chapter 6 textbook questions

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.

5.7 Cal /g

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 H rxn

6 CO2(g) + 6 H2O(l)-->C6H12O6(s) + 6 O2(g), Hrxn = 2803 kJ

A 12.5 g sample of granite initially at 82.0 C is immersed into 25.0 g of water initially at 22.0 C. What is the final temperature of both substances when they reach thermal equilibrium? (For water, Cs = 4.18 J>g C and for granite, Cs = 0.790 J/g C. ) a) 52.0 C b) 1.55 * 10^3 C c) 15.7 C d) 27.2 C

d

The citizens of the world burn the fossil fuel equivalent of 7 * 10^12 kg of petroleum per year. Assume that all of this petroleum is in the form of octane (C8H18) and calculate how much CO2 (in kg) is produced by world fossil fuel combustion per year. (Hint: Begin by writing a balanced equation for the combustion of octane.) If the atmosphere currently contains approximately 3 * 1015 kg of CO2 , how long will it take for the world's fossil fuel combustion to double the amount of atmospheric carbon dioxide?

2 * 10^13 kg CO2 produced per year, 150 years

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.

q = -2418 J, w = -5 kJ, H = -2418 J/mol, E = -2423 J/mol

Identify each process as endothermic or exothermic and indicate the sign of H . (a) sweat evaporating from skin (b) water freezing in a freezer (c) wood burning in a fire

(a) Sweat evaporating from skin cools the skin and is therefore endothermic, with a positive H . The skin must supply heat to the perspiration in order for it to continue to evaporate. (b) Water freezing in a freezer releases heat and is therefore exothermic, with a negative H . The refrigeration system in the freezer must remove this heat for the water to continue to freeze. (c) Wood burning in a fire releases heat and is therefore exothermic, with a negative H .

One way to evaluate fuels with respect to global warming is to determine how much heat they release during combustion relative to how much CO 2 they produce. The greater the heat relative to the amount of CO 2 , the better the fuel. Use the combustion reactions of carbon, natural gas, and octane, in combination with the enthalpy of combustion for each reaction (all given earlier), to calculate the heat (in kJ) released by each fuel per 1.00 kg of CO 2 produced.

-1.44 * 10^4 kJ

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

311 kJ

What mass of butane in grams is necessary to produce 1.5 * 10^3 kJ of heat? What mass of CO2 is produced? C4H10(g) + 13/2 O2(g)-->4 CO2(g) + 5 H2O(g) Hrxn = -2658 kJ

33 g C4H10 99 g CO2

Suppose you are cold-weather camping and decide to heat some objects to bring into your sleeping bag for added warmth. You place a large water jug and a rock of equal mass near the fire. Over time, both the rock and the water jug warm to about 38 C (100 F). If you could bring only one into your sleeping bag, which one should you choose to keep you the warmest? Why?

Bring the water; it has the higher heat capacity and will therefore release more heat as it cools.

When fuel is burned in a cylinder equipped with a piston, the volume expands from 0.255 L to 1.45 L against an external pressure of 1.02 atm. In addition, 875 J is emitted as heat. What is E for the burning of the fuel?

E = -998 J

When 1.550 g of liquid hexane 1C6H142 undergoes combustion in a bomb calorimeter, the temperature rises from 25.87 C to 38.13 C. Find Erxn for the reaction in kJ/mol hexane. The heat capacity of the bomb calorimeter, determined in a separate experiment, is 5.73 kJ/C.

Ereaction = -3.91 * 10^3 kJ/mol C6H14

Use standard enthalpies of formation to calculate the standard change in enthalpy for the melting of ice. (The H f 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.

H = 6.0 kJ/mol, 1.1 * 102 g

A cylinder equipped with a piston expands against an external pressure of 1.58 atm. If the initial volume is 0.485 L and the final volume is 1.245 L, how much work (in J) is done?

-122 J

Calculate Hrxn for the reaction: 5 C(s) + 6 H2(g)-->C5H12(l) Use the following reactions and given H 's. C5H12(l) + 8 O2(g)-->5 CO2(g) + 6 H2O(g) H = -3244.8 kJ C(s) + O2(g)-->CO2(g) H = -393.5 kJ 2 H2(g) + O2(g)-->2 H2O(g) H = -483.5 kJ

-173.2 kJ

To inflate a balloon you must do pressure-volume work on the surroundings. If you inflate a balloon from a volume of 0.100 L to 1.85 L against an external pressure of 1.00 atm, how much work is done (in joules)?

