THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 3

Those concepts, which do not originate from a “strong” theory but
clearly and vividly describe a series of relations among chemical phenom-
ena, are immensely important. Because of the simplicity and clarity of
these concepts, our structural thinking and the technical terms in scientifi c
language are immensely enriched. Sometimes, they can be improved on
the basis of some theories known later on. As a result, new concepts have
been introduced in chemistry from time to time for rationalization and
prediction and better elucidation of various physicochemical phenomena.
The theory approaches a chemical experiment via selective approxima-
tions and simplifi cations, which then serves as bridge between the rigor-
ous theory and experiment, the chemical reality. Two important concepts
that have been highly successful in providing a better understanding of
chemical binding and reactivity in a major class of molecular systems are
the concepts of acids and bases. Acid and base are two diametrically op-
posite concepts such that when they are mixed in equivalent proportion,
they destroy each other. It is very diffi cult to defi ne an acid and a base in
terms of their chemical composition or molecular structure [5].
This has been long recognized that acids and bases play a central role
in any unifi ed theory of chemical bonding. In this regard, their character-
ization has always been a conceptual challenge for chemists, with a rich
and epistemological history. Literature [6] shows that since the beginning
of seventeenth century, a growing interest to defi ne acids and bases initi-
ated with the pioneering works of Lémery and Boyle on the “Cartesian salt
theory” and the associate principle of reactivity driven by the “struggle be-
tween acids and alkali” as well. Next, the major contribution comes from
Rouelle in the eighteenth century, which consecrated the base concept as
the complement of that of an acid, and that of Black, with his pneumatic
theory of reactions. The effort of Rouelle and Black was being culminated
by the Lavoisier's contribution according to which the oxygen is directly
related to the acidic character of matter. At the beginning of the nineteenth
century, further insight was brought by the physicochemical experiments
of Volta, Gay-Lussac, and Liebig in elucidating the fact that acids have to
contain hydrogen to be exchanged with a metal and a “radical” of different
nature; they established the famous principle:
acid + base ↔ salt + water
The fi rst unifi cation of the acidic-basic character of a compound or
solution was formulated by Arrhenius, Van't Hoff, and Ostwald in the