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Chapter 9

Thermochemistry

Educators

KC
MC
ES
+ 7 more educators

Problem 1

What is thermochemistry? Why is it important?

KC
Kevin C.
Numerade Educator

Problem 2

What is energy? What is work? List some examples of each.

MC
Monique C.
University of Michigan - Ann Arbor

Problem 3

What is kinetic energy? What is potential energy? List some examples of each.

KC
Kevin C.
Numerade Educator

Problem 4

What is the law of conservation of energy? How does it relate to energy exchanges between a thermodynamic system and its surroundings?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 5

A friend claims to have constructed a machine that creates electricity but requires no energy input. Explain why you should be suspicious of your friend's claim.

KC
Kevin C.
Numerade Educator

Problem 6

What is a state function? List some examples of state functions.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 7

What is internal energy? Is internal energy a state function?

KC
Kevin C.
Numerade Educator

Problem 8

If energy flows out of a chemical system and into the surroundings, what is the sign of Esystem?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 9

If the internal energy of the products of a reaction is higher than the internal energy of the reactants, what is the sign of E for the reaction? In which direction does energy flow?

KC
Kevin C.
Numerade Educator

Problem 10

What is heat? Explain the difference between heat and temperature.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 11

How is the change in internal energy of a system related to heat and work?

KC
Kevin C.
Numerade Educator

Problem 12

Explain how the high specific heat capacity of water can affect the weather in coastal regions.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 13

What is heat capacity? Explain the difference between heat capacity and specific heat capacity.

KC
Kevin C.
Numerade Educator

Problem 14

Explain how the sum of heat and work can be a state function, even though heat and work are themselves not state functions.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 15

If two objects, A and B, of different temperature come into direct contact, what is the relationship between the heat lost by one object and the heat gained by the other? What is the relationship between the temperature changes of the two objects? (Assume that the two objects do not lose any heat to anything else.)

KC
Kevin C.
Numerade Educator

Problem 16

What is pressure-volume work? How is it calculated?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 17

What is calorimetry? Explain the difference between a coffee-cup calorimeter and a bomb calorimeter. What is each designed to measure?

KC
Kevin C.
Numerade Educator

Problem 18

What is the change in enthalpy (H) for a chemical reaction? How is H different from E?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 19

Explain the difference between an exothermic and an endothermic reaction. Give the sign of H for each type of reaction.

KC
Kevin C.
Numerade Educator

Problem 20

From a molecular viewpoint, where does the energy emitted in an exothermic chemical reaction come from? Why does the reaction mixture undergo an increase in temperature even though energy is emitted?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 21

From a molecular viewpoint, where does the energy absorbed in an endothermic chemical reaction go Why does the reaction mixture undergo a decrease in temperature even though energy is absorbed?

KC
Kevin C.
Numerade Educator

Problem 22

Is the change in enthalpy for a reaction an extensive property? Explain the relationship between H for a reaction and the amounts of reactants and products that undergo reaction.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 23

Explain how the value of H for a reaction changes upon:
a. multiplying the reaction by a factor
b. reversing the reaction
Why do these relationships hold?

KC
Kevin C.
Numerade Educator

Problem 24

What is Hess's law? Why is it useful?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 25

What is a standard state? What is the standard enthalpy change for a reaction?

KC
Kevin C.
Numerade Educator

Problem 26

How can bond energies be used to estimate H for a reaction?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 27

Explain the difference between exothermic and endothermic reactions in terms of the relative strengths of the bonds that are broken and the bonds that are formed.

KC
Kevin C.
Numerade Educator

Problem 28

What is the standard enthalpy of formation for a compound? For a pure element in its standard state?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 29

How do you calculate Hrxn from tabulated standard enthalpies of formation?

KC
Kevin C.
Numerade Educator

Problem 30

What is lattice energy? How does lattice energy depend on ion size? On ion charge?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 31

Which statement is true of the internal energy of a system and its surroundings during an energy exchange with a negative Esys?
a. The internal energy of the system increases, and the internal energy of the surroundings decreases.
b. The internal energy of both the system and the surroundings increases.
c. The internal energy of both the system and the surroundings decreases.
d. The internal energy of the system decreases, and the internal energy of the surroundings increases.

KC
Kevin C.
Numerade Educator

Problem 32

During an energy exchange, a chemical system absorbs energy from its surroundings. What is the sign of Esys for this process? Explain.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 33

Identify each energy exchange as primarily heat or work and determine the sign of E (positive or negative) for the system.
a. Sweat evaporates from skin, cooling the skin. (The evaporating sweat is the system.)
b. A balloon expands against an external pressure. (The contents of the balloon are the system.)
c. An aqueous chemical reaction mixture is warmed with an external flame. (The reaction mixture is the system.)