-177 J

Ammonia reacts with oxygen according to the equation: 4 NH3(g) + 5 O2(g)-->4 NO(g) + 6 H2O(g) Hrxn = -906 kJ Calculate the heat (in kJ) associated with the complete reaction of 155 g of NH3.

-2.06 * 10^3 kJ

Hydrazine (N2H4) is a fuel used by some spacecraft. It is normally oxidized by N2O4 according to the equation: N2H4(l) + N2O4(g-->h2 N2O(g) + 2 H2O(g) Calculate H rxn for this reaction using standard enthalpies of formation.

-382.1 kJ/mol

Consider the thermochemical equation for the combustion of acetone (C3H6O) , the main ingredient in nail polish remover. C3H6O(l) + 4 O2(g)-->3 CO2(g) + 3 H2O(g) H rxn = -1790 kJ 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.

-4.30 * 10^3 kJ

Magnesium metal reacts with hydrochloric acid according to the balanced equation: Mg(s) + 2 HCl(aq)-->MgCl2(aq) + H2(g) In an experiment to determine the enthalpy change for this reaction, 0.158 g of Mg metal is combined with enough HCl to make 100.0 mL of solution in a coffeecup calorimeter. The HCl is sufficiently concentrated so that the Mg completely reacts. The temperature of the solution rises from 25.6 C to 32.8 C as a result of the reaction. Find Hrxn for the reaction as written. Use 1.00 g/mL as the density of the solution and Cs,soln = 4.18 J/g # C as the specific heat capacity of the solution.

-4.6 * 10^5 J

Which statement is true of a reaction in which V is positive? Explain. a. H = E b. H > E c. H < E139

b. H > E

Write equations for the formation of (a) MgCO3(s) and (b) C6H12O6(s) from their respective elements in their standard states. Include the value of H f for each equation.

- 1273.3 kJ/mol

Find Hrxn for the reaction: 3 C(s) + 4 H2(g)-->C3H8(g) Use these reactions with known H 's: C3H8(g) + 5 O2(g)-->3 CO2(g) + 4 H2O(g) H = - 2043 kJ C(s) + O2(g)-->CO2(g) H = - 393.5 kJ 2 H2(g) + O2(g)-->2 H2O(g) H = - 483.6 kJ SOLUTION To work this and other Hess's law problems, manipulate the reactions with known H 's in such a way as to get the reactants of interest on the left, the products of interest on the right, and other species to cancel.

-105 kJ

Calculate Hrxn for the reaction: Fe2O3(s) + 3 CO(g)-->2 Fe(s) + 3 CO2(g) Use the following reactions and given H 's. 2 Fe(s) + 3>2 O2(g)-->Fe2O3(s) H = -824.2 kJ CO(g) + 1>2 O2(g)-->CO2(g) H = -282.7 kJ

-23.9 kJ

The H for the oxidation of S in the gas phase to SO3 is -204 kJ/mol and for the oxidation of SO2 to SO3 is 89.5 kJ/mol. Find the enthalpy of formation of SO2 under these conditions.

-294 kJ/mol

What mass of CO 2 (in kg) does the combustion of a 15-gallon tank of gasoline release into the atmosphere? Assume the gasoline is pure octane (C 8 H 18 ) and that it has a density of 0.70 g /mL.

1.2 * 10^2 kg CO2

Find Hrxn for the reaction: N2O(g) + NO2(g)-->3 NO(g) Use these reactions with known H 's: 2 NO(g) + O2(g)-->2 NO2(g) H = -113.1 kJ N2(g) + O2(g)-->2 NO(g) H = +182.6 kJ 2 N2O(g)-->2 N2(g) + O2(g) H = -163.2 kJ

N2O(g) + NO2(g)-->3 NO(g), Hrxn = +157.6 kJ

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

The aluminum is cooler because it has a lower heat capacity (specific heat).

