The Foundations of Chemistry

Most dissolving processes are accompanied by an overall increase in disorder. Thus,
the disorder factor is usually favorableto solubility. The determining factor, then, is whether
the heat of solution (energy) also favors dissolution or, if it does not, whether it is small
enough to be outweighed by the favorable effects of the increasing disorder. In gases, for
instance, the molecules are so far apart that intermolecular forces are quite weak. Thus,
when gases are mixed, changes in the intermolecular forces are very slight. So the very
favorable increase in disorder that accompanies mixing is always more important than
possible changes in intermolecular attractions (energy). Hence, gases that do not react
with one another can always be mixed in any proportion.
The most common types of solutions are those in which the solvent is a liquid. In the
next several sections we consider liquid solutions in more detail.

DISSOLUTION OF SOLIDS IN LIQUIDS

The ability of a solid to go into solution depends most strongly on its crystal lattice energy,
or the strength of attractions among the particles making up the solid. The crystal lattice
energyis defined as the energy change accompanying the formation of one mole of
formula units in the crystalline state from constituent particles in the gaseous state. This
process is always exothermic; that is, crystal lattice energies are always negative.For an
ionic solid, the process is written as

M
(g)
X(g)88nMX(s)
energy

The amount of energy involved in this process depends on the attraction between ions in
the solid. When these attractions are strong, a large amount of energy is released as the
solid forms, and so the solid is very stable.
The reverse of the crystal formation reaction is the separation of the crystal into ions.

MX(s)
energy88nM
(g)
X(g)

This process can be considered the hypothetical first step (step a in Figure 14-1) in forming
a solution of a solid in a liquid. It is always endothermic. The smaller the magnitude of
the crystal lattice energy (a measure of the solute–solute interactions), the more readily
dissolution occurs. Less energy must be supplied to start the dissolution process.
If the solvent is water, the energy that must be supplied to expand the solvent (step b
in Figure 14-1) includes that required to break up some of the hydrogen bonding between
water molecules.
The third major factor contributing to the heat of solution is the extent to which solvent
molecules interact with particles of the solid. The process in which solvent molecules
surround and interact with solute ions or molecules is called solvation.When the solvent
is water, the more specific term is hydration. Hydration energy(equal to the sum of
steps b and c in Figure 14-1) is defined as the energy change involved in the (exothermic)
hydration of one mole of gaseous ions.

Mn
(g)
xH 2 O88nM(OH 2 )xn

energy (for cation)

Xy(g)
rH 2 O88nX(H 2 O)ry
energy (for anion)

Hydration is usually highly exothermic for ionic or polar covalent compounds, because
the polar water molecules interact very strongly with ions and polar molecules. In fact,
the only solutes that are appreciably soluble in water either undergo dissociation or ioniza-
tion or are able to form hydrogen bonds with water.

14-2

One of the few exceptions is the

dissolution of NaF. The water

molecules become more ordered

around the small Fions. This is

due to the strong hydrogen bonding

between H 2 O molecules and Fions.

The amount of heat released on

mixing, however, outweighs the

disadvantage of this ordering.

F

H

O H H O

H

14-2 Dissolution of Solids in Liquids 545

Hydration energy is also referred to as

the heat of hydration.

A very negative crystal lattice energy

indicates very strong attractions within

the solid.