KC
Kevin C.
Numerade Educator

Problem 34

Identify each energy exchange as primarily heat or work and determine the sign of E (positive or negative) for the system.
a. A rolling billiard ball collides with another billiard ball. The first billiard ball (defined as the system) stops rolling after the collision.
b. A book is dropped to the floor. (The book is the system.)
c. A father pushes his daughter on a swing. (The daughter and the swing are the system.)

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 35

A system releases 622 kJ of heat and does 105 kJ of work on the surroundings. What is the change in internal energy of the system?

KC
Kevin C.
Numerade Educator

Problem 36

A system absorbs 196 kJ of heat, and the surroundings do 117 kJ of work on the system. What is the change in internal energy of the system?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 37

The gas in a piston (defined as the system) warms and absorbs 655 J of heat. The expansion performs 344 J of work on thesurroundings. What is the change in internal energy for the system?

Jennifer H.
Numerade Educator

Problem 38

The air in an inflated balloon (defined as the system) warms over a toaster and absorbs 115 J of heat. As it expands, it does 77 kJ of work. What is the change in internal energy for the system?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 39

A person packs two identical coolers for a picnic, placing twenty-four 12-ounce soft drinks and 5 pounds of ice in each. However, the drinks put into cooler A were refrigerated for several hours before they were packed in the cooler, while the drinks put into cooler B were at room temperature. When the picnickers open the two coolers three hours later, most of the ice in cooler A is still present, while nearly all of the ice in cooler B has melted. Explain this difference.

Allison P.
Numerade Educator

Problem 40

A kilogram of aluminum metal and a kilogram of water are each warmed to 75 C and placed in two identical insulated containers. One hour later, the two containers are opened, and the temperature of each substance is measured. The aluminum has cooled to 35 C, while the water has cooled only to 66 C. Explain this difference.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 41

How much heat is required to warm 1.50 L of water from 25.0 C to 100.0 C? (Assume a density of 1.00 g>mL for the water.)

KC
Kevin C.
Numerade Educator

Problem 42

How much heat is required to warm 1.50 kg of sand from 25.0 C to 100.0 C?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 43

Suppose that 25 g of each substance is initially at 27.0 C. What is the final temperature of each substance upon absorbing 2.35 kJ of heat?
a. gold
b. silver
c. aluminum
d. water

KC
Kevin C.
Numerade Educator

Problem 44

An unknown mass of each substance, initially at 23.0 C, absorbs 1.95 * 103 J of heat. The final temperature is recorded as indicated. Find the mass of each substance.
a. Pyrex glass (Tf = 55.4 C)
b. sand (Tf = 62.1 C)
c. ethanol (Tf = 44.2 C)
d. water (Tf = 32.4 C)

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 45

How much work (in J) is required to expand the volume of a pump from 0.0 L to 2.5 L against an external pressure of 1.1 atm?

KC
Kevin C.
Numerade Educator

Problem 46

The average human lung expands by about 0.50 L during each breath. If this expansion occurs against an external pressure of 1.0 atm, how much work (in J) is done during the expansion?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 47

The air within a piston equipped with a cylinder absorbs 565 J of heat and expands from an initial volume of 0.10 L to a final volume of 0.85 L against an external pressure of 1.0 atm. What is the change in internal energy of the air within the piston?

Charotte M.
Numerade Educator

Problem 48

A gas is compressed from an initial volume of 5.55 L to a final volume of 1.22 L by an external pressure of 1.00 atm. During the compression the gas releases 124 J of heat. What is the change in internal energy of the gas?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 49

When 1 mol of a fuel burns at constant pressure, it produces3452 kJ of heat and does 11 kJ of work. What are E and H for the combustion of the fuel?

KC
Kevin C.
Numerade Educator

Problem 50

The change in internal energy for the combustion of 1.0 mol ofoctane at a pressure of 1.0 atm is 5084.3 kJ. If the change in enthalpy is 5074.1 kJ, how much work is done during the combustion?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 51

Is each process exothermic or endothermic? Indicate the sign of H.
a. natural gas burning on a stove
b. isopropyl alcohol evaporating from skin
c. water condensing from steam

KC
Kevin C.
Numerade Educator

Problem 52

Is each process exothermic or endothermic? Indicate the signof H.
a. dry ice evaporating
b. a sparkler burning
c. the reaction that occurs in a chemical cold pack used to ice athletic injuries

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 52

What mass of natural gas (CH4) must burn to emit 267 kJ of heat?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 53

Consider the thermochemical equation for the combustion of acetone (C3H6O), the main ingredient in nail polish remover:

If a bottle of nail polish remover contains 177 mL of acetone, how much heat is released by its complete combustion? The density of acetone is 0.788 g>mL.

KC
Kevin C.
Numerade Educator

Problem 55

Nitromethane (CH3NO2) burns in air to produce significant amounts of heat:

How much heat is produced by the complete reaction of 5.56 kg of nitromethane?

