
4
OH” or “ammonium hydroxide” for a fictitious substance.
The Arrhenius definition was limited to the solvent water. A number of
substances were familiar that could indicate acids and bases with an indicator, but
did not exist in aqueous solution. The neutralization of ammonium salts and metal
amides was shown in liquid ammonia, using common indicators. It can be
expressed in analogy with neutralization in aqueous solution:
NH
4
þ
þ
NH
2
!
2NH
3
This problem could be resolved with the Brønsted definition: an excellent
example for the extension of mental models on one side and for the coexistence
of two theories on the other side. The Arrhenius definition is still valid today for
