Chapter 6: Thermochemistry: Energy Flow and Chemical
Change
1. A system that does no work but which transfers heat to the surroundings has
A) q < 0, E > 0. D) q > 0, E < 0.
B) q < 0, E < 0. E) q < 0, E = 0.
C) q > 0, E > 0.
2. A system that does no work but which receives heat from the surroundings has
A) q < 0, E > 0. D) q = –E.
B) q > 0, E < 0. E) w = E.
C) q = E.
3. A system which undergoes an adiabatic change (i.e., q = 0) and does work on the
surroundings has
A) w < 0, E = 0. D) w < 0, E > 0.
B) w > 0, E > 0. E) w < 0, E < 0.
C) w > 0, E < 0.
4. A system which undergoes an adiabatic change (i.e., q = 0) and has work done on it by
the surroundings has
A) w = E. D) w < 0, E > 0.
B) w = –E. E) w > E.
C) w > 0, E < 0.
5. A system receives 575 J of heat and delivers 425 J of work. Calculate the change in the
internal energy, E, of the system.
A) –150 J B) 150 J C) –1000 J D) 1000 J E) 575 J
6. A system delivers 225 J of heat to the surroundings while delivering 645 J of work.
Calculate the change in the internal energy, E, of the system.
A) –420 J B) 420 J C) –870 J D) 870 J E) –225 J
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
7. A system delivers 1275 J of heat while the surroundings perform 855 J of work on it.
Calculate E in J.
A) –2130 J B) –420 J C) 420 J D) 2130 J E) –1275 J
8. A system absorbs 21.6 kJ of heat while performing 6.9 kJ of work on the surroundings.
If the initial internal energy, E, is 61.2 kJ, what is the final value of E?
A) 32.7 kJ B) 46.5 kJ C) 75.9 kJ D) 82.8 kJ E) 89.7 kJ
9. A system initially has an internal energy E of 501 J. It undergoes a process during which
it releases 111 J of heat energy to the surroundings, and does work of 222 J. What is the final
energy of the system, in J?
A) 168 J
B) 390 J
C) 612 J
D) 834 J
E) cannot be calculated without more information
10. A system expands from a volume of 1.00 L to 2.00 L against a constant external pressure
of 1.00 atm. The work (w) done by the system, in J, is
A) 1.00 J. D) 1.01 × 105 J.
B) 2.00 J. E) none of the above.
C) 1.01 × 102 J.
11. A system contracts from an initial volume of 15.0 L to a final volume of 10.0 L under a
constant external pressure of 0.800 atm. The value of w, in J, is
A) –4.0 J. B) 4.0 J. C) –405 J. D) 405 J. E) 4.05 × 103 J.
12. An ideal gas (the system) is contained in a flexible balloon at a pressure of 1 atm and is
initially at a temperature of 20.°C. The surrounding air is at the same pressure, but its
temperature is 25°C. When the system has equilibrated with its surroundings, both systems and
surroundings are at 25°C and 1 atm. In changing from the initial to the final state, which one of
the following relationships regarding the system is correct?
A) E < 0 B) E = 0 C) H = 0 D) w > 0 E) q > 0
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
13. Which one of the following relationships is always correct?
A) potential energy + kinetic energy = constant
B) E = q + w
C) E = H – PV
D) H = E + PV
E) H = qv
14. In which of the following processes is H = E ?
A) Two moles of ammonia gas are cooled from 325°C to 300°C at 1.2 atm.
B) One gram of water is vaporized at 100°C and 1 atm.
C) Two moles of hydrogen iodide gas react to form hydrogen gas and iodine gas in a
40-L container.
D) Calcium carbonate is heated to form calcium oxide and carbon dioxide in a
container with variable volume.
E) One mole of solid carbon dioxide sublimes to the gas phase.
15. For which one of the following reactions will H be approximately (or exactly) equal to
E? A) H2(g) + Br2(g) → 2HBr(g)
B) H2O(l) → H2O(g)
C) CaCO3(s) → CaO(s) + CO2(g)
D) 2H(g) + O(g) → H2O(l)
E) CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
16. In which one of the following reactions would you expect H to be substantially greater
than E (i.e., H > E)?