KC
Kevin C.
Numerade Educator

Problem 56

Titanium reacts with iodine to form titanium(III) iodide, emitting heat:

Determine the masses of titanium and iodine that react if 1.55 * 103 kJ of heat is emitted by the reaction.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 57

The propane fuel (C3H8) used in gas barbeques burns according to a thermochemical equation:

If a pork roast must absorb 1.6 * 103 kJ to fully cook, and if only 10% of the heat produced by the barbeque is actually absorbed by the roast, what mass of CO2 is emitted into the atmosphere during the grilling of the pork roast?

Charotte M.
Numerade Educator

Problem 58

Charcoal is primarily carbon. Determine the mass of CO2 produced by burning enough carbon (in the form of charcoal) to produce 5.00 * 102 kJ of heat.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 59

We submerge a silver block, initially at 58.5 C, into 100.0 g of water at 24.8 C, in an insulated container. The final temperature of the mixture upon reaching thermal equilibrium is 26.2 C. What is the mass of the silver block?

Charotte M.
Numerade Educator

Problem 60

We submerge a 32.5-g iron rod, initially at 22.7 C, into an unknown mass of water at 63.2 C, in an insulated container. The final temperature of the mixture upon reaching thermal equilibrium is 59.5 C. What is the mass of the water?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 61

We submerge a 31.1-g wafer of pure gold initially at 69.3 C into 64.2 g of water at 27.8 C in an insulated container. What is the final temperature of both substances at thermal equilibrium?

KC
Kevin C.
Numerade Educator

Problem 62

We submerge a 2.85-g lead weight, initially at 10.3 C, in 7.55 g of water at 52.3 C in an insulated container. What is the final temperature of both substances at thermal equilibrium?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 63

Two substances, A and B, initially at different temperatures, come into contact and reach thermal equilibrium. The mass of substance A is 6.15 g, and its initial temperature is 20.5 C. The mass of substance B is 25.2 g, and its initial temperature is 52.7 C. The final temperature of both substances at thermal equilibrium is 46.7 C. If the specific heat capacity of substance B is 1.17 J>g # C, what is the specific heat capacity of substance A?

Charotte M.
Numerade Educator

Problem 64

A 2.74-g sample of a substance suspected of being pure gold is warmed to 72.1 C and submerged into 15.2 g of water initially at 24.7 C. The final temperature of the mixture is 26.3 C. What is the heat capacity of the unknown substance? Could the substance be pure gold?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 65

Exactly 1.5 g of a fuel burns under conditions of constant pressure and then again under conditions of constant volume. In measurement A the reaction produces 25.9 kJ of heat, and in measurement B the reaction produces 23.3 kJ of heat. Which measurement (A or B) corresponds to conditions of constant pressure? Which one corresponds to conditions of constant volume? Explain.

KC
Kevin C.
Numerade Educator

Problem 66

In order to obtain the largest possible amount of heat from a chemical reaction in which there is a large increase in the number of moles of gas, should you carry out the reaction under conditions of constant volume or constant pressure? Explain.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 67

When 0.514 g of biphenyl (C12H10) undergoes combustion in a bomb calorimeter, the temperature rises from 25.8 C to 29.4 C. Find Erxn for the combustion of biphenyl in kJ>mol biphenyl. The heat capacity of the bomb calorimeter, determined in a separate experiment, is 5.86 kJ>C.

Charotte M.
Numerade Educator

Problem 68

Mothballs are composed primarily of the hydrocarbon naphthalene (C10H8). When 1.025 g of naphthalene burns in a bomb calorimeter, the temperature rises from 24.25 C to 32.33 C. Find Erxn for the combustion of naphthalene. The heat capacity of the calorimeter, determined in a separate experiment, is 5.11 kJ>C.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 69

Zinc metal reacts with hydrochloric acid according to the balanced equation:

When 0.103 g of Zn(s) is combined with enough HCl to make 50.0 mL of solution in a coffee-cup calorimeter, all of the zinc reacts, raising the temperature of the solution from 22.5 C to 23.7 C. Find Hrxn for this reaction as written. (Use 1.0 g/mL for the density of the solution and 4.18 J/g # C as the specific heat capacity.)

KC
Kevin C.
Numerade Educator

Problem 70

Instant cold packs used to ice athletic injuries on the field contain ammonium nitrate and water separated by a thin plastic divider. When the divider is broken, the ammonium nitrate dissolves according to the endothermic reaction:

In order to measure the enthalpy change for this reaction, 1.25 g of NH4NO3 is dissolved in enough water to make 25.0 mL of solution. The initial temperature is 25.8 C, and the final temperature (after the solid dissolves) is 21.9 C. Calculate the change in enthalpy for the reaction in kJ. (Use 1.0 g/mL as the density of the solution and 4.18 J/g # C as the specific heat capacity.)

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 71

For each generic reaction, determine the value of H2 in terms ofH1.

