CHAPTER 6

Thermochemistry

1. A gas absorbs 0.0 J of heat and then performs 15.2 J of work. The change in internal energy of the gas isa) 30.4 Jb) 15.2 Jc) –30.4 Jd) –15.2 Je) none of these

2. What is the kinetic energy of a 1.50-kg object moving at 86.0 km/hr?a) 4.28 102 kJb) 5.55 103 kJc) 4.28 10–4 kJd) 8.56 102 kJe) 1.79 101 kJ

3. Which of the following statements correctly describes the signs of q and w for the following exothermic process at P = 1 atm and T = 370 K? H2O(g) H2O(l)

a) q and w are negative.b) q is positive, w is negative.c) q is negative, w is positive.d) q and w are both positive.

e) q and w are both zero.

4. For a particular process q = 20 kJ and w = 15 kJ. Which of the following statements is true?a) Heat flows from the system to the surroundings.b) The system does work on the surroundings.c) E = 35 kJd) All of the above are true.e) None of the above are true.

5. Which of the following statements is correct?a) The internal energy of a system increases when more work is done by the system than heat was flowing

into the system.b) The internal energy of a system decreases when work is done on the system and heat is flowing into the

system.c) The system does work on the surroundings when an ideal gas expands against a constant external

pressure.d) All statements are true.e) All statements are false.

6. For a particular process q = –17 kJ and w = 21 kJ. Which of the following statements is false?a) Heat flows from the system to the surroundings.b) The system does work on the surroundings.c) E = +4 kJd) The process is exothermic.e) None of the above is false.

7. One mole of an ideal gas is expanded from a volume of 1.00 liter to a volume of 4.90 liters against a constant external pressure of 1.00 atm. How much work (in joules) is performed on the surroundings? Ignore significant figures for this problem. (T = 300 K; 1 L·atm = 101.3 J)a) 198 J

b) 395 Jc) 1.19 103 Jd) 496 Je) none of these

8. Calculate the work for the expansion of CO2 from 1.0 to 5.8 liters against a pressure of 1.0 atm at constant temperature.a) 4.8 L·atmb) 5.8 L·atmc) 0 L·atmd) –4.8 L·atme) –5.8 L·atm

9. A fuel-air mixture is placed in a cylinder fitted with a piston. The original volume is 0.310-L. When the mixture is ignited, gases are produced and 815 J of energy is released. To what volume will the gases expand against a constant pressure of 635 mmHg, if all the energy released is converted to work to push the piston?a) 9.32 Lb) 7.03 Lc) 9.94 Ld) 9.63 Le) 1.59 L

10. Which statement is true of a process in which one mole of a gas is expanded from state A to state B?a) When the gas expands from state A to state B, the surroundings are doing work on the system.b) The amount of work done in the process must be the same, regardless of the path.c) It is not possible to have more than one path for a change of state.d) The final volume of the gas will depend on the path taken.e) The amount of heat released in the process will depend on the path taken.

11. Calculate the work associated with the expansion of a gas from 42.0 L to 79.0 L at a constant pressure of 19.0 atm.a) 703 L·atmb) –703 L·atmc) –1.50 103 L·atmd) 798 L·atme) 1.50 103 L·atm

22. Of energy, work, enthalpy, and heat, how many are state functions?a) 0b) 1c) 2d) 3e) 4

23. Which of the following properties is (are) intensive properties? I. mass

II. temperature

III. volume

IV. concentration

V. energy

a) I, III, and Vb) II onlyc) II and IVd) III and IVe) I and V

24. For the reaction H2O(l) H2O(g) at 298 K and 1.0 atm, H is more positive than E by 2.5 kJ/mol. This quantity of energy can be considered to bea) the heat flow required to maintain a constant temperatureb) the work done in pushing back the atmospherec) the difference in the H–O bond energy in H2O(l) compared to H2O(g)d) the value of H itselfe) none of these

25. Which one of the following statements is false?a) The change in internal energy, E, for a process is equal to the amount of heat absorbed at constant

volume, qv.b) The change in enthalpy, H, for a process is equal to the amount of heat absorbed at constant pressure,

qp.c) A bomb calorimeter measures H directly.d) If qp for a process is negative, the process is exothermic.e) The freezing of water is an example of an exothermic reaction.

