University of South Florida

CHM 2046

1)Calculate DG° for the reaction at 25°C.

A) –1.35 ´ 10⁵ kJ

B)     98.7 kJ

C)     –25.2 kJ

4. 135 kJ
5. 0

2. The reaction is allowed to proceed until all substances involved have reached their equilibrium concentrations. Under those conditions, what is DG for the reaction?

A) –1.35 ´ 10⁵ kJ

B)     98.7 kJ

C)     –25.2 kJ

4. 135 kJ
5. 0

3. Which of the following is true for this reaction?
   1. Both DH° and DS° favor the reaction's spontaneity.
   2. Both DH° and DS° oppose the reaction's spontaneity.
   3. DH° favors the reaction, but DS° opposes it.
   4. DH° opposes the reaction, but DS° favors it.
   5. The reaction cannot occur at room temperature.

4. Which of the following is not a state function?
   1. q
   2. G
   3. H
   4. E
   5. P

5. The standard free energy of formation of KCl(s) is –408.8 kJ/mol. DG° for the reaction 2KCl(s) ® 2K(s) + Cl2(g) is: A) –408.8 kJ

B)     817.6 kJ

C)     408.8 kJ

D)   –817.6 kJ

E)     none of these

6. Consider the following hypothetical reaction at 310 K. Standard free energies of formation are given in parentheses.

B ®              C                DG° = –32.6 kJ/mol (?)                  (176.4

kJ/mol)

Calculate the standard free energy of formation of compound B.

1. 1. 209.0 kJ/mol
   2. –209.0 kJ/mol
   3. 143.8 kJ/mol
   4. –143.8 kJ/mol
   5. none of these

7. 

   For the reaction                                                                     DH° = 126.4 kJ/mol and DS° = –74.9 J/K mol. At 361°C, what is DG ?
   1. 153.4 kJ/mol
   2. 47.6 kJ/mol
   3. 173.9 kJ/mol
   4. 78.9 kJ/mol
   5. 155.0 kJ/mol

8. Determine DG° for the following reaction: CH4(g) + 2O2(g) ® CO2(g) + 2H2O(l)

Substance                      DGf°(kJ/mol) CH4(g)                                –50.69

O2(g)                                      0

CO2(g)                                –394.4

H2O(l)                                 –237.4

A)   –581.1 kJ

B)     –919.9 kJ

C)     –818.5 kJ

D)   –682.5 kJ

E)     131.1 kJ

9. Of DS, DSsurr, DSuniv, and DG, which are state functions?
   1. DS, DSsurr, DSuniv, and DG are all state functions.
   2. Only DS, DSuniv, and DG are state functions.
   3. Only DS and DG are state functions.
   4. Only DS, DSsurr, and DSuniv are state functions.
   5. Only DSuniv and DG are state functions.

10. 

    At 699 K, DG° = –23.25 kJ for the reaction H2(g) + I2(g)     2HI(g). Calculate DG for this reaction if the reagents are both supplied at 10.0 atm pressure and the product is at 1.76 atm pressure.

A) –20.2 kJ

2. 20.2 kJ
3. 3.1 kJ

D) –43.4 kJ

E)     43.4 kJ

11. The standard molar free energies of formation of NO2(g) and N2O4(g) at 25°C are 51.84 and 98.00 kJ/mol, respectively. What is the value of DG for the reaction written as follows at 25°C if the pressures of both gases are

1.33 atm?

A) –4.97

B)     4.97

C)     –6.39

D)   –5.68

E)     –5.74

12. Elemental sulfur exists in two crystalline forms, rhombic and monoclinic. From the following data, calculate the equilibrium temperature at which monoclinic sulfur and rhombic sulfur are in equilibrium.

1. 1. 446 K
   2. 202 K

C)     –202 K

D)   –446 K

E)     0 K

13. Given the following data (DHf, S°, respectively) for N2O4(l) -20. kJ/mol, 209.0 J/K mol, and N2O4(g) 10. kJ/mol,

304.2 J/K mol. Above what temperature (in °C) is the vaporization of N2O4 liquid spontaneous?

1. Above –178 °C.
2. Above –231 °C.
3. Above 3 °C.
4. Above 30. °C.
5. Above 42 °C.

14. The reaction 2H2O(g) ® 2H2(g) + O2(g) has a positive value of DG°. Which of the following statements must be true?
    1. The reaction is slow.
    2. The reaction will not occur. (When H2O(g) is introduced into a flask, no O2 or H2 will form even over a long period of time.)
    3. The reaction is exothermic.
    4. The equilibrium lies far to the right.
    5. None of these is true.

15. Given the following free energies of formation:

DGf°

C2H2(g)                                    209.2 kJ/mol

C2H6(g)                                   –32.92 kJ/mol

B)     1.10

C)     7.95 ´ 10³⁰

D)   2.76 ´ 10⁴²

E)     None of these is within a factor of 10 of the correct answer.

