ACIDS AND BASES
The terms acid and base have been defined in different ways, depending on the particular way of
looking at the properties of acidity and basicity. Arrhenius first defined acids as compounds which
ionize to produce hydrogen ions, and bases as compounds which ionize to produce hydroxide ions.
According to the Lowry-Bronsted definition, an acid is a proton donor, and a base is a proton
acceptor.
According to the Lewis definition, acids are molecules or ions capable of coordinating with
unshared electron pairs, and bases are molecules or ions having unshared electron pairs available
for sharing with acids. To be acidic in the Lewis sense, a molecule must be electron deficient. This
is the most general acid base concept. All Lowery Bronsted acids are Lewis acids but, in addition,
the Lewis definition includes many other reagents such as boron trifluoride, aluminum chloride,
etc.
Theories of Acids and Bases
Three different theories have been put forth in order to define acids and bases. These theories
include the Arrhenius theory, the Bronsted-Lowry theory, and the Lewis theory of acids and bases.
Acids and bases can be defined via three different theories.
Arrhenius Concept of Acids and Bases
The Arrhenius concept of an acid and a base stated as follows:
An acid is a substance that, when dissolved in water, increases the concentration of hydronium
ion, H3O +(aq).
A base is a substance that, when dissolved in water, increases the concentration of hydroxide
ion, OH −(aq).
1
, Arrhenius proposed that acid-base reactions are characterized by acids if they dissociate
in aqueous solution to form hydrogen ions (H+) and bases if they form hydroxide (OH – ) ions in
aqueous solution.
Limitations of Arrhenius Concept
• The presence of water is absolutely necessary for acids and bases. Dry HCl can’t act as
an acid. HCl acts as an acid in water only and not any other solvent.
• The concept does not explain the acidic and basic character of substances in non-aqueous
solvents.
• The neutralization process is only possible for reactions which can occur in aqueous
solutions, although reactions involving salt formation can occur in the absence of a
solvent.
• The acidic character of some salts such as AlCl3 in aqueous solution can’t be explained.
• An extended as well as artificial explanation is needed to define the basic nature of NH 3 .
Brønsted–Lowry Concept of Acids and Bases
In 1923 the Danish chemist Johannes N. Brønsted (1879–1947) and, independently, the British
chemist Thomas M. Lowry (1874–1936) pointed out that acid–base reactions can be seen as
proton-transfer reactions and that acids and bases can be defined in terms of this proton (H +)
transfer. According to the Brønsted–Lowry concept, an acid is the species donating a proton in a
proton-transfer reaction. A base is the species accepting the proton in a proton-transfer reaction.
Consider, for example, the reaction of hydrochloric acid with ammonia, writing it as an ionic equation,
you have
After cancelling Cl− you obtain the net ionic equation
2
The terms acid and base have been defined in different ways, depending on the particular way of
looking at the properties of acidity and basicity. Arrhenius first defined acids as compounds which
ionize to produce hydrogen ions, and bases as compounds which ionize to produce hydroxide ions.
According to the Lowry-Bronsted definition, an acid is a proton donor, and a base is a proton
acceptor.
According to the Lewis definition, acids are molecules or ions capable of coordinating with
unshared electron pairs, and bases are molecules or ions having unshared electron pairs available
for sharing with acids. To be acidic in the Lewis sense, a molecule must be electron deficient. This
is the most general acid base concept. All Lowery Bronsted acids are Lewis acids but, in addition,
the Lewis definition includes many other reagents such as boron trifluoride, aluminum chloride,
etc.
Theories of Acids and Bases
Three different theories have been put forth in order to define acids and bases. These theories
include the Arrhenius theory, the Bronsted-Lowry theory, and the Lewis theory of acids and bases.
Acids and bases can be defined via three different theories.
Arrhenius Concept of Acids and Bases
The Arrhenius concept of an acid and a base stated as follows:
An acid is a substance that, when dissolved in water, increases the concentration of hydronium
ion, H3O +(aq).
A base is a substance that, when dissolved in water, increases the concentration of hydroxide
ion, OH −(aq).
1
, Arrhenius proposed that acid-base reactions are characterized by acids if they dissociate
in aqueous solution to form hydrogen ions (H+) and bases if they form hydroxide (OH – ) ions in
aqueous solution.
Limitations of Arrhenius Concept
• The presence of water is absolutely necessary for acids and bases. Dry HCl can’t act as
an acid. HCl acts as an acid in water only and not any other solvent.
• The concept does not explain the acidic and basic character of substances in non-aqueous
solvents.
• The neutralization process is only possible for reactions which can occur in aqueous
solutions, although reactions involving salt formation can occur in the absence of a
solvent.
• The acidic character of some salts such as AlCl3 in aqueous solution can’t be explained.
• An extended as well as artificial explanation is needed to define the basic nature of NH 3 .
Brønsted–Lowry Concept of Acids and Bases
In 1923 the Danish chemist Johannes N. Brønsted (1879–1947) and, independently, the British
chemist Thomas M. Lowry (1874–1936) pointed out that acid–base reactions can be seen as
proton-transfer reactions and that acids and bases can be defined in terms of this proton (H +)
transfer. According to the Brønsted–Lowry concept, an acid is the species donating a proton in a
proton-transfer reaction. A base is the species accepting the proton in a proton-transfer reaction.
Consider, for example, the reaction of hydrochloric acid with ammonia, writing it as an ionic equation,
you have
After cancelling Cl− you obtain the net ionic equation
2
