Modern inorganic chemistry

STRUCTURE AND BONDING 45

Smaller and more highly charged ions such as magnesium and

aluminium attract water strongly, and in these cases the attractive

forces between the water and the ions are so great that salts con-

taining water of crystallisation decompose when attempts are made

to dehydrate them by heating — the process being called hydrolysis.

For example,

[Mg(H 2 O) 2 ]^2 + [Mg(OH)]+Cr + HC1 4- H 2 O

or, as more commonly written.

MgCl 2. 2H 2 O -> Mg(OH) Cl + HC1 + H 2 O

The aluminium ion, charge + 3, ionic radius 0.045 nm, found in

aluminium trifluoride, undergoes a similar reaction when a soluble

aluminium salt is placed in water at room temperature. Initially the

aluminium ion is surrounded by six water molecules and the

complex ion has the predicted octahedral symmetry (see Table 2.8) :

H 2 O.

H 7 O

H?O

This complex ion behaves as an acid in water, losing protons, and
a series of equilibria are established (H+ is used, rather than H 3 O+.
for simplicity):

[A1(H 2 0) 6 ]3+ ^ [A1(OH)(H 2 0) 5 ]2+ +

11

[A10H) 2 (H 2 0) 4 ]+ -

II

[A1(OH) 3 (H 2 0) 3 ] +

11

[A1(OH) 4 (H 2 0) 2 ]- + H +

1;

[A1(OH) 5 (H,0)]^2 ~ + H

il

[A1(OH) 6 ]J- H +

4 heat

2 O + 2OH

These equilibria give rise to an acidic solution in water, to the
hexahydroxo-aluminate ion [A1(OH) 6 ]^3 ~ in a strongly alkaline