16. 

    The acid dissociation constant for a weak acid HX at 25°C is 4.3      10^–8. Calculate the free energy of formation for X^–(aq) at 25°C. The standard free energies of HX(aq) and H⁺(aq) at 25°C are –208.3 kJ/mol and 0, respectively.
    1. –205 kJ/mol
    2. 250 kJ/mol
    3. 0
    4. –166 kJ/mol
    5. –250 kJ/mol

17. The standard molar free energies of formation of NO2(g) and N2O4(g) at 25°C are 51.840 and 98.035 kJ/mol, respectively. What is the value of Kp (in atm) for the reaction written as follows at 25°C?

B)     1.00

C)     9.76

D) 4.38 ´ 10^–36

E)     7.99 ´ 10^–9

18. 

    Given that DGf° for NH3 = –16.668 kJ/mol, calculate the equilibrium constant for the following reaction at 298 K: N2(g) + 3H2(g)        2NH3(g)

A)   6.97 ´ 10⁵

B)     8.35 ´ 10²

C)     1.01

D) 4.51 ´ 10⁶⁹

E)     5.82 ´ 10⁸

19. Calculate Ksp for the salt NaCl at 25°C. Substance           DGf°(in kJ/mol)

Na⁺(aq)                                –262.0

Cl^–(aq)                                  –131.0

NaCl(s)                                  383.8

A)    41

B)     1.7 ´ 10¹⁹

C)    9.2

D)    4.1

E)    0.41

20. Determine DG° for the weak acid, HF, at 25°C. (Ka = 7.16 ´ 10^–4)
    1. 1.5 kJ
    2. 177 kJ
    3. 7.79 kJ
    4. 1.77 kJ
    5. 17.9 kJ

21. Assume that the reaction:

occurs in an ideal mixture of ideal gases. At 700. K, Kp = 3.24. At this temperature, DG° equals:

1. 1. 0 kJ
   2. 6.84 kJ

C)     –3.42 kJ

D)   –6.84 kJ

E)     –2.97 kJ

22. Consider the reaction:

for which DH° = –200. kJ and DS° = –187.1 J/K at 25°C. Assuming that DH° and DS° are independent of temperature, calculate the temperature where Kp = 1.

A) 969. K

B)     2069 K

C)     200. K

D)   1069 K

E)     none of these

23. 

    For the following reaction, CO2(g) + 2H2O(g)    CH4(g) + 2O2(g), DH° = 803 kJ which of the following will increase K?
    1. decrease number of moles of methane
    2. increase volume of system
    3. increase the temperature of system
    4. all of the above
    5. none of the above

24. For a particular reaction the equilibrium constant is 0.0122 at 370.°C and DH° is +16.0 kJ at 25°C. Assuming DH° and DS° are temperature independent, calculate DS° for the reaction.

A) 6.61 J/K

B)     –6.61 J/K

C)     –11.8 J/K

D) 11.8 J/K

E)     none of these

25. 

    

    

    

    Calculate DG° for                                                                       at 600.1 K, using the following data: H2(g) + O2(g)          H2O2(g)              Kp = 2.3 ´ 10⁶ at 600.1 K

1. 1. 141 kJ
   2. –501 kJ
   3. 501 kJ
   4. –287 kJ
   5. 287 kJ

26. 

    Consider the following hypothetical reaction (at 307.8 K). Standard free energies in kJ/mol are given in parentheses. A        B             +                       C DG° = ?

(-32.2)             (207.8)            (-237.0)

What is the value of the equilibrium constant for the reaction at 307.8 K? A) 0.31

B)     1.0

C)     9.0 ´ 10⁴

D) 273

E)     0.43

27. 

    The equilibrium constant K for the dissociation reaction of a molecule X2 X2(g)   2X(g)

was measured as a function of temperature (in K). A graph of ln K versus 1/T for this reaction gives a straight line with a slope of –1.352 ´ 10⁴ and an intercept of 15.59 K. The value of DS for this dissociation reaction is:

1. 1. 1.875 J/K mol
   2. 259.2 J/K mol
   3. 129.6 J/K mol
   4. 64.81 J/K mol
   5. none of these

28. 

    The following reaction has a DG° value of 48.47 kJ/mol at 25°C. HB(aq) + H2O(l) H3O⁺(aq) + B^–(aq)

Calculate the Ka for the acid HB. A) 0.981

B)     –19.6

C)     3.19 ´ 10^–9

D)   4.85 ´ 10^–5

E)     –233

29. The standard free energy of formation of nitric oxide, NO, at 1000. K (roughly the temperature in an automobile engine during ignition) is 77.9 kJ/mol. Calculate the equilibrium constant for the reaction

A) 1.56 ´ 10⁵

B)     8.53 ´ 10^–5

C)     –14.7

D) 7.27 ´ 10^-9

E)     0.948

30. 

