The Foundations of Chemistry

rable molecular weight boil at lower temperatures. In the absence of hydrogen bonding,

boiling points of analogous substances (CH 4 , SiH 4 , GeH 4 , SnH 4 ) increase fairly regularly

with increasing number of electrons and molecular size (molecular weight). This is due

to increasing effectiveness of dispersion forces of attraction in the larger molecules and

occurs even in the case of some polar covalent molecules. The increasing effectiveness of

dispersion forces, for example, accounts for the increase in boiling points in the sequences

HClHBrHI and H 2 SH 2 SeH 2 Te, which involve nonhydrogen-bonded polar

covalent molecules. The differences in electronegativities between hydrogen and other

nonmetals decreasein these sequences, and the increasing dispersion forces override the

decreasing permanent dipole–dipole forces. The permanentdipole–dipole interactions

therefore have very little effect on the boiling point trend of these compounds.

Let us compare the magnitudes of the various contributions to the total energy of inter-

actions in some simple molecules. Table 13-3 shows the permanent dipole moments and

the energy contributions for five simple molecules. The contribution from dispersion

forces is substantial in all cases. The permanent dipole–dipole energy is greatest for

substances in which hydrogen bonding occurs. The variations of these total energies of

interaction are closely related to molar heats of vaporization. As we shall see in Section

13-9, the heat of vaporization measures the amount of energy required to overcome the

attractive forces that hold the molecules together in a liquid.

The properties of a liquid or a solid are often the result of many forces. The proper-

ties of an ionic compound are determined mainly by the very strong ion–ion interactions,

even though other forces may also be present. In a polar covalent compound that contains

NXH, OXH, or FXH bonds, strong hydrogen bonding is often the strongest force

present. If hydrogen bonding is absent in a polar covalent compound, dispersion forces

are likely to be the most important forces. In a slightly polar or nonpolar covalent

compound or a monatomic nonmetal, the dispersion forces, though weak, are still the

strongest ones present, so they determine the forces. For large molecules, even the very

weak dispersion forces can total up to a considerable interactive force.

THE LIQUID STATE

We shall briefly describe several properties of the liquid state. These properties vary

markedly among various liquids, depending on the nature and strength of the attractive

forces among the particles (atoms, molecules, ions) making up the liquid.

The covalent bonding withina
polyatomic ion such as NH 4
or
SO 42 is very strong, but the forces
that hold the entire substancetogether
are ionic. Thus a compound that
contains a polyatomic ion is an ionic
compound (see Section 7-10).

492 CHAPTER 13: Liquids and Solids

TABLE 13-3 Approximate Contributions to the Total Energy of Interaction

Between Molecules, in kJ/mol

Permanent Permanent Molar Heat of

Dipole Moment Dipole–Dipole Dispersion Total Vaporization

Molecule (D) Energy Energy Energy (kJ/mol)

Ar 0 0 8.5 8.5 6.7

CO 0.1 
 0 8.7 8.7 8.0

HCl 1.03 3.3 17.8 21 16.2

NH 3 1.47 13* 16.3 29 27.4

H 2 O 1.85 36* 10.9 47 40.7

*Hydrogen-bonded.

Honey is a very viscous liquid.