Acid-base reaction theories

Originally, acids and bases were defined only by properties, such as the
fact that acids tasted sour and turned certain plant dyes from one color to
another, while bases tasted bitter, changed the colors of the same plant
dyes in the opposite direction, and neutralized the activity of acids. A
scientific definition was first proposed by the French chemist Antoine Lavoisier.

Lavoisier's definition

Since Lavoisier's knowledge of strong acids was mainly restricted to the
oxyacids, which tend to contain central atoms in high oxidation states
surrounded by oxygen, such as HNO3 and H2SO4, and since he was not aware of
the true composition of the hydrohalic acids, HCl, HBr, and HI, he defined
acids in terms of their containing oxygen, which in fact he named from Greek
words meaning "acid-former". When the elements chlorine, bromine, and iodine
were identified and the absence of oxygen in the hydrohalic acids was
established, this definition had to be rejected.

The Arrhenius definition

Svante Arrhenius provided the first modern definition of acids and bases in
1884. In water, a dissociation takes place:

     H2O → H+ + OH-

A compound causing an increase in H+ and a decrease in OH- is an acid and
one causing the reverse is a base.

An Arrhenius acid, when dissociated in water, typically yields
positively-charged hydrogen ion and a complementary negative ion.

An Arrhenius base, when dissociated in water, typically yields
negatively-charged hydroxide ion and a complementary positive ion.

The positive ion from a base can form a salt from the negative ion of an
acid. For example, two moles of the base sodium hydroxide (NaOH) can combine
with one mole of sulfuric acid (H2SO4) to form two moles of water and one
mole of sodium sulfate.

     2NaOH + H2SO4 → 2H2O + Na2SO4

The protonic (Br¿nsted-Lowry) definition

The Br¿nsted-Lowry definition, formulated independently by its two
proponents in 1923, revolves around an acid's ability to donate protons (H+)
to another compound, called a base, in a chemical reaction.

A base is a proton acceptor. In Br¿nsted-Lowry acid-base reactions, there is
a "competition" between two bases for a proton. so that if X and Y are two
species, the equilibrium

     HX + Y- ↔ HY + X-:

occurs. Both HX and HY are Br¿nsted-Lowry acids; both X- and Y- are
Br¿nsted-Lowry bases. If the reaction runs mostly to the left, then HY is
the stronger acid and X- the stronger base; if the reaction runs mostly to
the right, then HX is the stronger acid and Y- the stronger base.

Acids and bases in the Br¿nsted-Lowry system occur in conjugate pairs; in
the reaction

     HX → H+ + X-

HX is denoted the conjugate acid of the base X-, and X- is denoted the
conjugate base of the acid HX.

Some compounds, like water, can act either as an acid or a base, and are
called amphoteric compounds. Stronger acids also typically oxidize metals,
forming salts and releasing hydrogen.

See pH for a measure of proton concentration frequently used for measuring
acidity and alkalinity using this definition.

The solvent-system definition

This definition is based on a generalization of the earlier Arrhenius
definition. If we consider a solvent which can be dissociated into a
positive species X and a negative species Y:

     XY ↔ X+ + Y-

or

     2XY ↔ X2Y+ + Y-

or

     2XY ↔ X+ + XY2-

a compound causing an increase in X+ (or X2Y+) and a decrease in Y- (or
XY2-) is an acid and one causing the reverse is a base. For example in
liquid sulfur dioxide (SO2), thionyl compounds (formally supplying SO+2)
behave as acids, and sulfites (supplying SO3-2) behave as bases.

In this more general sense, aprotic compounds (those which do not donate
protons), can still react with bases, and the terms "acid" and "base" can
still be used for reactions in aprotic or non-aqueous environments.

The electronic (Lewis) definition

The more general definition offered by Lewis in 1923 (the same year as the
Br¿nsted-Lowry definition) describes the reactivity of an acid in terms of
its ability to accept a pair of electrons from a base, defined as an
electron-pair donor. In general, an acid reacts with a base by forming a new
covalent bond utilizing an empty orbital of the acid to share the extra
electron pair of the base.

The Usanovich definition

The most general definition is that of the Russian chemist Usanovich, and
can basically be summarized as defining an acid as anything that accepts
negative species or donates positive ones, and a base as the reverse. This
tends to overlap the concept of redox (oxidation-reduction), and so is not
highly favored by chemists.