KC
Kevin C.
Numerade Educator

Problem 72

Consider the generic reaction:

Determine the value of H for each related reaction:

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 73

Calculate Hrxn for the reaction:

Use the following reactions and given H values:

KC
Kevin C.
Numerade Educator

Problem 74

Calculate Hrxn for the reaction:

Use the following reactions and given H values:

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 75

Calculate Hrxn for the reaction:

Use the following reactions and given H values:

KC
Kevin C.
Numerade Educator

Problem 76

Calculate Hrxn for the reaction:

Use the following reactions and given H values:

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 77

Hydrogenation reactions are used to add hydrogen across double bonds in hydrocarbons and other organic compounds. Use average bond energies to calculate Hrxn for the hydrogenation reaction.

KC
Kevin C.
Numerade Educator

Problem 78

Ethanol is a possible fuel. Use average bond energies to calculate Hrxn for the combustion of ethanol:

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 79

Hydrogen, a potential future fuel, can be produced from carbon (from coal) and steam by this reaction:

Use average bond energies to calculate Hrxn for the reaction, and then use standard enthalpies to calculate Hrxn. Why are the two values different and which value is more accurate?

KC
Kevin C.
Numerade Educator

Problem 80

Hydroxyl radicals react with and eliminate many atmospheric pollutants. However, the hydroxyl radical does not clean up everything. For example, chlorofluorocarbons-which destroy stratospheric ozone-are not attacked by the hydroxyl radical. Consider the hypothetical reaction by which the hydroxyl radical might react with a chlorofluorocarbon:

Use bond energies to explain why this reaction is improbable.

ME
Matthew E.
Numerade Educator

Problem 81

Write an equation for the formation of each compound from its elements in their standard states, and find Hf for each from Appendix IIB.
a. NH3(g)
b. CO2(g)
c. Fe2O3(s)
d. CH4(g)

KC
Kevin C.
Numerade Educator

Problem 82

Write an equation for the formation of each compound from its elements in their standard states, and find Hrxn for each from Appendix IIB.
a. NO2(g)
b. MgCO3(s)
c. C2H4(g)
d. CH3OH(l)

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 83

Hydrazine (N2H4) is a fuel used by some spacecraft. It is normally oxidized by N2O4 according to the equation:

Calculate Hrxn for this reaction using standard enthalpies of formation.

KC
Kevin C.
Numerade Educator

Problem 84

Pentane (C5H12) is a component of gasoline that burns according to the following balanced equation:

Calculate Hrxn for this reaction using standard enthalpies of formation. (The standard enthalpy of formation of liquid pentane is -146.8 kJ>mol.)

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 85

Use standard enthalpies of formation to calculate Hrxn for each reaction.

Aadit S.
Numerade Educator

Problem 86

Use standard enthalpies of formation to calculate Hrxn for each reaction.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 87

During photosynthesis, plants use energy from sunlight to form glucose (C6H12O6) and oxygen from carbon dioxide and water. Write a balanced equation for photosynthesis and calculate Hrxn.

Ronald P.
Numerade Educator

Problem 88

Ethanol can be made from the fermentation of crops and has been used as a fuel additive to gasoline. Write a balanced equation for the combustion of ethanol and calculate Hrxn.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 89

Top fuel dragsters and funny cars burn nitromethane as fuel according to the balanced combustion equation:

The enthalpy of combustion for nitromethane is -709.2 kJ>mol. Calculate the standard enthalpy of formation (Hf) for nitromethane.

KC
Kevin C.
Numerade Educator

Problem 90

The explosive nitroglycerin (C3H5N3O9) decomposes rapidly upon ignition or sudden impact according to the balanced equation:

Calculate the standard enthalpy of formation (Hf ) for nitroglycerin.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 91

Explain the trend in the lattice energies (shown here) of the alkaline earth metal oxides.

KC
Kevin C.
Numerade Educator

Problem 92

Rubidium iodide has a lattice energy of -617 kJ>mol, while potassium bromide has a lattice energy of -671 kJ>mol. Why is the lattice energy of potassium bromide more exothermic than the lattice energy of rubidium iodide?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 93

The lattice energy of CsF is -744 kJ>mol whereas that of BaO is-3029 kJ>mol. Explain this large difference in lattice energy.

KC
Kevin C.
Numerade Educator

Problem 94

Arrange these compounds in order of increasing magnitude of lattice energy: KCl, SrO, RbBr, CaO.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 95

Use the Born-Haber cycle and data from Appendix IIB and Chapters 3 and 9 to calculate the lattice energy of KCl. (Hsub for potassium is 89.0 kJ>mol.)

Aadit S.
Numerade Educator

Problem 96

Use the Born-Haber cycle and data from Appendix IIB and Table 9.3 to calculate the lattice energy of CaO. (Hsub for calcium is 178 kJ>mol; IE1 and IE2 for calcium are 590 kJ>mol and 1145 kJ>mol, respectively; EA1 and EA2 for O are -141 kJ>mol and 744 kJ>mol, respectively.)