26. C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(l), H = –1.37 103 kJFor the combustion of ethyl alcohol as described in the above equation, which of the following is true?

I. The reaction is exothermic.

II. The enthalpy change would be different if gaseous water was produced.

III. The reaction is not an oxidation–reduction one.

IV. The products of the reaction occupy a larger volume than the reactants.

a) I, IIb) I, II, IIIc) I, III, IVd) III, IVe) only I

45. If a student performs an endothermic reaction in a calorimeter, how does the calculated value of H differ from the actual value if the heat exchanged with the calorimeter is not taken into account?a) Hcalc would be more negative because the calorimeter always absorbs heat from the reaction.b) Hcalc would be less negative because the calorimeter would absorb heat from the reaction.c) Hcalc would be more positive because the reaction absorbs heat from the calorimeter.d) Hcalc would be less positive because the reaction absorbs heat from the calorimeter.e) Hclac would equal the actual value because the calorimeter does not absorb heat.

46. A bomb calorimeter has a heat capacity of 2.47 kJ/K. When a 0.111-g sample of ethylene (C2H4) was burned in this calorimeter, the temperature increased by 2.26 K. Calculate the energy of combustion for one mole of ethylene.a) –5.29 kJ/molb) –50.3 kJ/molc) –624 kJ/mold) –0.274 kJ/mole) –1.41 103 kJ/mol

47. Consider the reaction:

When a 24.8-g sample of ethyl alcohol (molar mass = 46.07 g/mol) is burned, how much energy is released as heat?

a) 0.538 kJb) 0.737 kJc) 7.37 102 kJd) 3.40 104 kJe) 1.86 kJ

60. Consider the following processes:2A (1/2)B + C H1 = 5 kJ/mol

(3/2)B + 4C 2A + C + 3D H2 = –15 kJ/mol

E + 4A C H3 = 10 kJ/mol

Calculate H for: C E + 3D

a) 0 kJ/molb) 10 kJ/molc) –10 kJ/mold) –20 kJ/mole) 20 kJ/mol

61.Consider the following processes:

H (kJ/mol)

3B 2C + D –125.

(1/2)A B 150

E + A D 350

Calculate H for: B E + 2C

a) 325 kJ/molb) 525 kJ/molc) –175 kJ/mold) –325 kJ/mole) none of these

62. Consider the following numbered processes: 1. A 2B 2. B C + D 3. E 2DH for the process A 2C + E is

a) H1 + H2 + H3

b) H1 + H2

c) H1 + H2 – H3

d) H1 + 2H2 – H3

e) H1 + 2H2 + H3

63. At 25°C, the following heats of reaction are known:H (kJ/mol)

2ClF + O2 Cl2O + F2O 167.4

2ClF3 + 2O2 Cl2O + 3F2O 341.4

2F2 + O2 2F2O –43.4

At the same temperature, calculate H for the reaction: ClF + F2 → ClF3

a) –217.5 kJ/molb) –130.2 kJ/molc) +217.5 kJ/mold) –108.7 kJ/mole) none of these

64. Calculate H° for the reaction C4H4(g) + 2H2(g) C4H8(g), using the following data: H°combustion for C4H4(g) = –2341 kJ/mol

H°combustion for H2(g) = –286 kJ/mol

H°combustion for C4H8(g) = –2755 kJ/mol

a) –128 kJb) –158 kJc) 128 kJd) 158 kJe) none of these

68. The heat of formation of Fe2O3(s) is -826.0 kJ/mol. Calculate the heat of the reaction

when a 27.42-g sample of iron is reacted.a) –101.4 kJb) –202.8 kJc) –405.5 kJd) –811 kJe) –1.132 104 kJ

69. Which of the following does not have a standard enthalpy of formation equal to zero at 25°C and 1.0 atm?a) F2(g)b) Al(s)c) H2O(l)d) H2(g)e) They all have a standard enthalpy equal to zero.

