Exercises 533Surface tension The result of inward intermolecular forces of
attraction among liquid particles that must be overcome to
expand the surface area.
Triple point The point on a phase diagram that corresponds to
the only pressure and temperature at which three phases (usu-
ally solid, liquid, and gas) of a substance can coexist at
equilibrium.
Unit cell The smallest repeating unit showing all the structural
characteristics of a crystal.
Vapor pressure The partial pressure of a vapor in equilibrium
with its parent liquid or solid.
Viscosity The tendency of a liquid to resist flow; the inverse of
its fluidity.
Volatility The ease with which a liquid vaporizes.Exercises
General Concepts
*001.What causes dispersion forces? What factors determine
the strengths of dispersion forces between molecules?
*002.What is hydrogen bonding? Under what conditions can
strong hydrogen bonds be formed?
*003.Which of the following substances have permanent
dipole–dipole forces? (a) SiH 4 , (b) molecular MgCl 2 ,
(c) NBr 3 , (d) F 2 O.
*004.Which of the following substances have permanent
dipole–dipole forces? (a) Molecular AlCl 3 , (b) PCl 5 ,
(c) NO, (d) SeF 4.
*005.For which of the substances in Exercise 3 are dispersion
forces the only important forces in determining boiling
points?
*006.For which of the substances in Exercise 4 are dispersion
forces the only important forces in determining boiling
points?
*007.For each of the following pairs of compounds, predict
which compound would exhibit stronger hydrogen bond-
ing. Justify your prediction. It may help to write a Lewis
formula for each. (a) water, H 2 O, or hydrogen sulfide,
H 2 S; (b) dichloromethane, CH 2 Cl 2 or fluoroamine,
NH 2 F; (c) acetone, C 3 H 6 O (contains a CUO double
bond) or ethyl alcohol, C 2 H 6 O (contains one CXO sin-
gle bond).
*008.Hydrogen bonding is a very strong dipole–dipole inter-
action. Why is hydrogen bonding so strong in comparison
with other dipole–dipole interactions?
*009.What is the effect of pressure on sublimation?
*010.Which of the following substances exhibit strong
hydrogen bonding in the liquid and solid states?
(a) CH 3 OH (methyl alcohol), (b) PH 3 , (c) CH 4 ,
(d) CH 2 Cl 2 , (e) CH 3 NH 2.
*011.Which of the following substances exhibit strong hy-
drogen bonding in the liquid and solid states? (a) H 2 S,
(b) NH 3 , (c) SiH 4 , (d) HF, (e) HCl.
*012.The molecular weights of SiH 4 and PH 3 are nearly the
same. Account for the fact that the melting and boiling
points of PH 3 (133°C and 88°C) are higher than those
of SiH 4 (185°C and 112°C).0 *13.Imagine replacing one H atom of a methane molecule,
CH 4 , with another atom or group of atoms. Account for
the order in the normal boiling points of the resulting
compounds: CH 4 (161°C), CH 3 Br (3.59°C), CH 3 F
(78°C), CH 3 OH (65°C).
*014.For each of the following pairs of compounds, predict
which would exhibit stronger hydrogen bonding. Justify
your prediction. It may help to write a Lewis formula for
each. (a) ammonia, NH 3 , or phosphine, PH 3 ; (b) ethyl-
ene, C 2 H 4 , or hydrazine, N 2 H 4 ; (c) hydrogen fluoride,
HF, or hydrogen chloride, HCl.
*015.Describe the intermolecular forces that are present in
each of the following compounds. Which kind of force
would have the greatest influence on the properties
of each compound? (a) bromine pentafluoride, BrF 5 ,
(b) acetone, C 3 H 6 O (contains a central CUO double
bond); (c) formaldehyde, H 2 CO.
*016.Describe the intermolecular forces that are present in
each of the following compounds. Which kind of force
would have the greatest influence on the properties of
each compound? (a) ethyl alcohol, C 2 H 6 O (contains one
CXO single bond); (b) phosphine, PH 3 ; (c) sulfur hexa-
fluoride, SF 6.
*017.Give the correct names for these changes in state:
(a) Crystals of para-dichlorobenzene, used as a moth
repellent, gradually become vapor without passing
through the liquid phase. (b) As you enter a warm room
from the outdoors on a cold winter day, your eyeglasses
become fogged with a film of moisture. (c) On the same
(windy) winter day, a pan of water is left outdoors. Some
of it turns to vapor, the rest to ice.
*018.The normal boiling point of trichlorofluoromethane,
CCl 3 F, is 24°C, and its freezing point is 111°C. Com-
plete these sentences by supplying the proper terms that
describe a state of matter or a change in state. (a) At
standard temperature and pressure, CCl 3 F is a ________.
(b) In an arctic winter at 40°C and 1 atm pressure,
CCl 3 F is a ________. If it is cooled to 120°C, the
molecules arrange themselves in an orderly lattice, the
CCl 3 F ________ and becomes a ________. (c) If