Substances A and B, initially at different temperatures, come in contact with each other and reach thermal equilibrium. The mass of substance A is twice the mass of substance B. The specific heat capacity of substance B is twice the specific heat capacity of substance A. Which statement is true about the final temperature of the two substances once thermal equilibrium is reached? (a) The final temperature will be closer to the initial temperature of substance A than substance B. (b) The final temperature will be closer to the initial temperature of substance B than substance A. (c) The final temperature will be exactly midway between the initial temperatures of substances A and B.

(c) The specifi c heat capacity of substance B is twice that of A, but since the mass of B is half that of A, the quantity m * Cs will be identical for both substances so that the fi nal temperature is exactly midway between the two initial temperatures.

To determine whether a shiny gold-colored rock is actually gold, a chemistry student decides to measure its heat capacity. She first weighs the rock and finds it has a mass of 4.7 g. She then finds that upon absorption of 57.2 J of heat, the temperature of the rock rises from 25 C to 57 C. Find the specific heat capacity of the substance composing the rock and determine whether the value is consistent with the rock being pure gold.

Cs = 0.38 J/g # C The specific heat capacity of gold is 0.128 J/g # C; therefore, the rock cannot be pure gold.

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.

E = 0, H = 0, q = -w = 3.0 * 103 J

A cylinder and piston assembly (defined as the system) is warmed by an external flame. The contents of the cylinder expand, doing work on the surroundings by pushing the piston outward against the external pressure. If the system absorbs 559 J of heat and does 488 J of work during the expansion, what is the value of E ?

E = 71 J

Zinc metal reacts with hydrochloric acid according to the balanced equation: Zn(s) + 2 HCl(aq)-->ZnCl2(aq) + H2(g) 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.)

-1.6 * 10^5 J

Top fuel dragsters and funny cars burn nitromethane as fuel according to the balanced combustion equation: 2 CH3NO2(l) + 3>2O 2(g)-->2 CO2(g) + 3 H2O(l) + N2(g) H rxn = -1418 kJ The enthalpy of combustion for nitromethane is -709.2 kJ/mol. Calculate the standard enthalpy of formation 1H f 2 for nitromethane.

-113.0 kJ/mol

The heat of combustion of liquid octane (C8H18) to carbon dioxide and liquid water at 298 K is -1303 kJ/mol. Find E for this reaction.

-1292 kJ

The chemical hand warmers described in Section 6.1 produce heat when they are removed from their airtight plastic wrappers. Recall that they utilize the oxidation of iron to form iron oxide according to the reaction 4 Fe(s) + 3 O2(g)-->2 Fe2O3(s) . Calculate H rxn for this reaction and calculate how much heat is produced from a hand warmer containing 15.0 g of iron powder.

-1648.4 kJ 111 kJ emitted (-111 kJ)

The firing of a potato cannon provides a good example of the heat and work associated with a chemical reaction. A potato is stuffed into a long cylinder that is capped on one end and open at the other. Some kind of fuel is introduced under the potato at the capped end—usually through a small hole—and ignited. The potato then shoots out of the cannon, sometimes flying hundreds of feet, and the cannon emits heat to the surroundings. If the burning of the fuel performs 855 J of work on the potato and produces 1422 J of heat, what is E for the burning of the fuel? (Note: A potato cannon can be dangerous and should not be constructed without proper training and experience.)

-2277 J

When 1.010 g of sucrose 1C12H22O112 undergoes combustion in a bomb calorimeter, the temperature rises from 24.92 C to 28.33 C. Find Erxn for the combustion of sucrose in kJ/mol sucrose. The heat capacity of the bomb calorimeter, determined in a separate experiment, is 4.90 kJ/C. (You can ignore the heat capacity of the small sample of sucrose because it is negligible compared to the heat capacity of the calorimeter.)

-5.66 * 10^3 kJ/mol C12H22O11

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.

-6.3 * 10^3 kJ/mol

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?

-7.27 * 10^2 kJ

The thermite reaction, in which powdered aluminum reacts with iron oxide, is highly exothermic. 2 Al(s) + Fe2O3(s)-->Al2O3(s) + 2 Fe(s) Use standard enthalpies of formation to find H rxn for the thermite reaction.

