Modern inorganic chemistry

148 THE ELEMENTS OF GROUP 111

3LiH + A1C1 3 -* A1H 3 + 3LiCll

4LiH + A1C1 3 -* LiAlH 4 + 3LiCl|

In the absence of excess lithium hydride, aluminium hydride slowly

precipitates as a white polymer (AlH 3 )n. With excess lithium hydride,

the reaction:

A1H 3 + H- -»[A1H 4 ]-

may be assumed to occur, forming lithium tetrahydridoaluminate

(aluminium hydride), which remains in solution. In both cases, the

aluminium increases its covalency. The extent of this increase is

unknown in the polymer (AlH 3 )n (the structure of this compound

is not known with certainty but it is electron-deficient). In the tetra-

hedral ion [A1H 4 ] ~ the covalency has been increased to four.

Aluminium hydride loses hydrogen on heating. It reacts slowly

with diborane to give aluminium tetrahydridoborate:

2(AlH 3 )n + 3wB 2 H 6 -* 2nAl(BH 4 ) 3

OXIDES AND HYDROXIDES

Boron trioxide, B 2 O 3 is the anhydride of boric acid, H 3 BO 3 and

can be prepared by heating the acid:

2H 3 BO 3 -> B 2 O 3 + 3H 2 O

Boron trioxide is not particularly soluble in water but it slowly
dissolves to form both dioxo(HBO 2 )(meta) and trioxo(H 3 BO 3 )
(ortho) boric acids. It is a dimorphous oxide and exists as either a
glassy or a crystalline solid. Boron trioxide is an acidic oxide and
combines with metal oxides and hydroxides to form borates, some
of which have characteristic colours—a fact utilised in analysis as
the "borax bead test', cf. alumina p. 150. Boric acid. H 3 BO 3. properly
called trioxoboric acid, may be prepared by adding excess hydro-
chloric or sulphuric acid to a hot saturated solution of borax,
sodium heptaoxotetraborate, Na 2 B 4 O 7 , when the only moderately
soluble boric acid separates as white flaky crystals on cooling. Boric
acid is a very weak monobasic acid; it is, in fact, a Lewis acid since
its acidity is due to an initial acceptance of a lone pair of electrons
from water rather than direct proton donation as in the case of
Lowry-Br0nsted acids, i.e.