Modern inorganic chemistry

272 GROUP VI

HYDROLYSIS

The term hydrolysis is used widely to mean (a) the direct reaction

of water with a substance, for example the hydrolysis of an ion :

CH 3 COQ- + H 2 O ^ CH 3 COOH + OH~

H^ + H 2 O-»OH~ + H 2 t

or the hydrolysis of a molecule:

PC1 3 4- 3H 2 O -> H 3 PO 3 + 3HC1

CH 3 CN + H 2 O -» CH 3 CONH 2

CH 3 CONH 2 4- H 2 O ~» CH 3 COONH 4

(b) the dissociation of water co-ordinated to a cation to yield

hydroxonium ions, for example

[Fe(H 2 O) 6 ]^3 + + H 2 O ^ [Fe(H 2 O) 5 (OH)]^2 + + H 3 O +

This topic has been dealt with in depth previously, and it should be

particularly noted that in each type of hydrolysis the initial electro-

static attraction of the water molecule is followed by covalent

bond formation and (in contrast to hydration) the water molecule

is broken up.

WATER AS A CATALYST

Water appears to act as a catalyst in many chemical and physical
changes; but because a minute trace of water is often all that is
necessary to produce such a change, it is often very difficult to decide
whether water is used up in the process (i.e. is or is not a true catalyst)
and by what mechanism the 'catalysis' is accomplished. Thus, it
was once believed that ammonium chloride, vigorously dried, did
not undergo dissociation on heating into ammonia and hydrogen
chloride. In fact, presence of a trace of water assists the volatilisation
of the solid, which can occur much more rapidly in the presence of
water than when dry; the dissociation occurs with or without water.
Again, boron trifluoride, BF 3 (Chapter 7), is known to be a very
efficient catalyst for the polymerisation of unsaturated organic com-
pounds to form large polymer molecules; but catalysis only occurs
if a minute trace of water is present—hence water here is called a

(^4) cocatalyst'.
Other examples of water as an apparent catalyst are: (a) carbon
monoxide will not burn in oxygen unless a trace of water is present,