Chemistry Reference
In-Depth Information
Arrhenius Theory
The first acid-base theory that we will discuss is the Arrhenius Theory.
Proposed by a Swedish chemist named Svante Arrhenius in 1887, this was
the first acid-base theory, and it remains the most specific or limiting. Ac-
cording to the Arrhenius Theory, an acid is a substance that releases H
+
ions in aqueous solution. This would explain why the acids that you are
familiar with, such as HCl and H
2
SO
4
, have hydrogen in their formulas.
Arrhenius bases are only those substances that release OH
-
ions in aqueous
solutions. This would include common bases, such as NaOH and KOH, but
would exclude other substances, such as NH
3
, that also have the ability to
neutralize acids.
Arrhenius Acids and Bases
Arrhenius Acid
A substance that releases H
+
ions in aqueous
solutions. Examples include HI, HBr, HCl, and
H
2
SO
4
.
Arrhenius Base
A substance that release OH
-
ions in aqueous
solutions. Examples include KOH, NaOH,
Ca(OH)
2
, and Mg(OH)
2
.
Brønsted-Lowry Theory
The next theory of acids and bases is called the Brønsted-Lowry Theory,
proposed by Danish chemist Johannes Brønsted and the English chemist
Thomas Lowry, independently, in 1923. The definition of an acid, in this
theory, sounds essentially the same as the Arrhenius acid. A Brønsted-Lowry
acid is defined as a substance that donates a proton to another species.
Now, you might say, “A proton is a hydrogen ion (H
+
), so what is the differ-
ence between the two definitions for acids?” Most notably, this definition
doesn't require that the acid be in an aqueous solution. So a substance that
donates protons, even when in a solid or vapor phase, is still acting as a
Brønsted-Lowry acid.
The Brønsted-Lowry definition of a base is even more inclusive, incor-
porating substances that would never be considered bases by the Arrhenius
definition. A Brønsted-Lowry base is a substance that accepts protons from
another species. This definition allows substances such as ammonia NH
3
,
which not only doesn't release OH
-
ions, but doesn't even have oxygen in it,
to be considered bases. Look at the following chemical equation, and you
will see an example of ammonia acting as a base.