70. Given the following two reactions at 298 K and 1 atm, which of the statements is true?1. N2(g) + O2(g) 2NO(g) H1

2. NO(g) + O2(g) NO2(g) H2

a) ΔHf° for NO2(g) = H2

b) ΔHf° for NO(g) = H1

c) ΔHf° = H2

d) ΔHf° for NO2(g) = H2 + H1

e) none of these

71. Given: Cu2O(s) + O2(g) 2CuO(s) H° = –144 kJ

Cu2O(s) Cu(s) + CuO(s) H° = +11 kJ

Calculate the standard enthalpy of formation of CuO(s).

a) –166 kJb) –299 kJc) +299 kJd) +155 kJe) –155 kJ

72. Using the following data, calculate the standard heat of formation of the compound ICl in kJ/mol:H° (kJ/mol)

Cl2(g) 2Cl(g) 242.3

I2(g) 2I(g) 151.0

ICl(g) I(g) + Cl(g) 211.3

I2(s) I2(g) 62.8

a) –211 kJ/molb) –14.6 kJ/molc) 16.8 kJ/mold) 245 kJ/mole) 439 kJ/mol

80. This fossil fuel was formed from the remains of plants that were buried and exposed to high pressure and heat over time.a) coalb) natural gasc) diesel fueld) propanee) gasoline

81. The coal with the highest energy available per unit burned isa) Lignite.b) Subbituminous.c) Bituminous.d) Anthracite.e) They are equal in energy value.

82. All of the following statements about the greenhouse effect are true except:a) It occurs only on earth.b) The molecules H2O and CO2 play an important role in retaining the atmosphere's heat.c) Low humidity allows efficient radiation of heat back into space.d) The carbon dioxide content of the atmosphere is quite stable.e) A and D

83. Which of the following is both a greenhouse gas and a fuel?a) carbon dioxideb) coalc) freond) methanee) nitrogen

84. One of the main advantages of hydrogen as a fuel is that:a) The only product of hydrogen combustion is water.b) It exists as a free gas.c) It can be economically supplied by the world's oceans.d) Plants can economically produce the hydrogen needed.e) It contains a large amount of energy per unit volume of hydrogen gas.

85. Which of the following is not being considered as an energy source for the future?a) ethanolb) methanolc) seed oild) shale oile) carbon dioxide

86. The combustion of hydrogen gas releases 286 kJ per mol of hydrogen. If 10.0 L of hydrogen at STP was burned to produce electricity, how long would it power a 100-watt (W) light bulb? Assume no energy is lost to the surroundings. (1 W = 1 J/s)a) 21.3 minb) 1.48 daysc) 1.28 hrd) 7.94 hre) 3.55 hr

Spontaneity, Entropy, and Free Energy

1. For which process is S negative?a) evaporation of 1 mol of CCl4(l)b) mixing 5 mL ethanol with 25 mL waterc) compressing 1 mol Ne at constant temperature from 1.5 L to 0.5 Ld) raising the temperature of 100 g Cu from 275 K to 295 Ke) grinding a large crystal of KCl to powder

2. Ten identical coins are shaken vigorously in a cup and then poured out onto a table top. Which of the following distributions has the highest probability of occurrence? (T = Tails, H = Heads)a) T10H0

b) T8H2

c) T7H3

d) T5H5

e) T4H6

3. If two pyramid-shaped dice (with numbers 1 through 4 on the sides) were tossed, which outcome has the highest entropy?a) The sum of the dice is 3.b) The sum of the dice is 4.c) The sum of the dice is 5.d) The sum of the dice is 6.e) The sum of the dice is 7.

