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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

21C and the specific heat of sand is 0.19 cal/g C, so the heat is 250 g x 21C x 0.19 cal/g C= 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 10

4calories and

20.0 kcal; note that the first of these must be written in scientific notation to show threesignificant figures.)

4.4 Formaldehyde is a gas, based on its physical properties. Only gases expand and contractto any significant degree when they are compressed, and only gases have such low densities.

4.5 Ice is a solid, so the water molecules vibrate but remain essentially in the same placewithin the piece of ice.

4.6 From Table 4.6, we see that 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 in each container, but the sizes of thecontainers 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. Smiths tires will be less than 32 pounds per square inch, because thetires 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 (25C) is betweenthe melting point and the boiling point.

4.10 The calculator answer is 3240 calories.

6 g 540 cal

1 g= 3240 cal

Rounding this to one significant figure gives us 3000 calories.

4.11 The boiling point is the temperature where the graph levels off for the second time. Thisis somewhere between 300C and 400C, but closer to 400C. Any temperature between 360C

and 380C is a reasonable estimate.

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Chapter 4 Selected Answers 2

4.12 LiF boils at 1681C, and HF boils at 20C. LiF contains a metal and a nonmetal, so it isan 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 havethe lowest melting and boiling points. CBr4 contains the largest atoms, so it should have thehighest melting and boiling points. CCl4 is intermediate. Therefore, CF4 must be the gas, CCl4must be the liquid, and CBr4 must be the solid at room temperature.

4.14 NH3 molecules, like H2O molecules, can participate in hydrogen bonds, as shown inFigure 4.19. 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 moredifficult to pull 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 ofthese molecules can form hydrogen bonds. Both compound 1 and compound 3 contain at least

one hydrogen atom directly attached to the nitrogen atom.

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 theball decreases. (The ball slows down as it goes up, eventually coming to a stop before it starts fo

fall back to earth.)

4.2 a) This is potential energy. All types of chemical energy (the energy in foods, fuels, etc.)are potential energy, because they are a result of the locations of the atoms within the chemical

compounds.b) This is kinetic energy. Thermal energy (the energy in a hot object) is a result of the

rapid movement of atoms and molecules in the object.c) This is kinetic energy, because the top is in motion.

d) This is potential energy, because the water is higher than it would be if the dam werenot there. (Remember that energy is the ability to do work: the water can generate electricity if it

moves through a power plant.)

4.3 The 1000 mL sample of water has more thermal energy, because there are more watermolecules. The water molecules are moving at the same average speed in both samples, because

the speed of the molecules depends only on the temperature, not on the amount of water.

4.4 The 0C ice has more thermal energy than the 20C ice, and the molecules are movingfaster in the 0C ice.

4.5 These are typical heat calculations:

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a) The temperature change is 22C 18C = 4C. The heat calculation is then:

350 g 4C 0.40 cal/g C = 560 calories of heat.

b) The temperature change is 18C 12C = 6C. The heat calculation is then:

350 g 6C 0.40 cal/g C = 840 calories of heat.

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(0.26 g 100 mL = 0.0026 g/mL), which is typical for a gas. The density of sulfur trioxide

(192 g 100 mL = 1.92 g/mL) is typical for a liquid.

4.7 The helium atom is in constant, random motion. It moves throughout the balloon,colliding with other atoms 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 100C.

4.9 In a liquid, the molecules are in contact with one another, so there is no empty spacebetween 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 lotof 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, because the pressure of a gas is directly proportional to itstemperature (as one goes up or down, so must the other).

4.11 When air is cooled, its volume decreases but the mass of the air does not change (becausethe 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 togetherwhen 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 onit), or put more air into the tire.

4.14 Use the relationships in Table 4.6 to solve this problem.

a) 80 psi 1 atm

14.7 psi= 5.44217687 atm (calculator answer).

Rounding this answer to two significant figures gives us 5.4 atm.

