534 CHAPTER 13: Liquids and Solids
crystalline CCl 3 F is held at a temperature of 120°C
while a stream of helium gas is blown over it, the
crystals will gradually disappear by the process of
________. If liquid CCl 3 F is boiled at atmospheric
pressure, it is converted to a ________ at a temperature
of ________.
*019.Why does HF have a lower boiling point and lower heat
of vaporization than H 2 O, even though their molecular
weights are nearly the same and the hydrogen bonds
between molecules of HF are stronger?
0 *20.Many carboxylic acids form dimers in which two mole-
cules “stick together.” These dimers result from the
formation of twohydrogen bonds between the two mol-
ecules. Use acetic acid to draw a likely structure for this
kind of hydrogen-bonded dimer.The Liquid State
*021.Use the kinetic–molecular theory to describe the behav-
ior of liquids with changing temperature. Why are liquids
more dense than gases?
*022.Distinguish between evaporation and boiling. Use the
kinetic–molecular theory to explain the dependence of
rate of evaporation on temperature.
*023.Support or criticize the statement that liquids with high
normal boiling points have low vapor pressures. Give
examples of three common liquids that have relatively
high vapor pressures at 25°C and three that have low
vapor pressures at 25°C.
*024.Within each group, assign each of the boiling points
to the appropriate substance on the basis of intermolec-
ular forces. (a) Ne, Ar, Kr: 246°C, 186°C, 152°C;
(b) NH 3 H 2 O, HF: 33°C, 20°C, 100°C.
*025.Within each group, assign each of the boiling points
to the respective substances on the basis of intermolecu-
lar forces. (a) N 2 , HCN, C 2 H 6 : 196°C, 89°C, 26°C;
(b) H 2 , HCl, Cl 2 : 35°C, 259°C, 85°C.
*026.(a) What is the definition of the normal boiling point?
(b) Why is it necessary to specify the atmospheric pres-
sure over a liquid when measuring a boiling point?
*027.What factors determine how viscous a liquid is? How does
viscosity change with increasing temperature?
*028.What is the surface tension of a liquid? What causes this
property? How does surface tension change with increas-
ing temperature?
*029.What happens inside a capillary tube when a liquid “wets”
the tube? What happens when a liquid does not “wet” the
tube?C OHOCH 3*030.What are some of the similarities of the molecular-level
descriptions of the viscosity, surface tension, vapor pres-
sure and the rate of evaporation of a liquid?
*031.Dispersion forces are extremely weak in comparison to
the other intermolecular attractions. Explain why this
is so.
*032.Choose from each pair the substance that, in the liquid
state, would have the greater vapor pressure at a given
temperature. Base your choice on predicted strengths of
intermolecular forces. (a) BiBr 3 or BiCl 3 , (b) CO or CO 2 ,
(c) N 2 or NO, (d) CH 3 COOH or HCOOCH 3.
*033.Repeat Exercise 32 for (a) C 6 H 6 or C 6 Cl 6 , (b) F 2 CUO
or CH 3 OH, (c) He or H 2.
*034.The temperatures at which the vapor pressures of the fol-
lowing liquids are all 100 torr are given. Predict the order
of increasing boiling points of the liquids. Butane, C 4 H 10 ,
44.2°C; 1-butanol, C 4 H 10 O, 70.1°C; diethyl ether,
C 4 H 10 O, 11.5°C.
*035.Plot a vapor pressure curve for GaCl 3 from the follow-
ing vapor pressures. Determine the boiling point of GaCl 3
under a pressure of 250 torr from the plot:t(°C) 91 108 118 132 153 176 200
vp (torr) 20 40 60 100 200 400 760*036.Plot a vapor pressure curve for Cl 2 O 7 from the follow-
ing vapor pressures. Determine the boiling point of Cl 2 O 7
under a pressure of 125 torr from the plot:t(°C) 24 13 21029456279
vp (torr) 5 10 20 40 100 200 400 760*037.The vapor pressure of liquid bromine at room tempera-
ture is 168 torr. Suppose that bromine is introduced drop
by drop into a closed system containing air at 775 torr
and room temperature. (The volume of liquid bromine is
negligible compared to the gas volume.) If the bromine
is added until no more vaporizes and a few drops of liq-
uid are present in the flask, what would be the total
pressure? What would be the total pressure if the volume
of this closed system were decreased to one half its orig-
inal value at the same temperature?
*038.A closed flask contains water at 75.0°C. The total pres-
sure of the air-and-water-vapor mixture is 633.5 torr. The
vapor pressure of water at this temperature is given in
Appendix E as 289.1 torr. What is the partial pressure of
the air in the flask?
0 *39. Hvapis usually greater than Hfusionfor a substance,
yet the natureof interactions that must be overcome in
the vaporization and fusion processes are similar. Why
is Hvapgreater?
0 *40.The heat of vaporization of water at 100°C is 2.26 kJ/g;
at 37°C (body temperature) it is 2.41 kJ/g.