11 Acids and bases
11.1 Arrhenius Definition: Hydroxide and Hydronium Ions
The first and earliest definition of acids and bases was proposed in the 1800’s by Swedish sci-
entist Svante Arrhenius, who said that anacidwas anything that dissolved in water to yield
H+ions (like stomach acid HCl, hydrochloric acid), and abasewas anything that dissolved
in water to give upOH-ions (like soda lye NaOH, sodium hydroxide). Acids and bases
were already widely used in various occupations and activities of the time, so Arrhenius’
definition merely attempted to explained well-known and long-observed phenomenon.
Although simple, at the time this definition of the two types of substances was significant.
It allowed chemists to explain certain reactions as ion chemistry, and it also expanded the
ability of scientists of the time to predict certain chemical reactions. The definition left a
great deal wanting, however, in that many types of reactions that did not involve hydroxide
or hydronium ions directly remained unexplained.
Many general chemistry classes (especially in the lower grades or introductory levels) still
use this simple definition of acids and bases today, but modern organic chemists make fur-
ther distinctions between acids and bases than the distinctions provided under Arrhenius’s
definition.
11.2 Brønsted-Lowry Acids and Bases: Proton donors and acceptors.
A new definition for acids and bases, building upon the one already proposed by Arrhenius,
was brought forth independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry
in 1923. The new definition did not depend on a substance’s dissolution in water for
definition, but instead suggested that a substance was acidic if it readily donated a proton
(H+) to a reaction and a substance was basic if it accepted a proton in a reaction.
Definiton of Brønsted-Lowry Acid and BaseAnacidis anyproton donorand abase
is anyproton acceptor.The major advantage of the updated definition was that it was not limited to aqueous
solution. This definition of acids and bases allowed chemists to explain a great number
of reactions that took place in protic or aprotic solvents that were not water, and it also
allowed for gaseous and solid phase reactions (although those reactions are more rare).
For example, the hypothetical acid HA will disassociate into H+and A-: