Modern inorganic chemistry

112 HYDROGEN

molecule and allows reaction to proceed. The reactions of hydrogen

will now be examined in more detail.

REACTIONS WITH ELECTROPOSITIVE METALS

These give ionic or salt-like hydrides, for example

2Na + H 2 -> 2NaH

These solid ionic hydrides (having an ionic lattice and containing

the hydride ion H ~) react with water, for example

CaH 2 + 2H 2 O -» Ca(OH) 2 + 2H 2

i.e.

+ H

We can see that the hydride ion H ~ functions as a very strong base
(p, 89) withdrawing a proton from the water molecule and uniting
with it to give H 2 , i.e. H~ + H^ -> H 2 , a highly exothermic process.
It follows that we cannot use these ionic hydrides in aqueous
solutions; however, some of them (notably lithium hydride. LiH)
can be used in suspension in organic solvents as reducing agents,
and others can be converted to complex hydrides which can be used
in solution (see below),
The existence of the hydride ion is shown by electrolysis of the
fused salt when hydrogen is evolved at the anode. If calcium hydride
is dissolved in another fused salt as solvent, the amount of hydrogen
evolved at the anode on electrolysis is 1 g for each Faraday of
current (mole of electrons) passed, as required by the laws of
electrolysis.

REACTIONS WITH TRANSITION METALS

Most of these metals only react with hydrogen on heating; the first
stage of reaction is the taking of hydrogen on to the metal surface,
whereby the hydrogen molecules become attached as hydrogen
atoms—a process known as chemisorption, With some metals
reaction can proceed further, and hydrogen atoms penetrate into
the metal lattice and occupy positions between the metal atoms—
interstitial positions, as shown in Figure 5.1.
If all these 'holes' were filled, the hydrogen-metal ratio would be
a definite and fixed number; in practice, this rarely happens, and