A) H2(g) + Br2(g) → 2HBr(g)
B) CO2(s) → CO2(g)
C) C2H2(g) + H2(g) → C2H4(g)
D) H2O(s) → H2O(l)
E) HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
17. Cold packs, whose temperatures are lowered when ammonium nitrate dissolves in water,
are carried by athletic trainers when transporting ice is not possible. Which of the following is
true of this reaction?
A) H < 0, process is exothermic D) H > 0, process is endothermic
B) H > 0, process is exothermic E) H = 0, since cold packs are sealed
C) H < 0, process is endothermic
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
18. In a phase change of water between the liquid and the gas phases, 770.1 kJ of energy was
released by the system. What was the product, and how much of it was formed in the phase
change?
(Data: H2O(l) → H2O(g) H = 44.01 kJ/mol.)
A) 315 g water vapor was produced.
B) 17.5 g of water vapor was produced.
C) 17.5 mol of water vapor was produced.
D) 17.5 mol of liquid water was produced.
E) 17.5 g of liquid water was produced.
19. The dissolution of barium hydroxide in water is an exothermic process. Which of the
following statements is correct?
A) The enthalpy of solid barium hydroxide plus pure water is less than that of the
solution, at the same temperature.
B) The enthalpy of solid barium hydroxide plus pure water is greater than that of the
solution, at the same temperature.
C) The enthalpy of solid barium hydroxide plus pure water is the same as that of the
solution, at the same temperature.
D) The temperature of the solution is lower than of the barium hydroxide and water
before mixing.
E) When barium hydroxide dissolves in water, the system does work on the
surroundings.
20. Two solutions (the system), each of 25.0 mL volume and at 25.0°C, are mixed in a
beaker. A reaction occurs between them, and the temperature rises to 35.0°C. After the products
have equilibrated with the surroundings, the temperature is again 25.0°C and the total volume is
50.0 mL. No gases are involved in the reaction. Which one of the following relationships
concerning the change from initial to final states (both at 25.0°C) is correct?
A) E = 0 B) H = 0 C) E > 0 D) q = 0 E) w = 0
21. A Snickers® candy bar contains 280 Calories, of which the fat content accounts for 120
Calories. What is the energy of the fat content, in kJ?
A) 5.0 × 10–1 kJ D) 1.2 × 103 kJ
B) 29 kJ E) 5.0 × 105 kJ
C) 5.0 × 102 kJ
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
22. Your favorite candy bar, Gummy Beakers, contains 1.2 × 106 J of energy while your
favorite soft drink, Bolt, contains 6.7 × 105 J. If you eat two packs of Gummy Beakers a day and
drink 3 cans of Bolt, what percent of your 2000 Calorie daily food intake is left for broccoli,
beans, beef, etc.?
A) 53% B) 47% C) 27% D) 11% E) 0%
23. Natural gas, or methane, is an important fuel. Combustion of one mole of methane
releases 802.3 kilojoules of energy. How much energy does that represent in kilocalories?
A) 1.92 × 10–1 kcal D) 1.92 × 105 kcal
B) 1.92 × 102 kcal E) 3.36 × 106 kcal
C) 3.36 × 103 kcal
24. Which of the following is not a state function?
A) internal energy B) volume C) work D) pressure E) enthalpy
25. Calculate q when 28.6 g of water is heated from 22.0°C to 78.3°C.
A) 0.385 kJ B) 1.61 kJ C) 6.74 kJ D) 9.37 kJ E) 1.61 × 103 kJ
26. If, as a pioneer, you wished to warm your room by taking an object heated on top of a
pot-bellied stove to it, which of the following 15-pound objects, each heated to 100°C, would be
the best choice? The specific heat capacity (in J/(g·K)) for each substance is given in
parentheses. Iron (0.450), copper (0.387), granite (0.79), gold (0.129), water (4.18).