4. A two-bulbed flask contains 5 particles. What is the probability of finding all 5 particles on the left side?a) 2.50%b) 2.24%c) 3.13%d) 0.20%e) 6.25%

5. Which of the following shows a decrease in entropy?a) precipitationb) gaseous reactants forming a liquidc) a burning piece of woodd) melting icee) two of these

6. Which of the following result(s) in an increase in the entropy of the system?

I.

II. Br2(g) Br2(l)

III. NaBr(s) Na+(aq) + Br–(aq)

IV. O2(298 K) O2(373 K)

V. NH3(1 atm, 298 K) NH3(3 atm, 298 K)

a) Ib) II, Vc) I, III, IVd) I, II, III, IVe) I, II, III, V

10. A 100-mL sample of water is placed in a coffee cup calorimeter. When 1.0 g of an ionic solid is added, the temperature decreases from 21.5°C to 20.8°C as the solid dissolves. For the dissolving of the solida) H < 0b) Suniv > 0c) Ssys< 0d) Ssurr > 0e) none of these

11. A chemical reaction is most likely to be spontaneous if it is accompanied bya) increasing energy and increasing entropyb) lowering energy and increasing entropyc) increasing energy and decreasing entropyd) lowering energy and decreasing entropye) none of these (A-D)

12. Assume that the enthalpy of fusion of ice is 6020 J/mol and does not vary appreciably over the temperature range 270-290 K. If one mole of ice at 0°C is melted by heat supplied from surroundings at 286 K, what is the entropy change in the surroundings, in J/K?a) 22.1b) 21.0c) 0.0d) –21.0e) –22.1

13. If the change in entropy of the surroundings for a process at 451 K and constant pressure is -326 J/K, what is the heat flow absorbed by for the system?a) 326 kJb) 1.38 kJc) –147 kJd) 125 kJe) 147 kJ

29. Substance X has a heat of vaporization of 46.3 kJ/mol at its normal boiling point (423°C). For the process X(l) X(g) at 1 atm and 423°C calculate the value of S.

a) 0b) 66.5 J/K molc) 109 J/K mold) –66.5 J/K mole) –109 J/K mol

30. Substance X has a heat of vaporization of 54.1 kJ/mol at its normal boiling point (423°C). For the process X(l) X(g) at 1 atm and 423°C calculate the value of G.

a) 0 Jb) 77.7 Jc) 128 Jd) –77.7 Je) –128 J

31. he enthalpy of vaporization of isopropanol is 44.00 kJ/mol at its boiling point (82.3°C). Calculate the value of S when 1.00 mole of isopropanol is vaporized at 82.3°C and 1.00 atm.a) 0b) 5.35 102 J/K molc) 1.24 102 J/K mol

d) –1.24 102 J/K mole) –5.35 102 J/K mol

32. S is _______ for exothermic reactions and ______ for endothermic reactions.a) favorable, unfavorableb) unfavorable, favorablec) favorable, favorabled) unfavorable, unfavorablee) cannot tell

33. For the process CHCl3(s) CHCl3(l), H° = 9.15 kJ/mol and S° = 43.9 J/mol/K. What is the melting point of chloroform?a) –65 °Cb) 208 °Cc) 129 °Cd) 65 °Ce) –129 °C

34. Given that Hvap is 54.5 kJ/mol, and the boiling point is 83.4°C, 1 atm, if one mole of this substance is vaporized at 1 atm, calculate S.a) –153 J/K molb) 153 J/K molc) 653 J/K mold) –653 J/K mole) 0

Use the following to answer questions 36-37:

Consider two perfectly insulated vessels. Vessel #1 initially contains an ice cube at 0°C and water at 0°C. Vessel #2 initially contains an ice cube at 0°C and a saltwater solution at 0°C. In each vessel, consider the "system" to be the ice, and the "surroundings" to be the liquid.