-851.5 kJ

Find Hrxn for the reaction: 3 H2(g) + O3(g)-->3 H2O(g) Use these reactions with known H 's: 2 H2(g) + O2(g)-->2 H2O(g) H = -483.6 kJ 3 O2(g)-->2 O3(g) H = +285.4 kJ

-868.1 kJ

Use the standard enthalpies of formation to determine H rxn for the reaction: 4 NH3(g) + 5 O2(g)-->4 NO(g) + 6 H2O(g)

-902.0 kJ

How much heat is required to warm 1.50 L of water from 25.0 C to 100.0 C? (Assume a density of 1.0 g/mL for the water.)

4.7 * 10^5 J

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?

489 J

Suppose you find a penny (minted before 1982, when pennies were almost entirely copper) in the snow. How much heat is absorbed by the penny as it warms from the temperature of the snow, which is -8.0 C , to the temperature of your body, 37.0 C? Assume the penny is pure copper and has a mass of 3.10 g.

53.7 J

Nitromethane (CH 3 NO 2 ) burns in air to produce significant amounts of heat. 2 CH3NO2(l) + 3>2 O2(g)-->2 CO2(g) + 3 H2O(l) + N2(g) Hrxn = -1418 kJ How much heat is produced by the complete reaction of 5.56 kg of nitromethane?

6.46 * 10^4 kJ

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.

7.2 * 10^2 g

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 Ho rxn for the combustion of carbon.)

7.3 * 10^3 g H2SO4

Evaporating sweat cools the body because evaporation is an endothermic process: H2O(l)-->H2O(g) Horxn = +44.01 kJ 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 .

78 g

An ice cube of mass 9.0 g is added to a cup of coffee. The coffee's initial temperature is 90.0 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.

78.2 C

The propane fuel (C3H8) used in gas barbeques burns according to the thermochemical equation: C3H8(g)+5O2(g)-->3CO2(g)+4H2O(g) H rxn = -2217 kJ If a pork roast must absorb 1.6 * 10^3 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?

9.5 * 10^2 g CO2

A gaseous fuel mixture contains 25.3% methane (CH4) , 38.2% ethane (C2H6) , and the rest propane (C3H8) by volume. When the fuel mixture contained in a 1.55 L tank, stored at 755 mmHg and 298 K, undergoes complete combustion, how much heat is emitted? (Assume that the water produced by the combustion is in the gaseous state.)

94.0 kJ

The kinetic energy of a rolling billiard ball is given by KE = 1/2 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.

E = -1.7 J, q = -0.5 J, w = -1.2 J

Derive a relationship between H and E for a process in which the temperature of a fixed amount of an ideal gas changes.

H = E + nRT

The addition of hydrochloric acid to a silver nitrate solution precipitates silver chloride according to the reaction: AgNO3(aq) + HCl(aq)-->AgCl(s) + HNO3(aq) When 50.0 mL of 0.100 M AgNO3 is combined with 50.0 mL of 0.100 M HCl in a coffee-cup calorimeter, the temperature changes from 23.40 C to 24.21 C. Calculate Hrxn for the reaction as written. Use 1.00 g/mL as the density of the solution and C = 4.18 J/g # C as the specific heat capacity.

Hrxn = -68 kJ

Identify each energy exchange as primarily heat or work and determine whether the sign of E is 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 is the system.) c. An aqueous chemical reaction mixture is warmed with an external flame. (The reaction mixture is the system.)

a. heat, + b. work, - c. heat, +

How much heat must be absorbed by a 15.0 g sample of water to raise its temperature from 25.0 C to 55.0 C? (For water, C s = 4.18 J/g C.) a) 1.57 kJ b) 1.88 kJ c) 3.45 kJ d) 107 J

b

Hydrogen gas reacts with oxygen to form water. 2 H2(g) + O2(g)-->2 H2O(g) H = -483.5 kJ Determine the minimum mass of hydrogen gas required to produce 226 kJ of heat. a) 8.63 g b) 1.88 g c) 0.942 g d) 0.935 g