A) iron B) copper C) granite D) gold E) water
27. Ethylene glycol, used as a coolant in automotive engines, has a specific heat capacity of
2.42 J/(g·K). Calculate q when 3.65 kg of ethylene glycol is cooled from 132°C to 85°C.
A) –1900 kJ B) –420 kJ C) –99 kJ D) –0.42 kJ E) –4.2 × 10–6 kJ
28. A 275-g sample of nickel at 100.0°C is placed in 100.0 mL of water at 22.0°C. What is
the final temperature of the water? Assume that no heat is lost to or gained from the
surroundings. Specific heat capacity of nickel = 0.444 J/(g·K)
A) 39.6°C B) 40.8°C C) 61.0°C D) 79.2°C E) 82.4°C
29. Benzene is a starting material in the synthesis of nylon fibers and polystyrene
(styrofoam). Its specific heat capacity is 1.74 J/(g·K). If 16.7 kJ of energy is absorbed by a 225-
g sample of benzene at 20.0°C, what is its final temperature?
A) –22.7°C B) 36.7°C C) 42.7°C D) 62.7°C E) none of the above
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
30. When Karl Kaveman adds chilled grog to his new granite mug, he removes 10.9 kJ of
energy from the mug. If it has a mass of 625 g and was at 25°C, what is its new temperature?
Specific heat capacity of granite = 0.79 J/(g·K)
A) 3°C B) 14°C C) 22°C D) 47°C E) none of the above
31. The Starship Enterprise is caught in a time warp and Spock is forced to use the primitive
techniques of the 20th century to determine the specific heat capacity of an unknown mineral.
The 307-g sample was heated to 98.7°C and placed into a calorimeter containing 72.4 g of water
at 23.6°C. The heat capacity of the calorimeter was 15.7 J/K. The final temperature in the
calorimeter was 32.4°C. What is the specific heat capacity of the mineral?
A) 0.124 J/(g·K) D) 0.145 J/(g·K)
B) 0.131 J/(g·K) E) none of the above
C) 0.138 J/(g·K)
32. A piece of copper metal is initially at 100.0°C. It is dropped into a coffee cup calorimeter
containing 50.0 g of water at a temperature of 20.0°C. After stirring, the final temperature of
both copper and water is 25.0°C. Assuming no heat losses, and that the specific heat (capacity)
of water is 4.18 J/(g·K), what is the heat capacity of the copper in J/K?
A) 2.79 J/K
B) 3.33 J/K
C) 13.9 J/K
D) 209 J/K
E) none of the above
33. 15.0 g of ice cubes at 0.0°C are combined with 150. g of liquid water at 70.0°C in a
coffee cup calorimeter. Calculate the final temperature reached, assuming no heat loss or gain
from the surroundings. (Data: specific heat capacity of H2O(l), c = 4.18 J/g°C;
H2O(s) → H2O(l) H = 6.02 kJ/mol)
A) 0.0 B) 10.6 C) 30.7 D) 43.2 E) 56.4
34. 40.0 g of ice cubes at 0.0°C are combined with 150. g of liquid water at 20.0°C in a
coffee cup calorimeter. Calculate the final temperature reached, assuming no heat loss or gain
from the surroundings. (Data: specific heat capacity of H2O(l), c = 4.18 J/g°C;
H2O(s) → H2O(l) H = 6.02 kJ/mol)
A) 0.0 B) 10.6 C) 30.7 D) 43.2 E) 56.4
35. What is the final temperature when 20.0 g of water at 25°C is mixed with 30.0 g of water
at 80°C?
A) 35°C B) 42°C C) 53°C D) 58°C E) 70°C
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
36. A backpacker collects snow at 0°C, and places it in a cooking pot on a camp stove. It
takes 643 kJ of heat energy to melt the snow and bring the water to boiling. Assuming no heat
loss, and neglecting the specific heat capacity of the pot, calculate the mass of snow that the
backpacker collected.
