Chemical Engineering Chapter 20 1 Which of the following conditions will ensure that a chemical reaction will definitely proceed in the forward direction, toward products

Chapter 20: Thermodynamics: Entropy, Free Energy, and

the Direction of Chemical Reactions

1. Which of the following is true for pure oxygen gas, O2(g) at 25°C?

A) H° f > 0 B) H° f < 0 C) G° f > 0 D) G° f < 0 E) S° > 0

2. A certain process has Suniv > 0 at 25°C. What does one know about the process?

A) It is exothermic.

B) It is endothermic.

C) It is spontaneous at 25°C.

D) It will move rapidly toward equilibrium.

E) none of the above

3. Which of the following is necessary for a process to be spontaneous?

A) Hsys < 0 B) Ssys > 0 C) Ssurr< 0 D) Suniv > 0 E) Gsys = 0

4. Which of the following results in a decrease in the entropy of the system?

A) O2(g), 300 K → O2(g), 400 K

B) H2O(s), 0°C → H2O(l), 0°C

C) N2(g), 25°C → N2(aq), 25°C

D) NH3(l), –34.5°C → NH3(g), –34.5°C

E) 2H2O2(g) → 2H2O(g) + O2(g)

5. Which of the following should have the greatest molar entropy at 298 K?

A) CH4(g) B) H2O(l) C) NaCl(s) D) N2O4(g) E) H2(g)

6. Which of the following is true for a system at equilibrium?

A) S°sys = S°surr

B) S°sys = –S°surr

C) S°sys = S°surr = 0

D) S°univ > 0

E) None of the above is a sufficient condition.

Chapter 20: Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions

7. Which of the following is always true for an exothermic process?

A) q sys > 0, Ssurr < 0 D) q sys > 0, Ssurr > 0

B) q sys < 0, Ssurr > 0 E) w < 0

C) q sys < 0, Ssurr < 0

8. Which of the following is always true for an endothermic process?

A) q sys > 0, Ssurr < 0 D) q sys > 0, Ssurr > 0

B) q sys < 0, Ssurr > 0 E) w < 0

C) q sys < 0, Ssurr < 0

9. Which of the following values is based on the Third Law of Thermodynamics?

A) H° f = 0 for Al(s) at 298 K

B) G° f = 0 for H2(g) at 298 K

C) S° = 51.446 J/(mol·K) for Na(s) at 298 K

D) q sys < 0 for H2O(l) → H2O(s) at 0°C

E) none of the above

10. When a sky diver free-falls through the air, the process is

A) non-spontaneous because he is accelerating due to the force applied by gravity.

B) non-spontaneous because he is losing potential energy.

C) non-spontaneous, if he had planned the jump for two weeks.

D) spontaneous.

E) in equilibrium.

11. Which, if any, of the following processes is spontaneous under the specified conditions?

A) H2O(l) → H2O(s) at 25°C

B) CO2(s) → CO2(g) at 0°C

C) 2H2O(g) → 2H2(g) + O2(g)

D) C(graphite) → C(diamond) at 25°C and 1 atm pressure

E) None of the above is spontaneous.

12. Which relationship or statement best describes S° for the following reaction?

Pb(s) + Cl2(g) → PbCl2(s)

A) S°  0

B) S° < 0

C) S° > 0

D) S° = H°/T

E) More information is needed to make a reasonable prediction.

Chapter 20: Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions

13. Which relationship or statement best describes S° for the following reaction?

HgS(s) + O2(g) → Hg(l) + SO2(g)

A) S°  0

B) S° < 0

C) S° > 0

D) S° = H°/T

E) More information is needed to make a reasonable prediction.

14. Which relationship or statement best describes S° for the following reaction?

2H2S(g) + 3O2(g) → 2H2O(g) + 2SO2(g)

A) S°  0

B) S° < 0

C) S° > 0

D) S° = H°/T

E) More information is needed to make a reasonable prediction.

15. Which relationship best describes S° for the following reaction?

CO(g) + H2O(g) → CO2(g) + H2(g)

A) S° = H° B) S° = H°/T C) S° > 0 D) S° < 0 E) S°  0

16. Which relationship or statement best describes S° for the following reaction?

2NH3(g) + 2ClF3(g) → 6HF(g) + N2(g) + Cl2(g)

A) S°  0

B) S° < 0

C) S° > 0

D) S° = H°/T

E) More information is needed to make a reasonable prediction.