b

Which fuel is not a fossil fuel? a) coal b) hydrogen c) natural gas d) petroleum

b

Consider the reactions: A -->2 B H1 A --> 3 C H2 What is H for the reaction 2 B h 3 C ? a) Δ H 1 + Δ H 2 b) Δ H 1 - Δ H 2 c) Δ H 2 - Δ H 1 d) 2 * (Δ H 1 + Δ H 2 )

c

Which sample is most likely to undergo the smallest change in temperature upon the absorption of 100 kJ of heat? a) 15 g water b) 15 g lead c) 50 g water d) 50 g lead

c

A cylinder with a moving piston expands from an initial volume of 0.250 L against an external pressure of 2.00 atm. The expansion does 288 J of work on the surroundings. What is the fi nal volume of the cylinder? a) 1.42 L b) 1.17 L c) 144 L d) 1.67 L

d

The standard enthalpy of formation for glucose [C 6 H 12 O 6 ( s )] is -1273.3 kJ/mol. What is the correct formation equation corresponding to this H f ? a) 6C(s, graphite) + 6H2O(g)-->C6H12O6(s, glucose) b) 6C(s, graphite) + 6H2O(l) --> C6H12O6(s, glucose) c) 6C(s,graphite)+6H2(l)+3O2(l)-->C6H12O6(s, glucose) d) 6C(s, graphite)+6H2(g)+3O2(g)-->C6H12O6(s,glucose)

d

Use standard enthalpies of formation to determine Hrxn for the reaction: Fe2O3(s) + 3 CO(g) --> 2 Fe(s) + 3 CO2(g) a) -541.2 kJ b) -2336 kJ c) -541.2 kJ d) -24.8 kJ

d

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.

d

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

d

A 31.1 g wafer of pure gold, initially at 69.3 C, is submerged 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?

final temperature 28.4 C

A silver block, initially at 58.5 C, is submerged 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?

mass of silver 77.1 grams

An LP gas tank in a home barbeque contains 13.2 kg of propane, C3H8 . Calculate the heat (in kJ) associated with the complete combustion of all of the propane in the tank. C3H8(g) + 5 O2(g)-->3 CO2(g) + 4 H2O(g) Hrxn = -2044 kJ

- 6.12 * 10^5 kJ

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?

-2.8 * 10^2 J

A 32.5 g cube of aluminum initially at 45.8 C is submerged into 105.3 g of water at 15.4 C. What is the final temperature of both substances at thermal equilibrium? (Assume that the aluminum and the water are thermally isolated from everything else.)

17.3 C

A city of 100,000 people uses approximately 1.0 * 10^11 kJ of energy per day. Suppose all of that energy comes from the combustion of liquid octane 1C8H182 to form gaseous water and gaseous carbon dioxide. Use standard enthalpies of formation to calculate H rxn for the combustion of octane and then determine how many kilograms of octane would be necessary to provide this amount of energy.

2.3 * 10^6 kg C8H18

If an endothermic reaction absorbs heat, then why does it feel cold to the touch?

An endothermic reaction feels cold to the touch because the reaction (acting here as the system) absorbs heat from the surroundings. When you touch the vessel in which the reaction occurs, you, being part of the surroundings, lose heat to the system (the reaction), which makes you feel cold. The heat absorbed by the reaction (from your body, in this case) does not increase its temperature, but rather becomes potential energy stored in chemical bonds.

The combustion of toluene has a Erxn of -3.91 * 103^ kJ/mol . When 1.55 g of toluene 1C7H82 undergoes combustion in a bomb calorimeter, the temperature rises from 3.12 C to 37.57 C. Find the heat capacity of the bomb calorimeter.

Ccal = 4.55 kJ/C

Starting from the relationship between temperature and kinetic energy for an ideal gas, find the value of the molar heat capacity of an ideal gas when its temperature is changed at constant volume. Find its molar heat capacity when its temperature is changed at constant pressure.

Cv =3/2R, Cp =5/2R

When 1 mol of a fuel burns at constant pressure, it produces 3452 kJ of heat and does 11 kJ of work. What are E and H for the combustion of the fuel?

E = -3463 J, H = -3452 kJ

Lighters are usually fueled by butane 1C4H102 . When 1 mole of butane burns at constant pressure, it produces 2658 kJ of heat and does 3 kJ of work. What are the values of H and E for the combustion of one mole of butane?