(Data: specific heat capacity of liquid water, c = 4.18 J/gK; and: H2O(s) → H2O(l) H =
Hfusion = 6.02 kJ/mol)
A) 1.92 kg B) 1.90 kg C) 1.52 kg D) 855 g E) < 800 g
37. The specific heat capacity c of a metal is approximately related to its molar mass as
follows: c × = 3R, where R is the universal gas constant, 8.314 J/molK. Use this
relationship to identify the metal which has a specific heat capacity of 0.900 J/gK.
A) Li B) Sn C) Ca D) Al E) U
38. The combustion of glucose (C6H12O6) with oxygen gas produces carbon dioxide and
water. This process releases 2803 kJ per mole of glucose. When 3.00 mol of oxygen react in this
way with glucose, what is the energy release in kcal? (Hint: Write a balanced equation for the
combustion process.)
A) 223.5 kcal
B) 335.3 kcal
C) 1402 kcal
D) 2012 kcal
E) 5858 kcal
39. A common laboratory reaction is the neutralization of an acid with a base. When 50.0
mL of 0.500 M HCl at 25.0°C is added to 50.0 mL of 0.500 M NaOH at 25.0°C in a coffee cup
calorimeter, the temperature of the mixture rises to 28.2°C. What is the heat of reaction per mole
of acid? Assume the mixture has a specific heat capacity of 4.18 J/(g·K) and that the densities of
the reactant solutions are both 1.00 g/mL.
A) 670 J B) 1300 J C) 27 kJ D) 54 kJ E) > 100 kJ
40. Sand is converted to pure silicon in a three step process. The third step is
SiCl4(g) + 2Mg(s) → 2MgCl2(s) + Si(s) H = –625.6 kJ
What is the enthalpy change when 25.0 mol of silicon tetrachloride is converted to elemental
silicon?
A) –25.0 kJ D) –3.13 × 104 kJ
B) –7820 kJ E) none of the above
C) –1.56 × 104 kJ
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
41. Calcium hydroxide, which reacts with carbon dioxide to form calcium carbonate, was
used by the ancient Romans as mortar in stone structures. The reaction for this process is
Ca(OH)2(s) + CO2(g) → CaCO3(s) + H2O(g) H = –69.1 kJ
What is the enthalpy change if 3.8 mol of calcium carbonate is formed?
A) –18 kJ B) –69 kJ C) –73 kJ D) –260 kJ E) none of the above
42. Galena is the ore from which elemental lead is extracted. In the first step of the extraction
process, galena is heated in air to form lead(II) oxide.
2PbS(s) + 3O2(g) → 2PbO(s) + 2SO2(g) H = –827.4 kJ
What mass of galena is converted to lead oxide if 975 kJ of heat are liberated?
A) 203 g B) 282 g C) 406 g D) 478 g E) 564 g
43. The highly exothermic thermite reaction, in which aluminum reduces iron(III) oxide to
elemental iron, has been used by railroad repair crews to weld rails together.
2Al(s) + Fe2O3(s) → 2Fe(s) + Al2O3(s) H = –850 kJ
What mass of iron is formed when 725 kJ of heat are released?
A) 47 g B) 65 g C) 95 g D) 112 g E) 130 g
44. Use Hess’s Law to calculate the enthalpy change for the reaction
WO3(s) + 3H2(g) → W(s) + 3H2O(g)
from the following data:
2W(s) + 3O2(g) → 2WO3(s) H = –1685.4 kJ
2H2(g) + O2(g) → 2H2O(g) H = –477.84 kJ
A) 125.9 kJ
B) 252.9 kJ
C) 364.9 kJ
D) 1207.6 kJ
E) none of the above
45. Calculate the enthalpy change for the reaction
NO(g) + O(g) → NO2(g)
from the following data:
NO(g) + O3(g) → NO2(g) + O2(g) H = –198.9 kJ
O3(g) → 1.5O2(g) H = –142.3 kJ
O2(g) → 2O(g) H = 495.0 kJ
A) –551.6 kJ B) –304.1 kJ C) 190.9 kJ D) 153.8 kJ E) 438.4 kJ
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
46. Use the following data to calculate the standard heat (enthalpy) of formation, H°f , of
manganese(IV) oxide, MnO2 (s).
