Modern inorganic chemistry

ENERGETICS 77

covalent bonds are involved. Moreover, we must not assume that

magnesium trichloride cannot exist.

Early calculations of a similar kind indicated that the compound

A1C1 is unlikely to exist; but at temperatures above about HOOK

aluminium oxide A1 2 O 3 and the trichloride A1C1 3 react to form the

compound A1C1; on cooling this disproportionates to give the

trichloride and aluminium metal

3A1C1 -> A1C1 3 + 2A1 (p. 143)

There are many compounds in existence which have a considerable
positive enthalpy of formation. They are not made by direct union
of the constituent elements in their standard states, but by some
process in which the necessary energy is provided indirectly. Many
known covalent hydrides (Chapter 5) are made by indirect methods
(for example from other hydrides) or by supplying energy (in the
form of heat or an electric discharge) to the direct reaction to
dissociate the hydrogen molecules and also possibly vaporise the
other element. Other known endothermic compounds include
nitrogen oxide and ethyne (acetylene); all these compounds have
considerable kinetic stability.

REACTIONS IN AQUEOUS SOLUTION: IONIC

SUBSTANCES

Let us examine the enthalpy terms involved when an ionic crystal
MX is dissolved in water. The energy diagram for a Group I halide
is as shown in Figure 3.8.
In the diagram below A/is represents the heat (enthalpy) of
solution, which can be measured experimentally, and A/i 5 is the

MX(s)

M*(oq)+X"(g)

M+(aq)-HX-(aq)

Figure 3.8

Ahhyd(X-)