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 97

The kinetic energy of a rolling billiard ball is given by KE = 12 mv2. Suppose a 0.17-kg billiard ball is rolling down a pool table with an initial speed of 4.5 m>s. As it travels, it loses some of its energy as heat. The ball slows down to 3.8 m>s and then collides head-on with a second billiard ball of equal mass. The first billiard ball completely stops, and the second one rolls away with a velocity of 3.8 m>s. Assume the first billiard ball is the system and calculate w, q, and E for the process.

Aadit S.
Numerade Educator

Problem 98

A 100-W light bulb is placed in a cylinder equipped with a moveable piston. The light bulb is turned on for 0.015 hour, and the assembly expands from an initial volume of 0.85 L to a final volume of 5.88 L against an external pressure of 1.0 atm. Use the wattage of the light bulb and the time it is on to calculate E in joules (assume that the cylinder and light bulb assembly is the system and assume two significant figures). Calculate w and q.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 99

Evaporating sweat cools the body because evaporation is an endothermic process:

Estimate the mass of water that must evaporate from the skin to cool the body by 0.50 C. Assume a body mass of 95 kg and assume that the specific heat capacity of the body is 4.0 J>g # C.

KC
Kevin C.
Numerade Educator

Problem 100

LP gas burns according to the exothermic reaction:

What mass of LP gas is necessary to heat 1.5 L of water from room temperature (25.0 C) to boiling (100.0 C)? Assume that during heating, 15% of the heat emitted by the LP gas combustion goes to heat the water. The rest is lost as heat to the surroundings.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 101

Use standard enthalpies of formation to calculate the standard change in enthalpy for the melting of ice. (The Hf for H2O(s) is -291.8 kJ>mol.) Use this value to calculate the mass of ice required to cool 355 mL of a beverage from room temperature (25.0 C) to 0.0 C. Assume that the specific heat capacity and
density of the beverage are the same as those of water.

KC
Kevin C.
Numerade Educator

Problem 102

Dry ice is solid carbon dioxide. Instead of melting, solid carbon
dioxide sublimes according to the equation:

When dry ice is added to warm water, heat from the water causes the dry ice to sublime more quickly. The evaporating carbon dioxide produces a dense fog often used to create special effects. In a simple dry ice fog machine, dry ice is added to warm water in a Styrofoam cooler. The dry
ice produces fog until it evaporates away, or until the water gets too cold to sublime the dry ice quickly enough. A small Styrofoam cooler holds 15.0 L of water heated to 85 C. Use standard enthalpies of formation to calculate the change in enthalpy for dry ice sublimation, and calculate the mass of dry ice that should be added to the water so that the dry ice completely sublimes away when the water reaches 25 C. Assume no heat loss to the surroundings. (The Hf for CO2(s) is -427.4 kJ>mol.)

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 103

A 25.5-g aluminum block is warmed to 65.4 C and plunged into an insulated beaker containing 55.2 g water initially at 22.2 C. The aluminum and the water are allowed to come to thermal equilibrium. Assuming that no heat is lost, what is the final temperature of the water and aluminum?

KC
Kevin C.
Numerade Educator

Problem 104

We mix 50.0 mL of ethanol (density = 0.789 g>mL) initially at 7.0 C with 50.0 mL of water (density = 1.0 g>mL) initially at 28.4 C in an insulated beaker. Assuming that no heat is lost, what is the final temperature of the mixture?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 105

Palmitic acid (C16H32O2) is a dietary fat found in beef and butter. The caloric content of palmitic acid is typical of fats in general. Write a balanced equation for the complete combustion of palmitic acid and calculate the standard enthalpy of combustion. What is the caloric content of palmitic acid in Cal >g? Do the same calculation for table sugar (sucrose, C12H22O11). Which dietary substance (sugar or fat) contains more Calories per gram? The standard enthalpy of formation of palmitic acid is -208 kJ>mol, and that of sucrose is -2226.1 kJ>mol. (Use H2O(l) in the balanced chemical equations because the metabolism of these compounds produces liquid water.)

Aadit S.
Numerade Educator

Problem 106

Hydrogen and methanol have both been proposed as alternatives to hydrocarbon fuels. Write balanced reactions for the complete combustion of hydrogen and methanol and use standard enthalpies of formation to calculate the amount of heat released per kilogram of the fuel. Which fuel contains the most energy in the least mass? How does the energy of these fuels compare to that of octane (C8H18)?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 107

One tablespoon of peanut butter has a mass of 16 g. It is combusted in a calorimeter whose heat capacity is 120.0 kJ>C. The temperature of the calorimeter rises from 22.2 C to 25.4 C. Find the food caloric content of peanut butter.

