Modern inorganic chemistry

GROUP IV 201

The covalent carbides: These include boron carbide B 4 C and silicon

carbide SiC; the latter is made by heating a mixture of silica and

coke in an electric furnace to about 2000 K :

SiO 2 + 3C -> SiC + 2COT

The process is carried out alongside the similar one for producing

graphite. Silicon carbide when pure is colourless, but technical

silicon carbide (carborundum) is usually grey. These carbides have

a diamond-like structure, i.e. covalent bonds extend throughout

their crystals, and they are therefore of high melting point and

chemically inert. Both are used as abrasives, and boron carbide is

used in radiation shielding.

The interstitial carbides: These are formed by the transition metals

(e.g. titanium, iron) and have the general formula MXC. They are

often non-stoichiometric—the carbon atoms can occupy some or

all of the small spaces between the larger metal atoms, the arrange-

ment of which remains essentially the same as in the pure metal (cf.

the interstitial hydrides).

CARBON DISULPHIDE, CS 2

This was formerly manufactured by passing sulphur vapour over

white hot coal or charcoal. An equilibrium was established and the

carbon disulphide vapour was condensed, allowing the reaction to

proceed:

C + 2S ^ CS 2

Large quantities are now manufactured by the reaction between
sulphur vapour and methane at a temperature of 900-1000 K in the
presence of a clay catalyst:

CH 4 + 4S -> CS 2 + 2H 2 S

The CS 2 is then removed, after cooling, by a solvent. The molecule
has a covalent linear structure S=C=S.
Carbon disulphide is a volatile, evil-smelling liquid, although if
carefully purified, the unpleasant smell is removed, as it is due to
impurity. The vapour is inflammable and can form explosive
mixtures in air:

2CS 2 + 5O 2 -> 2CO + 4SO 2

It is also decomposed by water above 420 K:

CS 2 + 2H 2 O -» CO 2 + 2H 2 S