Modern inorganic chemistry

ACIDS AND BASES: OXIDATION AND REDUCTION 91

Hence, acids can be defined as substances producing cations
characteristic of the solvent (solvo-cations, for example H 3 O+,
NH4, NO*), and bases as substances producing anions character-
istic of the solvent (solvo-anions, for example OH~, NHJ, NOJ).
This concept has been applied to solvents such as liquid sulphur
dioxide, liquid hydrogen chloride and pure sulphuric acid.
We have seen that a base can be defined as combining with a
proton and, therefore, requires at least one lone pair of electrons. A
more general definition of acids and bases, due to G. N. Lewis,
describes a base as any species (atom, ion or molecule) which can
donate an electron pair, and an acid as any species which can
accept an electron pair—more simply, a base is an electron-pair
donor, an acid an electron-pair acceptor. Some examples of Lewis
acids and bases are:

Acid Base 'Neutralisation* reaction

A!C1 3

SO 3

Ag+

C0 2

NH 3

N(CH 3 ) 3

NH 3

o^2 -

A1C1 3 + NH 3 ^H 3 N:A1C1 3

S0 3 + N(CH 3 ) 3 -> (CH 3 ) 3 N:S0 3

Ag+ + 2NH 3 ->[Ag(NH 3 ) 2 ] +

CO 2 Hh O^2 ^COf

These other concepts of acids and bases are not so easily applied

quantitatively as the Lowry-Br0nsted concept. Nevertheless they

have proved to be very useful as ways of classifying chemical sub-

stances and—more importantly—these ideas have been a stimulus

to many advances in inorganic chemistry.

REDUCTION-OXIDATION PROCESSES

The term oxidation was originally applied to the formation of a

metal oxide by the direct combination of the metal and oxygen. For

example,

2Mg + O 2 -> 2MgO

The reverse of this process was termed reduction and reagents which

removed oxygen were termed reducing agents. Consider the reactions

1. CuO 4- H 2 -» Cu + H 2 O


2. ZnO + C -> Zn + CO

In reaction 1 hydrogen is the reducing agent, as it temoves
oxygen, but we should also note that the hydrogen, in accepting
oxygen, to form water, is itself oxidised. Carbon, in example 2, is the