36. Determine the sign of Ssys, Ssurr, and Suniv for the contents of Vessel #1.

Ssys Ssurr Suniv

a) 0 0 0b) + – 0c) + + +d) + – +e) + 0 +

37. Determine the sign of Ssys, Ssurr, and Suniv for the system (ice/saltwater) in Vessel #2.

Ssys Ssurr Suniv

a) 0 0 0b) + – 0c) + + +d) + – +e) + 0 +

38. The melting point of water is 0°C at 1 atm pressure because under these conditions:a) S for the process H2O(s) H2O(l) is positive.b) S and Ssurr for the process H2O(s) H2O(l) are both positive.c) S and Ssurr for the process H2O(s) H2O(l) are equal in magnitude and opposite in sign.d) G is positive for the process H2O(s) H2O(l).e) None of these is correct.

41. For a spontaneous exothermic process, which of the following must be true?a) G must be positive.b) S must be positive.c) S must be negative.d) Two of the above must be true.e) None of the above (A-C) must be true.

42. A mixture of hydrogen and chlorine remains unreacted until it is exposed to ultraviolet light from a burning magnesium strip. Then the following reaction occurs very rapidly:

H2(g) + Cl2(g) 2HCl(g) G = –45.54 kJ

H = –44.12 kJ

S = –4.76 J/K

Which of the following is consistent with this information?

a) The reactants are thermodynamically more stable than the products.b) The reaction has a small equilibrium constant.c) The ultraviolet light raises the temperature of the system and makes the reaction more favorable.d) The negative value for S slows down the reaction.e) The reaction is spontaneous, but the reactants are kinetically stable.

43. For a particular chemical reaction H = 6.8 kJ and S = –29 J/K. Under what temperature condition is the reaction spontaneous?a) When T < –234 K.b) When T < 234 K.c) The reaction is spontaneous at all temperatures.d) The reaction is not spontaneous at any temperature.e) When T > 234 K.

45. Consider the freezing of liquid water at –10°C. For this process what are the signs for H, S, and G?

H S G

a)+ – 0

b)+ – –

c)– + 0

d)– + –

e)– – –

46. For the process of a certain liquid vaporizing at 1 atm, Hvap = 66.8 kJ/mol and Svap= 74.1 J/mol K. Assuming these values are independent of T, what is the normal boiling point of this liquid?a) 901 °Cb) 1174 °Cc) 628 °Cd) 0.901 °Ce) none of these

Use the following to answer questions 53-55:

The following questions refer to the following reaction at constant 25°C and 1 atm.

2Fe(s) + (3/2)O2(g) + 3H2O(l) 2Fe(OH)3(s) H = –789 kJ/mol

Substance S° (J/mol K)

Fe(OH)3(s) 107

Fe(s) 27

O2(g) 205

H2O(l) 70

53. Determine Ssurr for the reaction (in kJ/mol K)a) 3.14b) 0.937c) 0.378d) 1.31e) 2.65

54. Determine Suniv for the reaction (in kJ/mol K)a) 0.23b) 2.3c) 0.36d) 2.8e) 3.6

55. What must be true about G for this reaction?a) G = Hb) G = 0c) G > 0d) G < 0e) G = Suniv

56. Given the following data, calculate the normal boiling point for formic acid (HCOOH).Hf°(kJ/mol) S°(J/mol K)

HCOOH(l) -410. 130.0

HCOOH(g) -363 250.9

a) 0.39 °Cb) 389 °Cc) 662 °Cd) 279 °Ce) 116 °C

57. The following reaction takes place at 120°C: H2O(l) H2O(g) H = 44.0 kJ/mol S = 0.119 kJ/mol KWhich of the following must be true?

a) The reaction is not spontaneous.b) The reaction is spontaneous.c) G = 0d) G < 0e) Two of these.

