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Intermolecular Forces, Solids and Liquids 165
Intermolecular forces can affect phase changes. Strong intermolecular forces
require more kinetic energy to convert a liquid into a gas. Stronger intermolecular
forces, make it easier to condense a gas into a liquid.
11-5 Utterly Confused About
Intermolecular Forces
Let’s begin by analyzing a typical problem. What is the strongest type of intermolecular
force present in each of the following: methane, CH 4, methyl alcohol,
CH 3OH, diamond, C, methyl fluoride, CH 3F, iron, Fe, ammonium fluoride,
NH 4F, krypton difluoride, KrF 2, and sodium chloride, NaCl? You may or may
not encounter a question with eight substances, such as this question, but
regardless of the number of substances, the procedure will be the same.
For each of the substances the possible answers are ionic bonding, covalent
bonding, metallic bonding, hydrogen bonding, dipole-dipole force, or London
force. Forces, such as ion-dipole forces and ion-induced dipole forces, are not
choices because these require the presence of two or more substances. For
example, sodium chloride cannot utilize either of these two forces, but sodium
chloride in water can. (Sodium chloride in water exhibits ion-dipole forces.)
Each of the eight substances will exhibit London forces since they are present
in everything containing electrons. London forces are only the strongest type of
intermolecular force if there are no other attractions present. The most convenient
method of analyzing this problem is to leave consideration of London
forces to the last.
We can begin with any of the intermolecular forces other than London forces.
It is usually easiest to begin with the “normal” bonds (covalent, ionic, and
metallic). Bonds only occur in specific circumstances. For example, metallic
bonds only occur in metals or metal alloys. The only metal or alloy in the seven
substance list is iron. For this reason, the strongest intermolecular force in iron
is metallic bonding.
Very few materials utilize covalent bonding as an important intermolecular
force. The best-known examples are silicon dioxide, SiO 2, graphite, C, and diamond,
C. One of these three common examples, diamond, is in our question.
Therefore, the strongest intermolecular force in diamond is covalent bonding.
Ionic bonding is present in compounds containing a metal and a nonmetal or in
a compound containing one or more polyatomic ions. There are a few exceptions
to this generalization, but these usually do not appear in this type of
question. An ionic substance must contain at least two different elements, so we