2MnO2(s) → 2MnO(s) + O2(g) H = 264 kJ
MnO2(s) + Mn(s) → 2MnO(s) H = –240 kJ
A) –504 kJ B) –372 kJ C) –24 kJ D) 24 kJ E) 504 kJ
47. The compound carbon suboxide, C3O2, is a gas at room temperature. Use the data
supplied to calculate the heat of formation of carbon suboxide.
(Data: 2CO(g) + C(s) → C3O2(g) H° = 127.3 kJ/mol
and: Hf° of CO(g) = –110.5 kJ/mol)
A) 116.8 B) –93.7 C) 227.8 D) –348.3 E) 93.7
48. Stoichiometric amounts of nitrogen gas and hydrogen gas react in a calorimeter to
produce 5.00 g of ammonia gas. The calorimeter temperature rises 0.42°C. The calorimeter and
water have a combined heat capacity of 32.16 kJ/K. Calculate the heat of formation of ammonia,
Hf°, in kJ/mol. The formation reaction for ammonia is:
0.5N2(g) + 1.5H2(g) → NH3(g).
A) –46 kJ/mol D) 3.97 kJ/mol
B) –13.5 kJ/mol E) 13.5 kJ/mol
C) –3.97 kJ/mol
49. Which one of the following statements about standard states is incorrect?
A) The standard state of a solid compound is the pure solid.
B) The standard state of a liquid compound is the pure liquid.
C) The standard state of a gaseous compound is the gas at a pressure of 1 atmosphere.
D) The standard state of an aqueous solute is a saturated solution in water.
E) The standard state of an element is the form in which it is stable at 1 atm and a
specified temperature, usually 25°C.
50. Which one of the following equations represents the formation reaction of CH3OH(l)?
A) C(g) + 2H2(g) +
12
O2 (g) → CH3OH(l)
B) C(g) + 4H(g) + O(g) → CH3OH(l)
C) C(graphite) + 4H(g) + O(g) → CH3OH(l)
D) C(diamond) + 4H(g) + O(g) → CH3OH(l)
E) C(graphite) + 2H2(g) +
12
O2(g) → CH3OH(l)
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
51. Which one of the following is not a correct formation reaction? (products are correct)
A) H2(g) + O(g) → H2O(l)
B)
12
H2(g) +
12
Cl2(g) → HCl(g)
C) 6C(graphite) + 3H2(g) → C6H6(l)
D) C(graphite) → C(diamond)
E) 6C(graphite) + 6H2(g) + 3O2(g) → C6H12O6(s)
52. Which one of the following is a correct formation reaction?
A) C(diamond) → C(graphite)
B) H2(g) + O(g) → H2O(l)
C) C(graphite) + 4H(g) → CH4(g)
D) 6C(graphite) + 6H2O(s) → C6H12O6(s)
E) 2C(graphite) + 3H2(g) +
12
O2(g) → C2H5OH(l)
53. Calculate the H°rxn for the decomposition of calcium carbonate to calcium oxide and
carbon dioxide. H°f [CaCO3(s)] = –1206.9 kJ/mol; H°f [CaO(s)] = –635.1 kJ/mol; H°f
[CO2(g)] = –393.5 kJ/mol
CaCO3(s) → CaO(s) + CO2(g)
A) –2235.5 kJ B) –1448.5 kJ C) –178.3 kJ D) 178.3 kJ E) 2235.5 kJ
54. Nitric acid, which is among the top 15 chemicals produced in the United States, was first
prepared over 1200 years ago by heating naturally occurring sodium nitrate (called saltpeter)
with sulfuric acid and collecting the vapors produced. Calculate H°rxn for this reaction. H°f
[NaNO3(s)] = –467.8 kJ/mol; H°f [NaHSO4(s)] = –1125.5 kJ/mol; H°f
[H2SO4(l) = –814.0 kJ/mol; H°f [HNO3(g)] = –135.1 kJ/mol
NaNO3(s) + H2SO4(l) → NaHSO4(s) + HNO3(g)
A) –644.2 kJ B) –291.4 kJ C) –21.2 kJ D) 21.2 kJ E) 644.2 kJ
55. An important step in the synthesis of nitric acid is the conversion of ammonia to nitric
oxide.