65. In which reaction is S° expected to be positive?a) I2(g) I2(s)b) H2O(l) H2O(s)c) CH3OH(g) + O2(g) CO2(g) + 2H2O(l)d) 2O2(g) + 2SO(g) 2SO3(g)e) none of these

66. For the dissociation reaction of the acid HF: HF(aq) H+(aq) + F–(aq)

S is observed to be negative. The best explanation is:

a) This is the expected result since each HF molecule produces two ions when it dissociates.b) Hydration of the ions produces the negative value of S.c) The reaction is expected to be exothermic and thus S should be negative.d) The reaction is expected to be endothermic and thus S should be negative.e) None of these can explain the negative value of S.

67. Consider the dissociation of hydrogen: H2(g) 2H(g)

One would expect that this reaction:

a) will be spontaneous at any temperatureb) will be spontaneous at high temperaturesc) will be spontaneous at low temperaturesd) will not be spontaneous at any temperaturee) will never happen

68. When a stable diatomic molecule spontaneously forms from its atoms, what are the signs of H°, S°, and G°?

H S G

a)+ + +

b)+ – –

c)– + +

d)– – +

e)– – –

1983

CO(g) + 2 H2(g) <===> CH3OH(l) For this reaction, H° = -128.1 kilojoules

Hf° (kJ mol¯1) Gf° (kJ mol¯1) S° (J mol¯1 K¯1) CO(g) -110.5 -173.3 +197.9

CH3OH(l) -238.6 -166.2 +126.8

The data in the table above were determined at 25 °C.

(a) Calculate G° for the reaction above at 25 °C.

(b) Calculate Keq for the reaction above at 25 °C.

(c) Calculate S° for the reaction above at 25 °C.

(d) In the table above, there are no data for H2. What are the values of Hf°, Gf°, and of the absolute entropy, S°, for H2 at 25 °C?

1989

Br2 (l) ---> Br2 (g)

At 25 °C the equilibrium constant, Kp, for the reaction above is 0.281 atmosphere.

(a) What is G°298 for this reaction?

(b) It takes 193 joules to vaporize 1.00 gram of Br2(l) at 25 °C and 1.00 atmosphere pressure. What are the values of H°298 and of S°298 for this reaction?

(c) Calculate the normal boiling point of bromine. Assume that H° and S° remain constant as the temperature is changed.

(d) What is the equilibrium vapor pressure of bromine at 25 °C ?

2011

3. Answer the following questions about glucose, C6H12O6 , an important biochemical energy source.(a) Write the empirical formula of glucose.In many organisms, glucose is oxidized to carbon dioxide and water, as represented by the following equation.C6H12O6(s) + 6 O2(g) → 6 CO2(g) + 6 H2O(l)A 2.50 g sample of glucose and an excess of O2(g) were placed in a calorimeter. After the reaction was initiated and proceeded to completion, the total heat released by the reaction was calculated to be 39.0 kJ.

(b) Calculate the value of ΔH°, in kJ mol−1, for the combustion of glucose.

(c) When oxygen is not available, glucose can be oxidized by fermentation. In that process, ethanol and carbon dioxide are produced, as represented by the following equation.

C6H12O6(s) → 2 C2H5OH(l) + 2 CO2(g) ΔH° = −68.0 kJ mol−1 at 298 K

The value of the equilibrium constant, Kp , for the reaction at 298 K is 8.9 × 1039.(i) Calculate the value of the standard free-energy change, ΔG°, for the reaction at 298 K. Include unitswith your answer.

(ii) Calculate the value of the standard entropy change, ΔS°, in J K−1 mol−1, for the reaction at 298 K.

(iii) Indicate whether the equilibrium constant for the fermentation reaction increases, decreases, or remains the same if the temperature is increased. Justify your answer.

(d) Using your answer for part (b) and the information provided in part (c), calculate the value of ΔH° for the following reaction.

C2H5OH(l) + 3 O2(g) → 2 CO2(g) + 3 H2O(l)