17. Which relationship best describes S° for the following reaction?

8H2(g) + S8(s) → 8H2S(g)

A) S° = H° B) S° = H°/T C) S°  0 D) S° < 0 E) S° > 0

Chapter 20: Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions

18. Which relationship or statement best describes S° for the following reaction?

O3(g) + NO(g) → O2(g) + NO2(g)

A) S°  0

B) S° < 0

C) S° > 0

D) S° = H°/T

E) More information is needed to make a reasonable prediction.

19. Which relationship or statement best describes S° for the following reaction?

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

A) S°  0

B) S° < 0

C) S° > 0

D) S° = H°/T

E) More information is needed to make a reasonable prediction.

20. Which relationship or statement best describes S° for the following reaction?

KCl(s) → K+(aq) + Cl–(aq)

A) S°  0

B) S° < 0

C) S° > 0

D) S° = H°/T

E) More information is needed to make a reasonable prediction.

21. Which relationship or statement best describes S° for the following reaction?

BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)

A) S°  0

B) S° < 0

C) S° > 0

D) S° = H°/T

E) More information is needed to make a reasonable prediction.

Chapter 20: Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions

22. Which relationship or statement best describes S° for the following reaction?

CaO(s) + CO2(g) → CaCO3(s)

A) S°  0

B) S° < 0

C) S° > 0

D) S° = H°/T

E) More information is needed to make a reasonable prediction.

23. Which of the following pairs has the member with the greater molar entropy listed first?

All systems are at 25°C.

A) CO(g), CO2(g) D) Li(s), Pb(s)

B) NaCl(s), NaCl(aq) E) H2(g), H2O(g)

C) H2S(g), H2S(aq)

24. You are given pure samples of ethane, C2H6(g), and toluene, C7H8(l). What prediction

would you make concerning their standard molar entropies at 298 K?

A) S°ethane > S°toluene

B) S°ethane < S°toluene

C) S°ethane  (S°toluene) ÷ 3

D) S°ethane  S°toluene

E) Since toluene is much more complex than ethane, but ethane is in the gas phase

while toluene is a liquid, none of the above predictions can be confidently made without

further information or calculations.

25. You are given pure samples of pentane, CH3CH2CH2CH2CH3(l), and 1,3-pentadiene,

CH2=CHCH=CHCH3(l). What prediction would you make concerning their standard molar

entropies at 298 K?

A) S°pentane > S°1, 3-pentadiene

B) S°pentane < S°1, 3-pentadiene

C) S°pentane  S°1, 3-pentadiene

D) S°pentane = S°1, 3-pentadiene + 2

2

H

S

E) More information is needed to make reasonable predictions.

Chapter 20: Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions

26. You are given pure samples of ammonia, NH3(g), and nitrogen trifluoride, NF3(g). What

prediction would you make concerning their standard molar entropies at 298 K?

A) S°ammonia > S°nitrogen trifluoride

B) S°ammonia < S°nitrogen trifluoride

C) S°ammonia  S°nitrogen trifluoride

D) Other conditions need to be specified before a reliable prediction can be made.

E) Even if more conditions are specified, a reliable prediction cannot be made.

27. In which one of the following pairs will the first system have a higher entropy than the

second?

Assume P and T are the same for each pair, unless stated otherwise.

A) 1 mole He(g); 1 mole Kr(g)

B) 1 mole O2(g); 2 mole O(g)

C) 1 mole CH4(g); 1 mole C2H6(g)

D) 1 mole Xe(g) at 1 atmosphere; 1 mole Xe(g) at 0.5 atmosphere

E) 20 one-dollar bills distributed randomly among 20 people; 20 one-dollar bills

distributed randomly among 10 people

28. In which one of these pairs will the entropy of the first substance be greater than that of

the second? Assume P and T are the same for each pair, unless stated otherwise.

