The Foundations of Chemistry

The overall heat of solution for a solid dissolving in a liquid is equal to the heat of

solvation minus the crystal lattic energy.

Hsolution(heat of solvation)(crystal lattice energy)

Remember that both terms on the right are always negative.

Nonpolar solids such as naphthalene, C 10 H 8 , do not dissolve appreciably in polar

solvents such as water because the two substances do not attract each other significantly.

This is true despite the fact that crystal lattice energies of solids consisting of nonpolar

molecules are much less negative (smaller in magnitude) than those of ionic solids. Naph-

thalene dissolves readily in nonpolar solvents such as benzene because there are no strong

attractive forces between solute molecules or between solvent molecules. In such cases,

the increase in disorder controls the process. These facts help explain the observation that

“like dissolves like.”

The statement “like dissolves like” means that polar solvents dissolve ionic and polar

molecular solutes, and nonpolar solvents dissolve nonpolar molecular liquids.

Consider what happens when a piece of sodium chloride, a typical ionic solid, is placed

in water. The 
ends of water molecules attract the negative chloride ions on the surface

of the solid NaCl, as shown in Figure 14-2. Likewise, the ends of H 2 O molecules (O

atoms) orient themselves toward the Na
ions and solvate them. These attractions help

to overcome the forces holding the ions in the crystal, and NaCl dissolves in the H 2 O.

H 2 O

NaCl(s)8888nNa
(aq)
Cl(aq)

A cube of sugar is slowly lifted
through a solution.

546 CHAPTER 14: Solutions

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 14.4, Energetics of Solution
Formation.

Figure 14-2 The role of electrostatic attractions in the dissolution of NaCl in water. The


H of the polar H 2 O molecule helps to attract Claway from the crystal. Likewise, Na


is attracted by the O. Once they are separated from the crystal, both kinds of ions are

surrounded by water molecules, to complete the hydration process.

Cl–

H

Cl–

• O

• HH +
  Cl–

Na+

Na+

H

+ +

O–

H O–

+ H+

Cl–

Na+ Cl–

Na+

Na+

Cl–

Cl–