4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g)
Calculate H°rxn for this reaction.
H°f [NH3(g)] = –45.9 kJ/mol; H°f [NO(g)] = 90.3 kJ/mol; H°f [H2O(g)] = –241.8 kJ/mol
A) –906.0 kJ B) –197.4 kJ C) –105.6 kJ D) 197.4 kJ E) 906.0 kJ
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
56. Calculate the H°rxn for the following reaction. (H°f [SiO2(s)] = –910.9 kJ/mol; H°f
[SiCl4(g)] = –657.0 kJ/mol; H°f [HCl(g)] = –92.3 kJ/mol; H°f [H2O (g)] = –241.8 kJ/mol)
SiO2(s) + 4HCl(g) → SiCl4(g) + 2H2O(g)
A) –139.5 kJ B) –137.4 kJ C) –104.4 kJ D) 104.4 kJ E) 139.5 kJ
57. Calculate the H°rxn for the following reaction. (H°f [AsH3(g)] = 66.4 kJ/mol; H°f
[H3AsO4(aq)] = –904.6 kJ/mol; H°f [H2O(l)] = –285.8 kJ/mol)
H3AsO4(aq) + 4H2(g) → AsH3(g) + 4H2O(l)
A) –1981.4 kJ B) –685.2 kJ C) –172.2 kJ D) 172.2 kJ E) 685.2 kJ
58. Ethanol, C2H5OH, is being promoted as a clean fuel and is used as an additive in many
gasoline mixtures. Calculate the H°rxn for the combustion of ethanol.
(H°f [C2H5OH(l)] = –277.7 kJ/mol; H°f [CO2(g)] = –393.5 kJ/mol; H°f [H2O(g)] = –241.8
kJ/mol)
A) –1234.7 kJ B) –751.1 kJ C) –357.6 kJ D) 357.6 kJ E) 1234.7 kJ
59. Consider the equation E = q + w
Explain fully the meaning of all three terms in the equation, and also the implied sign
convention for q and w.
60. Starting from equations relating pressure to force and force to work, derive the
relationship w = – PV, explaining the steps in your argument.
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
61. a. A gas sample absorbs 53 kJ of heat and does 18 kJ of work. Calculate the change in its
internal energy.
b. A system expands against a constant pressure of 1.50 atm, from an initial volume of 1.00 L to
a final volume of 10.0 L. Calculate the work (w) involved in this process, in kJ.
62. Calculate, in J, the work done by 10.0 g of CO2 when it sublimes against a pressure of
1.00 atm to form gaseous CO2 at 0.0°C. The volume of CO2(s) can be neglected; CO2(g) can be
assumed to behave ideally. The process occurring is
CO2(s) → CO2(g)
63. 1.00 mol of an ideal gas (the system) is heated from 20.0°C to 100.0°C at a constant
pressure of 1.00 atm.
a. Given that the internal energy of an ideal gas is E =
12
RT, calculate E for this change, in J.
b. Calculate w for this change, in J.
c. Hence, calculate q for this change, in J.
64. a. Starting from the equation H = E + PV, show how the relationship H = qp is derived.
Clearly indicate any necessary assumptions or conditions.
b. In one sentence, state in full what is meant by the equation: H = qp.
65. Although internal energy (E) is more fundamental and conceptually easier than enthalpy
(H), in most chemical applications H is more relevant and useful than E. Why?
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
66. a. Explain fully what is meant by the term “state function.”
b. (i) Give two examples of thermodynamic quantities which state functions.
(ii) Give two examples of thermodynamic quantities which are not state functions.
67. When 1.00 g of solid NH4Cl is dissolved in 25.00 g of water contained in a coffee cup
calorimeter, both reagents initially being at 25.0°C, the temperature falls to 22.4°C. Assuming
that the heat capacity of the ammonium chloride solution is 4.18 J/(g·K), calculate the heat
(enthalpy) of solution of NH4Cl, (a) in J/g and (b) in kJ/mol.