A) 1 mole of F2(g); 1 mole of Cl2(g)

B) 1 mole of I2(s); 1 mole of I2(g)

C) 1 mole of CaCO3(s); 1 mole of CaO(s) plus 1 mole of CO2(g)

D) 1 mole of H2(g) at 25°C; 1 mole of H2(g) at 50°C

E) 1 mole of O3(g); 1 mole of O2(g)

29. Which one of the following phase changes decreases the entropy of the system?

A) melting

B) heating a gas

C) vaporization

D) condensation

E) sublimation

30. Which one of the following changes of state increases the entropy of the system?

A) condensation D) crystallization

B) cooling a gas E) sublimation

C) freezing

Chapter 20: Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions

31. A sample of water is heated at a constant pressure of one atmosphere. Initially, the

sample is ice at 260 K, and at the end the sample consists of steam at 400 K. In which of the

following 5K temperature intervals would there be the greatest increase in the entropy of the

sample?

A) from 260 K to 265 K D) 370 K to 375 K

B) from 275 K to 280 K E) from 395 K to 400 K

C) from 360 K to 365 K

32. Calculate S° for the reaction

SiCl4(g) + 2Mg(s) → 2MgCl2(s) + Si(s)

Substance: SiCl4(g) Mg(s) MgCl2(s) Si(s)

S°(J/Kmol): 330.73 32.68 89.62 18.83

A) –254.96 J/K D) 254.96 J/K

B) –198.02 J/K E) 471.86 J/K

C) 198.02 J/K

33. Calculate S° for the reaction

4Cr(s) + 3O2(g) → 2Cr2O3(s)

Substance: Cr(s) O2(g) Cr2O3(s)

S°(J/Kmol): 23.77 205.138 81.2

A) –548.1 J/K

B) –147.7 J/K

C) 147.7 J/K

D) 310.1 J/K E

) 548.1 J/K

34. Calculate S° for the reaction

2Cl2(g) + SO2(g) → SOCl2(g) + Cl2O(g)

Substance: Cl2(g) SO2(g) SOCl2(g) Cl2O(g)

S°(J/Kmol) 223.0 248.1 309.77 266.1

A) –118.2 J/K

B) –104.8 J/K

C) 104.8 J/K

D) 118.2 J/K

E) 1270.0 J/K

Chapter 20: Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions

35. Calculate S° for the combustion of propane.

C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)

Substance: C3H8(g) O2(g) CO2(g) H2O(g)

S°(J/Kmol): 269.9 205.138 213.74 18.825

A) –100.9 J/K B) –72.5 J/K C) 72.5 J/K D) 100.9 J/K E) 877.5 J/K

36. Elemental boron can be formed by reaction of boron trichloride with hydrogen.

BCl3(g) + 1.5H2(g) → B(s) + 3HCl(g)

Substance: BCl3(g) H2(g) B(s) HCl(g)

S°(J/Kmol): ? 130.6 5.87 186.8

If S° = 80.3 J/K for the reaction above, what is S° for BCl3(g)?

A) –18.2 J/Kmol D) 355.4 J/Kmol

B) 18.2 J/Kmol E) 450.6 J/Kmol

C) 290.1 J/Kmol

37. For a chemical reaction to be spontaneous only at high temperatures, which of the

following conditions must be met?

A) S° > 0, H° > 0 D) S° < 0, H° > 0

B) S° > 0, H° < 0 E) G° > 0

C) S° < 0, H° < 0

38. For a chemical reaction to be spontaneous only at low temperatures, which of the

following conditions must be met?

A) S° > 0, H° > 0 D) S° < 0, H° > 0

B) S° > 0, H° < 0 E) G° > 0

C) S° < 0, H° < 0

39. For a chemical reaction to be spontaneous at all temperatures, which of the following

conditions must be met?

A) S° > 0, H° > 0

B) S° > 0, H° < 0

C) S° < 0, H° < 0

D) S° < 0, H° > 0

E) It is not possible for a reaction to be spontaneous at all temperatures.

Chapter 20: Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions

40. For a chemical reaction to be non-spontaneous at any temperature, which of the

following conditions must be met?

A) S° > 0, H° > 0

B) S° > 0, H° < 0

C) S° < 0, H° < 0

D) S° < 0, H° > 0

E) All reactions are spontaneous at some temperature.

41. For a process with S < 0, which one of the following statements is correct?

A) The process will definitely be spontaneous if H < 0.

B) The process will be definitely be spontaneous if H < TS.

C) The process can never be spontaneous.

D) The process will definitely be spontaneous, regardless of H.

E) The process will definitely be spontaneous if Ssurr > 0.

