CHAPTER 4: ANSWERS TO SELECTED PROBLEMS
SAMPLE PROBLEMS (“Try it yourself”)
4.1
The molecules are moving faster in the hot cup of tea.
4.2
The sand absorbs 997.5 calories of heat (calculator answer). The temperature change is
21ºC and the specific heat of sand is 0.19 cal/g ºC, so the heat is 250 g x 21ºC x 0.19 cal/g ºC
= 997.5 calories. Rounding this number to two significant figures gives 1000 calories, and we
need to write this as 1.0 x 103 calories to show the correct number of significant figures.
4.3
83.5 kJ equals 83,500 J, which equals 19,957 calories, which equals 19.957 kcal.
(Rounding the last two numbers to three significant figures gives you 2.00 x 104 calories and
20.0 kcal; note that the first of these must be written in scientific notation to show three
significant figures.)
4.4
Formaldehyde is a gas, based on its physical properties.
4.5
Ice is a solid, so the water molecules vibrate but remain in fixed positions.
4.6
One millibar equals 100 Pa, so 1.8 Pa is the same as 0.018 millibars.
4.7
In this case, we have the same amount of oxygen, but the sizes of the containers are
different. Gases exert more pressure when we put them into smaller containers, so the oxygen
exerts more pressure in the 10 liter container than it does in the 20 liter container.
4.8
The pressure in Mr. Smith’s tires will be less than 32 pounds per square inch, because the
tires are cooler at midnight than they are at noon. The lower the temperature, the lower the
pressure.
4.9
Acetic acid is a liquid at room temperature, because room temperature (25ºC) is between
the melting point and the boiling point.
4.10 The calculator answer is 3240 calories. Rounding this to one significant figure gives us
3000 calories.
4.11
Any temperature between 360ºC and 380ºC is a reasonable estimate.
4.12 LiF boils at 1681ºC, and HF boils at 20ºC. LiF contains a metal and a nonmetal, so it is
an ionic compound and is very difficult to boil. HF contains two nonmetals, so it is a molecular
compound and is much easier to boil.
4.13 All of these compounds are molecular substances, and they all contain the same number
of atoms, so we look at the sizes of the atoms. CF4 contains the smallest atoms, so it should have
the lowest melting and boiling points. CBr4 contains the largest atoms, so it should have the
highest melting and boiling points. CCl4 is intermediate. Therefore, CF4 must be the gas, CCl4
must be the liquid, and CBr4 must be the solid at room temperature.
4.14 NH3 molecules, like H2O molecules, can participate in hydrogen bonds. The positively
charged hydrogen atoms in each NH3 molecule are attracted to the negatively charged nitrogen
atoms in other NH3 molecules. These hydrogen bonds make it more difficult to NH3 molecules
away from one another. Therefore, the boiling point of NH3 is higher than that of PH3, which
cannot form hydrogen bonds.
4.15 The boiling point of compound 3 should be closer to that of compound 1, because both of
these molecules can form hydrogen bonds.
END OF SECTION PROBLEMS
Section 4.1
4.1
As the ball rises, the potential energy of the ball increases, and the kinetic energy of the
ball decreases. (The ball slows down as it goes up, and eventually stops rising and begins to fall
back to earth.)
4.2
a) potential energy
b) kinetic energy
c) kinetic energy
d) potential energy (the water can generate electricity if it moves through a power plant)
4.3
The 1000 mL sample of water has more thermal energy. The water molecules are
moving the same speed in both samples, because the speed of the molecules depends only on the
temperature, not on the amount of water.
4.4
The 0ºC ice has more thermal energy than the –20ºC ice, and the molecules are moving
faster in the 0ºC ice.
4.5
a) You must add 560 calories.
b) You must remove 840 calories.
Section 4.2
4.6
Sulfur dioxide is a gas and sulfur trioxide is a liquid. Both of these substances can
change their shapes, so neither one is a solid. The density of sulfur dioxide is extremely low
(0.0026 g/mL), which is typical for a gas. The density of sulfur trioxide (1.92 g/mL) is typical
for a liquid.
4.7
The helium atoms are in constant, random motion. They move throughout the balloon,
colliding with one another and with the walls of the balloon.
4.8
To change a substance into a solid, we should cool it, so carbon dioxide is most likely to
be a solid at –100ºC.