KC
Kevin C.
Numerade Educator

Problem 108

A mixture of 2.0 mol of H2(g) and 1.0 mol of O2(g) is placed in a sealed evacuated container made of a perfect insulating material at 25 C. The mixture is ignited with a spark, and it reacts to form liquid water. Determine the temperature of the water.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 109

A 20.0-L volume of an ideal gas in a cylinder with a piston is at a pressure of 3.0 atm. Enough weight is suddenly removed from the piston to lower the external pressure to 1.5 atm. The gas then expands at constant temperature until its pressure is 1.5 atm. Find E, H, q, and w for this change in state.

KC
Kevin C.
Numerade Educator

Problem 110

When we burn 10.00 g of phosphorus in O2(g) to form P4O10(s), we generate enough heat to raise the temperature of 2950 g of water from 18.0 C to 38.0 C. Calculate the enthalpy of formation of P4O10(s) under these conditions.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 111

The H for the oxidation of S in the gas phase to SO3(g) is -204 kJ>mol, and for the oxidation of SO2(g) to SO3(g) it is 89.5kJ>mol. Find the enthalpy of formation of SO2(g) under these conditions.

KC
Kevin C.
Numerade Educator

Problem 112

The Hof of TiI3(s) is -328 kJ>mol, and the H for the reaction 2 Ti(s) + 3 I2(g) 2 TiI3(s) is -839 kJ. Calculate the H of sublimation (the state transition from solid to gas) of I2(s), which is a solid at 25 C.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 113

A copper cube measuring 1.55 cm on edge and an aluminum cube measuring 1.62 cm on edge are both heated to 55.0 C and submerged in 100.0 mL of water at 22.2 C. What is the final temperature of the water when equilibrium is reached? (Assume a density of 0.998 g>mL for water.)

KC
Kevin C.
Numerade Educator

Problem 114

A pure gold ring and pure silver ring have a total mass of 14.9 g. We heat the two rings to 62.0 C and drop them into 15.0 mL of water at 23.5 C. When equilibrium is reached, the temperature of the water is 25.0 C. What is the mass of each ring? (Assume a density of 0.998 g>mL for water.)

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 115

The reaction of Fe2O3(s) with Al(s) to form Al2O3(s) and Fe(s) is called the thermite reaction and is highly exothermic. What role does lattice energy play in the exothermicity of the reaction?

KC
Kevin C.
Numerade Educator

Problem 116

NaCl has a lattice energy -787 kJ>mol. Consider a hypothetical salt XY. X3+ has the same radius as Na+, and Y3- has the same radius as Cl-. Estimate the lattice energy of XY.

BW
Brandon W.
The University of Tennessee - Knoxville

Problem 117

If hydrogen were used as a fuel, it could be burned according to this reaction:

Use average bond energies to calculate Hrxn for this reaction and also for the combustion of methane (CH4). Which fuel yields more energy per mole? Per gram?

KC
Kevin C.
Numerade Educator

Problem 118

Calculate Hrxn for the combustion of octane (C8H18), a component of gasoline, by using average bond energies, and then calculate it using enthalpies of formation from Appendix IIB. What is the percent difference between your results? Which result would you expect to be more accurate?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 119

The heat of atomization is the heat required to convert a molecule in the gas phase into its constituent atoms in the gas phase. The heat of atomization is used to calculate average bond energies. Without using any tabulated bond energies, calculate the average C-Cl bond energy from the following data: The heat of atomization of CH4 is 1660 kJ>mol, and that of CH2Cl2 is 1495 kJ>mol.

RI
Rezoanul I.
Numerade Educator

Problem 120

Calculate the heat of atomization (see previous problem) of C2H3Cl, using the average bond energies in Table 9.3.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 121

A typical frostless refrigerator uses 655 kWh of energy per year in the form of electricity. Suppose that all of this electricity is generated at a power plant that burns coal containing 3.2% sulfur by mass and that all of the sulfur is emitted as SO2 when the coal is burned. If all of the SO2 goes on to react with rainwater to form H2SO4, what mass of H2SO4 does the annual operation of the refrigerator produce? (Hint: Assume that the remaining percentage of the coal is carbon and begin by calculating Hrxn for the combustion of carbon.)

Aadit S.
Numerade Educator

Problem 122

A large sport utility vehicle has a mass of 2.5 * 103 kg. Calculate the mass of CO2 emitted into the atmosphere upon accelerating the SUV from 0.0 mph to 65.0 mph. Assume that the required energy comes from the combustion of octane with 30% efficiency. (Hint: Use KE = 12 mv2 to calculate the kinetic energy required for the acceleration.)

