two blocks of different solids, such as copper and lead, that have been pressed together
for a period of years shows that each block contains some atoms of the other element.
This demonstrates that solids do diffuse, but very slowly (Figure 13-2).
INTERMOLECULAR ATTRACTIONS AND PHASE
CHANGESWe have seen (Section 12-15) how the presence of strong attractive forces between gas
molecules can cause gas behavior to become nonideal when the molecules get close
together. In liquids and solids the molecules are much closer together than in gases. As a
result, properties of liquids, such as boiling point, vapor pressure, viscosity, and heat of
vaporization, depend markedly on the strengths of the intermolecular attractive forces.
These forces are also directly related to the properties of solids, such as melting point and
heat of fusion. Let us preface our study of these condensed phases with a discussion of
the types of attractive forces that can exist between molecules and ions.
Intermolecular forces refer to the forces betweenindividual particles (atoms, molecules,
ions) of a substance. These forces are quite weak relative to intramolecular forces, that is,
covalent and ionic bonds withincompounds. For example, 920 kJ of energy is required to
decompose one mole of water vapor into H and O atoms. This reflects the strength of
intramolecular forces (covalent bonds).
Only 40.7 kJ is required to convert one mole of liquid water into steam at 100°C.
H 2 O()88nH 2 O(g) (absorbs 40.7 kJ/mol)This reflects the strength of the intermolecular forces of attraction between the water
molecules, mainly hydrogen bonding.
If it were not for the existence of intermolecular attractions, condensed phases (liquids
and solids) could not exist. These are the forces that hold the particles close to one another
in liquids and solids. As we shall see, the effects of these attractions on melting points of
solids parallel those on boiling points of liquids. High boiling points are associated with
compounds that have strong intermolecular attractions. Let us consider the effects of the
general types of forces that exist among ionic, covalent, and monatomic species.
Ion–Ion Interactions
According to Coulomb’s Law, the force of attractionbetween two oppositely charged ions
is directly proportional to the charges on the ions, qand q, and inversely proportional
to the square of the distance between them, d.
F Energy has the units of forcedistance, Fd, so the energy of attractionbetween two
oppositely charged ions is directly proportional to the charges on the ions and inversely
proportional to the distance of separation.
E qq
dqq
d^2H O H(g) 2H (g) O (g) (absorbs 920 kJ/mol)13-2
It is important to be able to tell
whether a substance is ionic, nonpolar
covalent, or polar covalent. You should
review the discussion of bonding in
Chapters 7 and 8.13-2 Intermolecular Attractions and Phase Changes 487See the Saunders Interactive
General Chemistry CD-ROM,
Screen 13.3, Intermolecular Forces (1).When oppositely charged ions are
close together, d(the denominator)
is small, so F, the attractive force
between them, is large.