4.9
In a liquid, the molecules are in contact with one another, so there is no empty space
between molecules. The molecules themselves cannot be compressed significantly, so the
volume of water does not change when you squeeze it. (Gases can contract because there is a lot
of empty space between molecules.)
Section 4.3
4.10 a) The pressure decreases, because there are fewer oxygen molecules remaining in the
cylinder.
b) The pressure decreases.
4.11 When air is cooled, its volume decreases, but the mass of the air does not change
(because the number of molecules does not change). Packing the same mass into a smaller
volume gives us a higher density. (Another way to think of this is that the molecules move
closer together when we cool a gas, so we can pack more molecules into the same volume.)
4.12 When you heat a gas, its pressure increases. If you heat an aerosol can to a high
temperature, the pressure inside the can becomes so high that the can bursts.
4.13 You can increase the temperature of the tire, reduce the volume of the tire (by pressing on
it), or put more air into the tire.
4.14
a) 5.4 atm (the calculator answer is 5.44217687 atm)
b) 4100 torr (the calculator answer is 4136.054422 torr)
Section 4.4
4.15
a) Naphthalene is a solid at room temperature.
b) At 100ºC, naphthalene is a liquid.
4.16 Boiling can be seen (the liquid bubbles), while evaporation cannot. Also, boiling occurs
at one specific temperature, while evaporation occurs at all temperatures.
4.17
Substances melt and freeze at the same temperature, so the liquid NaCl freezes at 801ºC.
4.18 At 20ºC, water is normally a liquid, but there is always some water vapor present in the
surrounding air. (There are other ways to express this idea.)
4.19 The calculator answer is 3975 calories. To two significant figures, the answer is
4.0 x 103 calories.
4.20 Isopropyl alcohol freezes at –89ºC. While it is freezing, the temperature does not change.
Once all of the isopropyl alcohol has become a solid, the temperature can begin to drop once
again.
4.21
Mercury is a liquid at 150ºC.
Section 4.5
4.22 NF3 is a molecular compound, whereas CrF3 is an ionic compound. NF3 molecules are
only weakly attracted to one another, and at room temperature this attraction is not strong enough
to keep the molecules in contact, so NF3 is a gas. In CrF3, the individual Cr3+ and F– ions are
attracted to one another very strongly, so they remain in fixed positions at room temperature. As
a result, CrF3 is a solid.
4.23 The hydrogen from one molecule is attracted to the nitrogen from another molecule, as
shown below.
H
H
H
C
H
H
C
H
N
H
C
H
H
δ–
δ+
H
H
N
H
C
H
H
4.24 In general, molecules with more atoms have higher boiling points than molecules with
fewer atoms, because the attractive forces between molecules (dispersion forces) depend on the
size of the molecules.
4.25 Na2SO4 is an ionic compound, so we expect it to be a solid at room temperature, because
of the powerful attraction between ions. H2SO4 is a molecular compound, and the attractive
forces between molecules are much weaker than those between ions, so H2SO4 is the liquid.
4.26 Compound 1 contains an O–H bond, so this molecule can participate in hydrogen
bonding. Compound 2 does not contain an O–H bond, so it cannot form hydrogen bonds.
Therefore, compound 1 should have the higher boiling point. Compound 1 boils at 95ºC, and
compound 2 boils at 49ºC.
4.27 Butane cannot form hydrogen bonds, so the attraction between butane molecules is very
weak, giving it a low boiling point. 1-propanol and ethylene glycol can form hydrogen bonds, so
their boiling points are higher. Ethylene glycol has two hydrogen atoms that can participate in
hydrogen bonds, while 1-propanol has only one, so ethylene glycol molecules are attracted to
one another more strongly than are 1-propanol molecules. This added attraction gives ethylene
glycol a higher boiling point than 1-propanol.
4.28 Only the hydrogen atoms that are bonded to nitrogen or oxygen can participate in
hydrogen bonding.
H
H
S
H
C
H
N
C
C
H
H
O
O
H
CUMULATIVE PROBLEMS (Odd-numbered problems only)
4.29 The thermal energy of the cake increases. Thermal energy depends on temperature, so as
the temperature goes up, the thermal energy goes up as well.
4.31
a) The car moving at 40 miles per hour.
b) The car.
c) The atoms in the 80ºC water.
d) Both have the same amount of kinetic energy.
4.33
a) Both have the same amount of potential energy.
c) The new battery.
d) The large stone.
4.35
Two liters of water at 20ºC.
