Modern inorganic chemistry

80 ENERGETICS

REACTIONS IN AQUEOUS SOLUTION: COVALENT

SUBSTANCES

Metals in higher oxidation states form halides which are essentially

covalent, for example A1C1 3 , SnCl 4 , FeCl 3 ; when these compounds

dissolve in water they do so by a strongly exothermic process.

Indeed it is perhaps incorrect to think of this only as a dissolution

process, since it is more like a chemical reaction—but to differentiate

for a particular substance is not easy, as we shall see. The steps

involved in the case of aluminium chloride can be represented as

AlCl 3 (s) -* AlCl 3 (g) -» Al(g) + 3Cl(g) -> Al3+(g) + 3CP(g)

i

Al3+(aq)+ 3Cr(aq)

Obviously sufficient energy is available to break the Al—Cl co-

valent bonds and to remove three electrons from the aluminium

atom. Most of this energy comes from the very high hydration

enthalpy of the Al3+(g) ion (p. 78). Indeed it is the very high

hydration energy of the highly charged cation which is responsible

for the reaction of other essentially covalent chlorides with water

(for example, SnQ 4 ).

Essentially the same processes occur when chlorides (for example)

of non-metallic elements 'dissolve' in water. Thus, the enthalpy

changes for hydration chloride can be represented:

HCl(g) -> H(g) + Cl(g) -> H+(g) + CT(g) -> H+(aq) + Cl"(aq)

This is an exothermic process, due largely to the large hydration
enthalpy of the proton. However, unlike the metallic elements, non-
metallic elements do not usually form hydrated cations when their
compounds 'dissolve' in water; the process of hydrolysis occurs
instead. The reason is probably to be found in the difference in
ionisation energies. Compare boron and aluminium in Group III:

lonisation energies (kJ mol~ ] t

1st 2nd 3rd Total

B

Al

801

578

2428

1817

3660

2745

6889

5140

Clearly the hydration of the^4 B^3 + ' ion would have to produce an
enormous amount of energy to compensate for that necessary to