ACIDS AND BASES: OXIDATION AND REDUCTION 89
lone pair of electrons is a prerequisite, since an electron pair is
needed to attach a proton. In general, base strength (a) decreases as
the number of lone pairs increases, (b) increases as the size of the
base molecule or ion decreases, and (c) increases as the negative
charge on the base increases. As an example of the effect of lone
pairs, consider the sequence NH 3 , H 2 O, HF. All are neutral mole-
cules and are of similar size; but the marked decrease of base
strength from NH 3 to HF occurs as the number of electron pairs
increases from one to three. The effect of size has already been
observed; both ions F~ and Cl~, with four lone pairs each, are
weak bases, but F~ is a stronger base (loses its proton less readily)
than is Cl~ because F~ is smaller. The effect of charge can be
considerable: of the two species H 2 O and OH~, the latter is by far
the stronger base, even though it has three lone pairs as against two
in H 2 O. It we consider O^2 "" (for example, in K 2 O), with four lone
pairs, but a double negative charge, this is so strongly basic that it
reacts with water thus :
As an example of a really strong base, the hydride ion H~ (for
example in NaH) is unique ; it has one lone pair, a negative charge
and a very small size. Like O^2 ", it is too strong a base to exist in
water :H + H 2 O-*H 2 + OH"Since, generally, any base stronger than OH" will react with
water to produce OH~ we must use another solvent to 'observe'
very strong bases. The high b^se strengths of the hydride ion and
the oxide ion can best be observed in molten salts as solvents*, since
hydrides and ionic oxides are either insoluble in ordinary solvents
or attack them.
For very strong acids, it is usually possible to use a solvent of a
more conventional kind; thus, for example, the acid HBF 4 , tetra
fluoroboric acid, is extremely strong, because attachment of the
hydrogen to the tetrafluoroborate group BF 4 is essentially ionic,
H + BF^ and hence dissociation to an acid is very easy. Hence
HBF 4 behaves as a strong acid in, for example, an organic solvent,
in which it can be used.
- Thus, the strongly basic oxide ion O^2 attacks the weakly acidic SiO 2 in a molten
salt as solvent (p. 187):
SiO + O^2 ^ -> SiO-