H represents only the heat exchanged; therefore, H = -2658 kJ . E represents the heat and work exchanged; therefore, E = -2661 kJ . The signs of both H and E are negative because heat and work are fl owing out of the system and into the surroundings. Notice that the values of H and E are similar in magnitude, as is the case in many chemical reactions.

A 55.0 g aluminum block initially at 27.5 C absorbs 725 J of heat. What is the final temperature of the aluminum?

Tf = 42.1 C

We pack two identical coolers for a picnic, placing twenty-four 12-ounce soft drinks and 5 pounds of ice in each. However, the drinks that we put into cooler A were refrigerated for several hours before they were packed in the cooler, while the drinks that we put into cooler B were at room temperature. When we open the two coolers 3 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.

The drinks that went into cooler B had more thermal energy than the refrigerated drinks that went into cooler A. The temperature difference between the drinks in cooler B and the ice was greater than the difference between the drinks and the ice in cooler A. More thermal energy was exchanged between the drinks and the ice in cooler B, which resulted in more melting.

The same reaction, with exactly the same amount of reactant, is conducted in a bomb calorimeter and in a coffee-cup calorimeter. In one measurement, qrxn = -12.5 kJ and in the other qrxn = -11.8 kJ . Which value was obtained in the bomb calorimeter? (Assume that the reaction has a positive V in the coffee-cup calorimeter.)

The value of qrxn with the greater magnitude (-12.5 kJ) must have come from the bomb calorimeter. Recall that Erxn = qrxn + wrxn. In a bomb calorimeter, the energy change that occurs in the course of the reaction all takes the form of heat ( q ). In a coffee-cup calorimeter, the amount of energy released as heat may be smaller because some of the energy may be used to do work ( w ).

A chemical system produces 155 kJ of heat and does 22 kJ of work. What is E for the surroundings ? a) 177 kJ b) -177 kJ c) 133 kJ d) -133 kJ

a

Manganese reacts with hydrochloric acid to produce manganese(II) chloride and hydrogen gas. Mn(s) + 2 HCl(aq)-->MnCl2(aq) + H2(g) When 0.625 g Mn is combined with enough hydrochloric acid to make 100.0 mL of solution in a coffee-cup calorimeter, all of the Mn reacts, raising the temperature of the solution from 23.5 C to 28.8 C. Find H rxn for the reaction as written. (Assume that the specific heat capacity of the solution is 4.18 J/g C and the density is 1.00 g/mL.) a) -195 kJ b) -3.54 kJ c) -1.22 kJ d) -2.21 kJ

a

Natural gas burns in air to form carbon dioxide and water, releasing heat. CH4(g) + O2(g)-->CO2(g) + H2O(g) H rxn = -802.3 kJ What minimum mass of CH 4 is required to heat 55 g of water by 25 C? (Assume 100% heating efficiency.) a) 0.115 g b) 2.25 * 10^3 g c) 115 g d) 8.70 g

a

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?

specific heat capacity of substance A 1.10 J/g # C

Identify each energy exchange as heat or work and determine whether the sign of heat or work (relative to the system) is positive or negative. (a) An ice cube melts and cools the surrounding beverage. (The ice cube is the system.) (b) A metal cylinder is rolled up a ramp. (The metal cylinder is the system.) (c) Steam condenses on skin, causing a burn. (The condensing steam is the system.)

(a) heat, sign is positive (b) work, sign is positive (c) heat, sign is negative

A block of copper of unknown mass has an initial temperature of 65.4 C. The copper is immersed in a beaker containing 95.7 g of water at 22.7 C. When the two substances reach thermal equilibrium, the final temperature is 24.2 C. What is the mass of the copper block?