4.37
2300 calories
4.39
36,000 calories (the calculator answer is 35,941.66667 calories)
b) The airplane at 30,000 feet.
4.41 There are several reasons for this. In particular, though, you should recognize that the
specific heat of sand is much lower than that of water, so the temperature of sand goes up much
more than the temperature of water when you add the same amount of heat to both.
4.43 Water has the higher specific heat. Specific heat is the amount of energy you must put
into a substance to change its temperature. The higher the specific heat, the smaller the
temperature change you observe.
4.45 Oxygen is a gas. Gases always fill their entire container, so the oxygen will occupy the
entire 1000 mL volume. Water, by contrast, is a liquid and has a fixed volume.
4.47 The density of propane is 0.0018 g/mL, which is much too low for a solid or liquid, but is
a reasonable value for a gas. Propane is a gas at room temperature.
4.49 In both solids and liquids, the molecules are in contact with one another, with little empty
space between molecules. When you melt a solid, the amount of empty space only changes very
slightly, and the size of the molecules themselves does not change, so the liquid and solid form
of a substance take up the same amount of space.
4.51 This is not a reasonable conclusion, because the individual grains of sand do not change
their shapes when you pour sand from one container to another. Only the space between the
grains (which is filled with air) changes its shape. Sand is actually a solid.
4.53 At 2000ºC, the thermal energy of the molecules or ions is so high that it can overcome
even a very powerful attraction between molecules or ions.
4.55 Sugar is a solid at room temperature, so sugar molecules vibrate but do not move around
within a sugar crystal. Each molecule remains in a fixed position.
4.57 The atmospheric pressure in Denver is lower than it is in San Francisco. When the
pressure on the outside of the balloon decreases, it becomes lower than the pressure of the air
inside the balloon, so the balloon expands. (However, as the air in the balloon expands, its
pressure drops, and once the internal pressure equals the external pressure, the balloon does not
expand further.)
4.59 The temperature of the pavement is very hot on a typical summer day in the desert. As a
result, the air inside the tires heats up. As the temperature increases, the pressure inside the tire
also increases. If the tire is already very weak, this added pressure is enough to make the tire
burst.
4.61
82 pounds per square inch (the calculator answer is 82.32 pounds per square inch)
4.63 a) Lactic acid is normally a liquid at 25ºC. (A small amount of vapor can also be
present.)
b) Lactic acid is a solid at 0ºC. (A small amount of vapor can also be present.)
4.65
520,000 calories (520 kilocalories)
4.67 18,600 calories. (It takes 2400 calories to heat the water from 20ºC to 100ºC, and an
additional 16,200 calories to boil the water.)
4.69 The boiling point of methane is lower than 25ºC. Methane is a gas when its temperature
is higher than the boiling point, so room temperature must be higher than the boiling point.
4.71
A reasonable estimate would be 5ºC or 10ºC, based on the graph.
4.73
Benzene is a gas at 100ºC, because 100ºC is above the boiling point.
4.75 You will begin with gaseous benzene. Once you cool the benzene to its boiling point, the
benzene will begin to condense. The temperature will remain constant until the benzene has
turned into a liquid. Once the benzene has condensed, the temperature will drop to 75ºC.
4.77 Calcium chloride is an ionic compound, held together by the attractive forces between
oppositely charged ions. These forces are very strong, so calcium chloride has a high melting
point.
4.79
a) covalent bonds
b) dispersion forces and hydrogen bonds
c) dispersion forces and hydrogen bonds
4.81
a) covalent bonds
b) dispersion forces
c) dispersion forces
4.83 The second and third molecules (HOCl and C2H7N) can form hydrogen bonds, because
they contain at least one hydrogen atom that is covalently bonded to oxygen or nitrogen.
4.85 a) Methyl iodide has the strongest dispersion force, because iodine atoms are larger than
bromine or chlorine atoms.
b) Methyl iodide has the highest boiling point, because it has the strongest attraction
between molecules.
c) Methyl chloride is most likely to be a gas, because it has the weakest attraction
between molecules.
4.87 Only the hydrogen atoms that are directly bonded to nitrogen or oxygen can participate in
hydrogen bonding.
H
O
H
C
C
N
H
N
C
C
O
H
H
H
O
H
H
4.89 The student has forgotten that NaOH is not a molecular substance. NaOH is an ionic
compound, containing Na+ and OH– ions. The attraction between ions in an ionic compound is
much stronger than hydrogen bonds, so NaOH should have the higher melting point.