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 123

Combustion of natural gas (primarily methane) occurs in most household heaters. The heat given off in this reaction is used to raise the temperature of the air in the house. Assuming that all the energy given off in the reaction goes to heating up only the air in the house, determine the mass of methane required to heat the air in a house by 10.0 C. Assume that the house dimensions are 30.0 m * 30.0 m * 3.0 m; specific heat capacity of air is 30 J>K # mol; and 1.00 mol of air occupies 22.4 L for all temperatures concerned.

Aadit S.
Numerade Educator

Problem 124

When backpacking in the wilderness, hikers often boil water to sterilize it for drinking. Suppose that you are planning a backpacking trip and will need to boil 35 L of water for your group. What volume of fuel should you bring? Assume that the fuel has an average formula of C7H16; 15% of the heat generated from combustion goes to heat the water (the rest is lost to the surroundings); the density of the fuel is 0.78 g>mL; the initial temperature of the water is 25.0 C; and the standard enthalpy of formation of C7H16 is -224.4 kJ>mol.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 125

An ice cube of mass 9.0 g is added to a cup of coffee. The coffee's initial temperature is 90.0 degree C and the cup contains 120.0 g of liquid. Assume the specific heat capacity of the coffee is the same as that of water. The heat of fusion of ice (the heat associated with ice melting) is 6.0 kJ/mol. Find the temperature of the coffee after the ice melts.

KC
Kevin C.
Numerade Educator

Problem 126

Find H, E, q, and w for the freezing of water at -10.0 C. The specific heat capacity of ice is 2.04 J/g # C and its heat of fusion (the quantity of heat associated with melting) is -332 J/g.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 127

The heat of vaporization of water at 373 K is 40.7 kJ>mol. Find q, w, E, and H for the evaporation of 454 g of water at this temperature at 1 atm.

Aadit S.
Numerade Educator

Problem 128

Find H for the combustion of ethanol (C2H6O) to carbon dioxide and liquid water from the following data. The heat capacity of the bomb calorimeter is 34.65 kJ/K, and the combustion of 1.765 g of ethanol raises the temperature of the calorimeter from 294.33 K to 295.84 K.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 129

The main component of acid rain (H2SO4) forms from SO2, a pollutant in the atmosphere, via these steps:

Draw the Lewis structure for each of the species in these steps and use bond energies and Hess's law to estimate Hrxn for the overall process. (Use 265 kJ/mol for the S-O single-bond energy.)

Aadit S.
Numerade Educator

Problem 130

Use average bond energies together with the standard enthalpy of formation of C(g) (718.4 kJ>mol) to estimate the standard enthalpy of formation of gaseous benzene, C6H6(g). (Remember that average bond energies apply to the gas phase only.) Compare the value you obtain using average bond energies to the actual standard enthalpy of formation of gaseous benzene, 82.9 kJ>mol. What does the difference between these two values tell you about the stability of benzene?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 131

The standard heat of formation of CaBr2 is -675 kJ>mol. The first ionization energy of Ca is 590 kJ>mol, and its second ionization energy is 1145 kJ/mol. The heat of sublimation of Ca [Ca(s) Ca(g)] is 178 kJ>mol. The bond energy of Br2 is 193 kJ/mol, the heat of vaporization of Br2(l) is 31 kJ/mol, and the electron affinity of Br is -325 kJ>mol. Calculate the lattice energy of CaBr2.

Aadit S.
Numerade Educator

Problem 132

The standard heat of formation of PI3(s) is -24.7 kJ/mol, and the PI bond energy in this molecule is 184 kJ/mol. The standard heat of formation of PI is 334 kJ/mol, and that of I2(g) is 62 kJ/mol. The I2 bond energy is 151 kJ/mol. Calculate the heat of sublimation of PI3[PI3(s) PI3(g)].

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 133

Which statement is true of the internal energy of the system and its surroundings following a process in which Esys = +65 kJ? Explain.
a. The system and the surroundings both lose 65 kJ of energy.
b. The system and the surroundings both gain 65 kJ of energy.
c. The system loses 65 kJ of energy, and the surroundings gain 65 kJ of energy.
d. The system gains 65 kJ of energy, and the surroundings lose 65 kJ of energy.

KC
Kevin C.
Numerade Educator

Problem 134

Which expression describes the heat emitted in a chemical reaction when the reaction is carried out at constant pressure? Explain.
a. E - w
b. E
c. E - q

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 135

Two identical refrigerators are plugged in for the first time. Refrigerator A is empty (except for air), and refrigerator B is filled with jugs of water. The compressors of both refrigerators immediately turn on and begin cooling the interiors of the refrigerators. After two hours, the compressor of refrigerator A turns off while the compressor of refrigerator B continues to run. The next day, the compressor of refrigerator A can be heard turning on and off every few minutes, while the compressor of refrigerator B turns off and on every hour or so (and stays on longer each time). Explain these observations.

KC
Kevin C.
Numerade Educator

Problem 136

A 1-kg cylinder of aluminum and a 1-kg jug of water, both at room temperature, are put into a refrigerator. After one hour, the temperature of each object is measured. One of the objects is much cooler than the other. Which one is cooler and why?