37.8 grams Cu

Two substances, A and B, of equal mass but at different temperatures come into thermal contact. The specific heat capacity of substance A is twice the specific heat capacity of substance B. Which statement is true of the temperature of the two substances when they reach thermal equilibrium? (Assume no heat loss other than the thermal transfer between the substances.) a) The final temperature of both substances will be closer to the initial temperature of substance A than the initial temperature of substance B. b) The final temperature of both substances will be closer to the initial temperature of substance B than the initial temperature of substance A. c) The final temperature of both substances will be exactly midway between the initial temperatures of substance A and substance B. d) The final temperature of substance B will be greater than the final temperature of substance A.

a

When a 3.80 g sample of liquid octane (C 8 H 18 ) is burned in a bomb calorimeter, the temperature of the calorimeter rises by 27.3 C. The heat capacity of the calorimeter, measured in a separate experiment, is 6.18 kJ/ C. Determine the enthalpy of combustion for octane in units of kJ/mol octane. a) -5.07 * 10 3 kJ/mol b) 5.07 * 10 3 kJ/mol c) -44.4 * 10 3 kJ/mol d) -16.7 * 10 3 kJ/mol

a

Which process is endothermic? a) The evaporation of water from the skin. b) The burning of candle wax. c) The oxidation of iron in a chemical hand warmer. d) The combustion of natural gas in a stove.

a

Use standard enthalpies of formation to calculate H rxn for each reaction: a. C2H4(g) + H2(g)-->C2H6(g) b. CO(g) + H2O(g)-->H2(g) + CO2(g) c. 3 NO2(g) + H2O(l)-->2 HNO3(aq) + NO(g) d. Cr2O3(s) + 3 CO(g)-->2 Cr(s) + 3 CO2(g)

a. -137.1 kJ b. -41.2 kJ c. -137 kJ d. 290.7 kJ

Convert between energy units: a. 534 kWh to J b. 215 kJ to Cal c. 567 Cal to J d. 2.85 * 103 J to cal

a. 1.92 * 10^9 J b. 5.14 * 10^4 cal c. 2.37 * 10^6 J d. 0.681 Cal

Determine the mass of CO2 produced by burning enough of each of the following fuels to produce 1.00 * 102 kJ of heat. Which fuel contributes least to global warming per kJ of heat produced? a. CH4(g) + 2 O2(g)-->CO2(g) + 2 H2O(g) H rxn = -802.3 kJ b. C3H8(g) + 5 O2(g)-->3 CO2(g) + 4 H2O(g) H rxn = -2043 kJ c. C8H18(l) + 25>2 O2(g-->h8 CO2(g) + 9 H2O(g) H rxn = -5074.1 kJ

a. 5.49 g CO2 b. 6.46 g CO2 c. 6.94 g CO2 Natural gas, CH4(g), contributes the least to global warming by producing the least CO2(g) per kJ of heat produced.

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

a. 7.6 * 10^2 C b. 4.3 * 10^2 C c. 1.3 * 10^2 C d. 49 C

Suppose that a person eats 2387 Calories per day. Convert this amount of energy into each unit: a. J b. kJ c. kWh

a. 9.987 * 10^6 J b. 9.987 * 10^3 kJ c. 2.78 kWh

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

a. At constant pressure, heat can be added and work can be done on the system. E = q + w; therefore, q = E - w.

Identify each process as endothermic or exothermic and indicate the sign of H . (a) an ice cube melting (b) nail polish remover quickly evaporating after it is accidentally spilled on the skin (c) gasoline burning within the cylinder of an automobile engine

a. endothermic, positive H b. endothermic, positive H c. exothermic, negative H

Determine whether each process is exothermic or endothermic and indicate the sign of H . a. natural gas burning on a stove b. isopropyl alcohol evaporating from skin c. water condensing from steam

a. exothermic, - b. endothermic, + c. exothermic, -

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.)

23.9 C

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?

26.1 C

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.

Measurement B corresponds to conditions of constant pressure. Measurement A corresponds to conditions of constant volume. When a fuel is burned under constant pressure some of the energy released does work on the atmosphere by expanding against it. Less energy is manifest as heat due to this work. When a fuel is burned under constant volume, all of the energy released by the combustion reaction is evolved as heat.

Palmitic acid 1C16H32O22 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 H2O1l2 in the balanced chemical equations because the metabolism of these compounds produces liquid water.]

palmitic acid: 9.9378 Cal/g, sucrose: 3.938 Cal/g, fat contains more Cal/g than sugar

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.

q= 1030 kJ, H = 1030 kJ, E = 952 kJ, w = -78 kJ