68. 2.53 g of solid NaOH is dissolved in 100.00 g of water in a coffee cup calorimeter, all the
reagents initially being at 20.0°C. Calculate the final temperature of the solution obtained, given
the following information:
NaOH(s) → NaOH(aq) H° = – 43.0 kJ
Heat capacity of NaOH solution = 4.18 J/(g·K)
69. A mass of 1.250 g of benzoic acid (C7H6O2) was completely combusted in a bomb
calorimeter. If the heat capacity of the calorimeter was 10.134 kJ/K and the heat of combustion
of benzoic acid is –3226 kJ/mol, calculate (to three decimal places) the temperature increase that
should have occurred in the apparatus.
70. The reaction
2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)
was studied in a coffee cup calorimeter. 100. mL portions of 1.00 M aqueous NaOH and H2SO4,
each at 24.0°C, were mixed. The maximum temperature achieved was 30.6°C. Neglect the heat
capacity of the cup and the thermometer, and assume that the solution of products has a density
of exactly 1 g/mL and a specific heat capacity of 4.18 J/(g·K).
a. Calculate the heat of reaction, q, in J.
b. Calculate H, the heat (enthalpy) of reaction, in kJ/mol of Na2SO4 produced.
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
71. a. State Hess’s Law.
b. Use the H° data given below to calculate H° for the reaction:
C2H4(g) + H2(g) → C2H6(g)
Data: H° (kJ)
C2H6(g) + 3.5O2(g) → 2CO2(g) + 3H2O(l) –1560
C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l) –1411
2H2(g) + O2(g) → 2H2O(l) –572
72. Given the following data:
H° (kJ)
2O3(g) → 3O2(g) –427
O2(g) → 2O(g) 495
NO(g) + O3(g) → NO2(g) + O2(g) –199
calculate H° for the reaction
NO(g) + O(g) → NO2(g)
73. Diborane (B2H6) has been considered as a possible rocket fuel. Calculate H° for the
reaction
B2H6(g) → 2B(s) + 3H2(g)
using the following data:
H° (kJ)
2B(s) + 1.5O2(g) → B2O3(s) –1273
B2H6(g) + 3O2(g) → B2O3(s) + 3H2O(g) –2035
H2(g) +
12
O2(g) → H2O(g) –242
74. Clearly state the thermodynamic standard state of
a. an element or compound.
b. a solute.
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
75. a. Define, or explain fully what is meant by the standard enthalpy of formation of a
substance, H°f..
b. What is the standard state of the element oxygen?
c. Write down in full the formation reaction for liquid ethanol, C2H5OH(l). The equation should
be balanced and should indicate the physical state of each substance.
76. a. Write a balanced equation for the combustion of benzene, C6H6(l) in oxygen.
b. The standard heat of combustion of benzene is –3271 kJ/mol. Calculate its standard heat of
formation, H°f , given the data:
H°f [CO2(g)] = –394 kJ; H°f [H2O(l)] = –286 kJ
77. In a reaction with high energy reactants and low energy products, q is negative.
78. The only way in which a system can do work on the surroundings is by expansion
against the external pressure.
79. Different chemical bonds have different potential energies.
80. For all processes, both q and w will have the same sign.
81. For all processes, both q and E will have the same sign.
82. For a reaction in a sealed, rigid container, H is always greater than E.
83. H does not depend on the path of a reaction, but E does.
84. The enthalpy (H) of liquid water is greater than that of the same quantity of ice at the
same temperature.
Chapter 6: Thermochemistry: Energy Flow and Chemical Change
85. In an endothermic reaction, in going from the reactants to the products at the same
temperature, the value of q is negative.
86. E values obtained by bomb calorimetry can be converted to give accurate H values.
87. The standard state of a substance in aqueous solution is a 1 M solution.
88. The standard heat (enthalpy) of formation of graphite is zero.
89. Standard heats (enthalpies) of formation of compounds, H°f, may be positive or
negative.