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 137

Two substances A and B, initially at different temperatures, are thermally isolated from their surroundings and allowed to come into thermal contact. The mass of substance A is twice the mass of substance B, but the specific heat capacity of substance B is four times the specific heat capacity of substance A. Which substance will undergo a larger change in temperature?

KC
Kevin C.
Numerade Educator

Problem 138

When 1 mol of a gas burns at constant pressure, it produces 2418 J of heat and does 5 J of work. Determine E, H, q, and w for the process.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 139

In an exothermic reaction, the reactants lose energy and the reaction feels hot to the touch. Explain why the reaction feels hot even though the reactants are losing energy. Where does the energy come from?

KC
Kevin C.
Numerade Educator

Problem 140

Which statement is true of a reaction in which V is positive?
Explain.
a. H = E
b. H 7 E
c. H 6 E

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 141

Which statement is true of an endothermic reaction?
a. Strong bonds break and weak bonds form.
b. Weak bonds break and strong bonds form.
c. The bonds that break and those that form are of approximately the same strength.

KC
Kevin C.
Numerade Educator

Problem 142

When a firecracker explodes, energy is obviously released. The compounds in the firecracker can be viewed as being "energy rich." What does this mean? Explain the source of the energy in terms of chemical bonds.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 143

Have each group member write a problem involving the transfer of heat from one material in Table 9.2 to another material in the table. Working as a group, solve each problem. The group member who wrote the problem in question may act as the group facilitator when the group is working on that problem. What do all of your problems have in common? How do they differ?

Check back soon!

Problem 144

Classify each process as endothermic or exothermic. What is the sign of H for each process? Explain your answers.
a. gasoline burning in an engine
b. steam condensing on a mirror
c. water boiling in a pot
Have each member of your group provide an additional example. Provide at least two examples of exothermic processes and two additional examples of endothermic processes.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 145

A propane tank on a home barbeque contains 10.4 * 103 g of propane.
a. Write the balanced chemical reaction for the combustion of gaseous propane (C3H8) to form water vapor and gaseous carbon dioxide.
b. Use the value for Hrxn provided in the text to calculate the total amount of heat produced when the entire contents of the tank of propane is burned.
c. What mass of water could be warmed from 25 C to 100 C with this much heat?

KC
Kevin C.
Numerade Educator

Problem 146

Solid carbon-C(s, graphite), gaseous hydrogen-H2(g), and the sugar glucose-C6H12O6(s) are all burned with oxygen in a bomb calorimeter, and the amount of heat given off is determined for each process. How can these data obtained be used to determine the heat of formation of glucose? Your answer should include both chemical reactions and complete sentences.

ES
Eugene S.
University of Minnesota - Twin Cities

Problem 147

Consider the decomposition of liquid hydrogen peroxide (H2O2) to form water and oxygen.
a. What is the heat of formation for hydrogen peroxide?
b. What is the heat of formation for liquid water?
c. What is the heat of formation for gaseous oxygen? Why?
d. Write the balanced chemical equations that correspond to the H values you looked up for parts a, b, and c.
e. Write the balanced chemical equation for the decomposition of hydrogen peroxide to form water and oxygen. (Write the equation such that the coefficient on oxygen is 1.)
f. What is the heat of reaction for the process in part e?
g. Draw a scale diagram of this reaction in which 1 cm = 100 kJ showing the relative energies of reactants (on the left), products (on the right), and the elements in their most stable states (in the middle). Label all the energies you know.

Aadit S.
Numerade Educator

Problem 148

The heating value of combustible fuels is evaluated based on the quantities known as the higher heating value (HHV) and the lower heating value (LHV). The HHV has a higher absolute value and assumes that the water formed in the combustion reaction is formed in the liquid state. The LHV has a lower absolute value and assumes that the water formed in the combustion reaction is formed in the gaseous state. The LHV is therefore the sum of the HHV (which is negative) and the heat of vaporization of water for the number of moles of water formed in the reaction (which is positive). The table on the right lists the enthalpy of combustion-which is equivalent to the HHV-for several closely related hydrocarbons.
Use the information in the table at right to answer the following questions:
a. Write two balanced equations for the combustion of C3H8;= one equation assuming the formation of liquid water and the other equation assuming the formation of gaseous water.
b. Given that the heat of vaporization of water is 44.0 kJ/mol, what is Hrxn for each of the reactions in part a? Which quantity is the HHV? The LLV?
c. When propane is used to cook in an outdoor grill, is the amount of heat released the HHV or the LLV? What amount of heat is released upon combustion of 1.00 kg of propane in an outdoor grill?
d. For each CH2 unit added to a linear alkane, what is the average increase in the absolute value of Hcomb?

ES
Eugene S.
University of Minnesota - Twin Cities