Page 50 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 FUNDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

A, -BI and A, - B, are two conjugate acid-base pairs. This is the most
important expression for reactions involving acids and bases; it represents the
transfer of a proton from A, to B, or from A, to B,. The stronger the acid A,
and the weaker A,, the more complete will be the reaction (b). The stronger
acid loses its proton more readily than the weaker; similarly, the stronger base
accepts a proton more readily than does the weaker base. It is evident that the
base or acid conjugate to a strong acid or a strong base is always weak, whereas
the base or acid conjugate to a weak acid or weak base is always strong.
In aqueous solution a Br~nsted-Lowry acid A

is strong when the above equilibrium is virtually complete to the right so that
[A] is almost zero. A strong base is one for which [BI, the equilibrium
concentration of base other than hydroxide ion, is almost zero.
Acids may thus be arranged in series according to their relative combining
tendencies with a base, which for aqueous solutions (in which we are largely
interested) is water:
HC1+ H,O = H30+ +Cl-
Acid, Base, Acid, Base,
This process is essentially complete for al1 typical 'strong' (i.e. highly ionised)
acids, such as HC1, HBr, HI, HNO,, and HC10,. In contrast with the 'strong'
acids, the reactions of a typical 'weak' or slightly ionised acid, such as acetic
acid or propionic (propanoic) acid, proceeds only slightly to the right in the
equation:
CH3COOH + H,O = H30+ + CH3COO-
Acid , Base, Acid, Base,
The typical strong acid of the water system is the hydrated proton H,O+,
and the role of the conjugate base is minor if it is a sufficiently weak base, e.g.
Cl-, Br-, and CIO;. The conjugate bases have strengths that Vary inversely
as the strengths of the respective acids. It can easily be shown that the basic
ionisation constant of the conjugate base KB,conj. is equal to Kw/K,,conj,, where
K, is the ionic product of water.
Scheme (b) includes reactions formerly described by a variety of names, such
as dissociation, neutralisation, hydrolysis and buffer action (see below). One
acid-base pair may involve the solvent (in water H, O + - H, O or H, O - OH - ),
showing that ions such as H,O+ and OH- are in principle only particular
examples of an extended class of acids and bases though, of course, they do
occupy a particularly important place in practice. It follows that the properties
of an acid or base may be greatly influenced by the nature of the solvent
employed.
Another definition of acids and bases is due to G. N. Lewis (1938). From
the experimental point of view Lewis regarded al1 substances which exhibit
'typical' acid- base properties (neutralisation, replacement, effect on indicators,
catalysis), irrespective of their chemical nature and mode of action, as acids or
bases. He related the properties of acids to the acceptance of electron pairs, and
bases as donors of electron pairs, to form covalent bonds regardless of whether
protons are involved. On the experimental side Lewis' definition brings
together a wide range of qualitative phenomena, e.g. solutions of BF,, BCl,,