Acids are one of the largest classes of chemicals whose
aqueous solutions have one or more of the following
properties: (a) sour taste, (b) ability to turn litmus paper
red and cause other indicator dyes to change to
characteristic colours, (c) ability to react with and
dissolve some metals to form salts, and (d) ability to react
with bases or alkali forming salts. All acids contain
hydrogen. In water, acids ionize and form hydronium
ions (H3O+), usually written simply as hydrogen ions.
Acids are classified as strong acids or weak acids
according to the concentration of hydrogen ions that
results from ionization. Hydrochloric acid, sulphuric
acid, nitric acid and perchloric acid are strong acids since
they ionize almost completely in dilute aqueous solutions.
Acetic acid and carbonic acid are weak acids. The pH
range of acids is from 6.9 to 1 and is a measure of the acid
character in an aqueous solution.
In solvents other than water, acid is defined as a
substance that ionizes to release the positive ion of the
solvent. The Lowry-Bronsted definition of an acid as a
substance that can give up a proton is useful in the
understanding of a base. The most significant
contribution to the theory of acids was the electron-pair
concept enunciated by G. N. Lewis. According to Lewis,
any molecule or ion that can combine with another
molecule by forming a covalent bond with two electrons
from the second molecule or ion is an acid. Thus, acid is
an electron acceptor like BF3 or AlCl3. The hydrogen ion
is the simplest substance that will do this.
The terms 'hard' and 'soft' acids and bases refer to
the ease with which their electron orbitals can be
distorted. Hard acids have positive oxidation state, and
their valence electrons are not readily excited. Soft acids
have little or no positive charge and possess easily excited
valence electrons. Hard acids combine with hard bases
and soft acids with soft bases. Soft acids tend to accept
electrons and form covalent bonds more readily than hard
acids. For example, halogen acids show a progression
from hard (HF) to soft (HI).
Major groups of acids are as follows:
(i)Inorganic mineral acids like hydrochloric acid and
phosphoric acid.
(ii)Organic acids, whose four major groups are
(a) mono carboxylic acids which contain one COOH
group (examples include acetic acid), (b)
dicarboxylic acid, which contain two COOH groups
(examples include pthalic, sebacic and adipic acids),
(c) fatty acids which contain long chain COOH
groups (examples include oleic, palmitic and stearic
acids), and (d) amino acids which contain NIE, and
COOH groups (examples include glycine.) Organic
acids can also be classified into (a) aliphatic acids,
such as acetic acid and formic acid, and (b) aromatic
acids, such as benzoic acid and salicylic acid.