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b) 80 psi 51.7 torr

1 psi= 4136 torr (calculator answer)

Rounding this answer to two significant figures gives us 4100 torr.

Section 4.4

4.15 a) Naphthalene is a solid at room temperature, because the melting point of naphthalene

is higher than room temperature (25C).b) 100C is between the melting and boiling points of naphthalene, so naphthalene is a

liquid at 100C.

4.16 Boiling can be seen (the liquid bubbles), while evaporation cannot. Also, boiling occursat 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 801C.

4.18 At 20C, water is normally a liquid, but there is always some water vapor present in thesurrounding air. (There are other ways to express this idea.)

4.19 The heat of vaporization tells us that it takes 159 calories of heat to evaporate (or boil)one gram of isopropyl alcohol. We can use this relationship as a conversion factor:

25 g 159 cal

1 g= 3975 calories (calculator answer)

To express this answer with only two significant figures, we must use scientific notation:4.0 x 10

3calories.

4.20 Isopropyl alcohol freezes at 89C. While it is freezing, the temperature does not change,

because we must remove heat in order to change the liquid into a solid. Once all of the isopropylalcohol has become a solid, the temperature can begin to drop once again.

4.21 Mercury is a liquid at 150C. The melting point of mercury is around 40C and the

boiling point is around 360C. At any temperature between these, mercury is a liquid. (Therewill also be a small amount of gaseous mercury present at this temperature.)

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 areattracted to one another very strongly, so they remain in fixed positions at room temperature. Asa result, CrF3 is a solid.

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4.23 The hydrogen from one molecule is attracted to the nitrogen from another molecule, asshown below.

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) becomestronger as the size of the molecules increases.

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 attractiveforces between molecules are much weaker than those between ions, so H2SO4 is the liquid.

4.26 Compound 1 contains an OH bond, so this molecule can participate in hydrogen

bonding. Compound 2 does not contain an OH bond, so it cannot form hydrogen bonds.Therefore, compound 1 should have the higher boiling point. Compound 1 boils at 95C, and

compound 2 boils at 49C.

4.27 Butane cannot form hydrogen bonds, so the attraction between butane molecules is veryweak, 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 inhydrogen 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 ethyleneglycol 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.

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.

H

C HH

N H

C H

H

H

H

C HH

N H

C H

H

H

+

H N

H

C

C

H

C

O

O H

S H

HH

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4.31 a) The car moving at 40 miles per hour has more kinetic energy.b) The car has more kinetic energy, because it is heavier. The heavier a moving object is,

the more kinetic energy it has (because it can do more work if it hits something).c) The atoms in the 80C water have more kinetic energy. The atoms and molecules in a

hot object have more kinetic energy than the atoms and molecules in a cool object.

d) Both batteries have the same amount of kinetic energy. (They have different amountsof potential energy.)

4.33 a) Both pieces of bread have the same amount of potential energy (chemical energy).b) The airplane at 30,000 feet has more potential energy.

c) The new battery has more potential energy (chemical energy).d) The large stone has more potential energy, because it can do more work if it falls than

the smaller stone can.

4.35 Two liters of water at 20C has more thermal energy.

4.37 To calculate the heat, we need the temperature change (in Celsius degrees) and the massof the copper (in grams). The temperature change is 30C 20C = 10C, and the mass of the

copper is 2500 g (remember that there are 1000 grams in a kilogram). The heat calculation is:2500 g x 10C x 0.092 cal/gC = 2300 calories

4.39 To calculate the heat, we need the temperature change (in Celsius degrees) and the mass

of the copper (in grams). For the temperature change, we must first change the two temperaturesfrom Fahrenheit to Celsius:

70F 32 = 38 38 1.8 = 21.11111111C

40F 32 = 8 8 1.8 = 4.44444444C

Now we can get the temperature change: 21.11111111C 4.44444444C = 16.66666667C

For the mass, we must convert 25 pounds into grams. There are 454 grams in a pound, so

25 pounds 454 g

1 pound= 11350 g

Finally, we can do the heat calculation:11350 g x 16.66666667C x 0.19 cal/gC = 35,941.667 calories

Rounding this answer to two significant figures gives us 36,000 calories

4.41 There are several reasons for this. In particular, though, you should recognize that thespecific 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 putinto a substance to change its temperature. Raising the temperature of water takes more heat

than raising the temperature of oil. If we use the same amount of heat, then, the oil will becomehotter than the water.