42. Consider the following quantities used in thermodynamics: E, H, q, w, S, G. How many

of them are state functions?

A) 0 B) 1 C) 2 D) 3 E) 4

43. In order for a process to be spontaneous,

A) H must be less than zero.

B) S must be greater than zero.

C) G must be greater than zero.

D) it should be rapid.

E) Ssys + Ssurr must be greater than zero.

44. Which of the following conditions will ensure that a chemical reaction will definitely

proceed in the forward direction, toward products?

A) H > 0 B) H < 0 C) Ssys > 0 D) Ssurr > Ssys E) S > H/T

45. Given: H2O(l) → H2O(g) H° = 40.7 kJ at 373K

What is the entropy change in the system (S) when one mole of water vaporizes at 100 °C and

a pressure of one atmosphere?

A) 407 J/K B) –407 J/K C) 109 J/K D) –109 J/K E) J/K

Chapter 20: Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions

46. Given: H2O(l) → H2O(s) H° = –6.02 kJ at 273K

Calculate the entropy change of the surroundings (Ssurr) when one mole of water freezes at 0

°C and a pressure of one atmosphere.

A) 22.1 J/K B) –22.1 J/K C) 397 J/K D) –397 J/K E) 0.022 J/K

47. A certain process has H° > 0, S° < 0, and G° > 0. The values of H° and S° do not

depend on the temperature. Which of the following is a correct conclusion about this process?

A) It is non-spontaneous at all T.

B) It is spontaneous at high T.

C) It is spontaneous at low T.

D) It is spontaneous at all T.

E) None of the above conclusions is correct.

48. The second law of thermodynamics tells us that

A) the entropy of the universe is constant.

B) entropy is neither created nor destroyed.

C) the universe proceeds toward a state of lower entropy.

D) the universe proceeds toward a state of higher entropy.

E) the universe cannot create entropy.

49. In order for a process to be spontaneous,

A) the entropy of the system must increase.

B) the entropy of the surroundings must increase.

C) the entropy of the universe must decrease.

D) the entropy of the surroundings must decrease.

E) the entropy change of the surroundings plus the entropy change of the system must

be positive.

50. Calculate G° for the reaction

SiCl4(g) + 2Mg(s) → 2MgCl2(s) + Si(s)

Substance: SiCl4(g) Mg(s) MgCl2(s) Si(s)

G° f (kJ/mol): –616.98 0 –591.79 0

A) 566.60 kJ B) 50.38 kJ C) 25.19 kJ D) –25.19 kJ E) –566.60 kJ

Chapter 20: Thermodynamics: Entropy, Free Energy, and the Direction of Chemical Reactions

51. Calculate G° for the combustion of propane.

C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)

Substance: C3H8(g) O2(g) CO2(g) H2O(g)

G° f (kJ/mol): –24.5 0 –394.4 –228.6

A) –2073.1 kJ B) –1387.3 kJ C) –598.5 kJ D) 598.5 kJ E) 2073.1 kJ

52. Elemental boron can be formed by reaction of boron trichloride with hydrogen.

BCl3(g) + 1.5H2(g) → B(s) + 3HCl(g)

Calculate G° for the reaction.

Substane: BCI3(g) H2(g) B(s) HCl(g)

G° f (kJ/mol): –388.7 0 0 –95.3

A) –293.4 kJ D) 102.8 kJ

B) 293.4 kJ E) none of the above

C) –102.8 kJ

53. Calculate G° for the reaction of ammonia with fluorine.

2NH3(g) + 5F2(g) → N2F4(g) + 6HF(g)

Substance: NH3(g) F2(g) N2F4(g) HF(g)

G° f (kJ/mol): –16.4 0 79.9 –275.4

A) 179.1 kJ D) –1539.7 kJ

B) –179.1 kJ E) none of the above

C) 1539.7 kJ

54. Use the given data at 298 K to calculate G° for the reaction

2Cl2(g) + SO2(g) → SOCl2(g) + Cl2O(g)

Substance: Cl2(g) SO2(g) SOCl2(g) Cl2O(g)

H° f (kJ/mol): 0 –296.8 –212.5 80.3

S°(J/Kmol): 223.0 248.1 309.77 266.1

A) 129.3 kJ B) 133.6 kJ C) 196.0 kJ D) 199.8 kJ E) 229.6 kJ