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.

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4.47 From the mass and volume, we can calculate the density of propane. We should convert

the mass from milligrams to grams first, so we can compare our answer with the typical densitiesof solids, liquids and gases (all of which are expressed in g/mL). 90 mg is the same is 0.09 g, so

the density is: 0.09 g 50 mL = 0.0018 g/mL

This density is much too low for a solid or liquid, but is a reasonable value for a gas. Wecan conclude that 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 veryslightly, 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 changetheir shapes when you pour sand from one container to another. Only the air between the grains

changes its shape. Sand is actually a solid.

4.53 At 2000C, the thermal energy of the molecules or ions is so high that it can overcomeeven a very powerful attraction between these particles.

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 the same position within the crystal.

4.57 The atmospheric pressure in Denver is lower than it is in San Francisco. When thepressure 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, itspressure 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 tirealso increases. If the tire is already very weak, this added pressure is enough to make the tire

burst.

4.61 In table 4.6, we see that one atm equals 14.7 pounds per square inch (psi). We can usethis relationship as a conversion factor:

5.6 atm 14.7 psi

1 atm= 82.32 psi (calculator answer)

Rounding this answer to two significant figures gives us 82 pounds per square inch.

4.63 a) Lactic acid is normally a liquid at 25C, because 25C is between the melting point andthe boiling point of lactic acid. (A small amount of vapor can also be present.)

b) Lactic acid is a solid at 0C, because this temperature is below the freezing point oflactic acid. (A small amount of vapor can also be present.)

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4.65 It takes 80 calories to melt one gram of ice. We can use this relationship to convert themass of ice into the corresponding amount of heat, but we must first convert 6.5 kg into grams.

Since a kilogram equals 1000 grams, 6.5 kg is the same as 6500 grams. Now we calculate theheat:

6500 g 80 cal

1 g

= 520,000 calories of heat (which equals 520 kilocalories)

4.67 We must do this problem in two steps. First, we calculate the amount of heat required toraise the temperature from 20C to 100C (a temperature change of 80C):

30 g x 80C x 1 cal/gC = 2400 caloriesThen we calculate the amount of heat required to boil the water. To do so, we need the heat of

vaporization of water: it takes 540 calories to boil a gram of water.30 g x 540 cal/g = 16,200 calories

The total is 2400 cal + 16,200 cal = 18,600 calories

4.69 The boiling point of methane is lower than 25C. A substance can only be a gas if its

temperature is higher than its boiling point.

4.71 A reasonable estimate would be 5C or 10C, based on the graph.

4.73 Benzene is a gas at 100C, because 100C 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 hasturned into a liquid. Once the benzene has condensed, the temperature will drop to 75C.

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 meltingpoint.

4.79 a) covalent bonds

b) dispersion forces and hydrogen bondsc) dispersion forces and hydrogen bonds

4.81 a) covalent bonds

b) dispersion forcesc) dispersion forces

4.83 The second and third molecules (HOCl and C2H7N) can form hydrogen bonds, becausethey 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. The larger the atoms, the stronger the dispersion force.b) Methyl iodide has the highest boiling point, because it has the strongest attraction (the

dispersion force) between molecules.

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c) Methyl chloride is most likely to be a gas, because it has the weakest attractionbetween molecules.

4.87 Only the hydrogen atoms that are directly bonded to nitrogen or oxygen can participate in

hydrogen bonding.

4.89 The student has forgotten that NaOH is not a molecular substance. NaOH is an ioniccompound, 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.

H N

H

C

C

H

C

O

O H

C

OH

H N

H

H

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