    Consider the reaction 2NO2(g)     N2O4(g); DH° = –56.8 kJ and DS° = –175 J/K. In a container (at 298 K) N2O4(g) and NO2(g) are mixed with initial partial pressures of 2.4 atm and 0.42 atm, respectively. Which of the following statements is correct?
    1. Some N2O4(g) will decompose into NO2(g).
    2. Some NO2(g) will dimerize to form N2O4(g).
    3. The system is at equilibrium at these initial pressures.
    4. The final total pressure must be known to answer this question.
    5. None of these.

31. 

    Calculate DH° at 25°C for the following reaction: 2NO + O2           2NO2

A) 56.91 kJ

B)     –113.8 kJ

C)     –28.5 kJ

4. 3239 kJ
5. none of these

32. 

    Calculate K for the following reaction at 25°C: 2NO + O2           2NO2

A) 2.92 ´ 10⁶

B)     2.13 ´ 10¹²

C)     7.30 ´ 10⁵

D)   1.21 ´ 10³

E)     1.46 ´ 10⁶

33. 

    Calculate DG° at 25°C for the following reaction: 2NO + O2           2NO2

A)   –70.3 kJ

B)     –5.90 kJ

C)     –35.2 kJ

4. 5.90 kJ
5. 70.3 kJ

34. Calculate DS° at 25°C for the following reaction:

1. 1. 236 J/K

B)     –146 J/K

C)     –236 J/K

D)   –264 J/K

E)     264 J/K

35. For this system at equilibrium, how will raising the temperature affect the amount of NO present?
    1. The amount of NO will increase.
    2. The amount of NO will decrease.
    3. The amount of NO will remain the same.
    4. Cannot be determined.
    5. Answer depends on the value of K.

36. What would be the effect on the amount of NO present of compressing the equilibrium system to a smaller volume, while keeping the temperature constant?
    1. The amount of NO will increase.
    2. The amount of NO will decrease.
    3. The amount of NO will remain the same.
    4. Cannot be determined.
    5. Answer depends on the value of K.

2. 27.0 kJ
3. 2.27 kJ

D) –2.27 kJ

E)     27.0 J

A) –74.6 kJ

2. 74.6 kJ
3. 20.3 kJ

D) –20.3 kJ

E)     27.1 kJ

39. The equilibrium constant of a certain reaction was measured at various temperatures to give the plot shown below. What is DS° for the reaction in J/mol × K?

A) 0.20

B)     3.0

C)     25

D) –50.

E)     –8.3 ´ 10³

40. Consider a weak acid, HX. If a 0.10 M solution of HX has a pH of 5.64 at 25°C, what is DG° for the acid's dissociation reaction at 25°C?

A) –58.6 kJ

2. 4.92 kJ
3. 0

D) –4.92 kJ

E)     58.6 kJ

41. 

    For the reaction CO(g) + 2H2(g)       CH3OH(g) DG°700K = –13.462 kJ. The Kp for this reaction at 700. K is: A) 10.1

B)     1.00

C)     1.54

D)   2.31

E)     none of these

42. 

    For the reaction 2HF(g)        H2(g) + F2(g), DG° = 38.3 kJ, at 1000 K. If, at this temperature, 5.00 moles of HF(g),

0.500 moles of H2(g), and 0.75 moles of F2(g) are mixed in a 1.00-L container:

1. Some HF will decompose (to yield H2 and F2).
2. The system is at equilibrium.
3. Some HF will be formed (from H2 and F2).
4. Not enough data are given to answer this question.
5. None of these (A-D).

43. If the temperature of the system is raised, the ratio of the partial pressure of PCl5 to the partial pressure of PCl3 will
    1. increase
    2. decrease
    3. stay the same
    4. impossible to tell without more information
    5. none of these

44. When some Cl2(g) is added at constant volume and temperature, the ratio of the partial pressure of PCl5 to the partial pressure of PCl3 will
    1. increase
    2. decrease
    3. stay the same
    4. impossible to tell without more information
    5. none of these

45. When the volume is decreased at constant temperature, the ratio of the partial pressure of PCl5 to the partial pressure of PCl3 will
    1. increase
    2. decrease
    3. stay the same
    4. impossible to tell without more information
    5. none of these

46. 

    Water gas, a commercial fuel, is made by the reaction of hot coke carbon with steam. C(s) + H2O(g)              CO(g) + H2(g)

When equilibrium is established at 844°C the concentrations of CO, H2, and H2O are 4.00 ´ 10^–2, 4.00 ´ 10^–2, and

1.00 ´ 10^–2 mole/liter, respectively. Calculate the value of DG° for this reaction at 844°C.

A) 12.9 kJ

B)     –12.87 kJ

C)     55.64 kJ

D)   17.02 kJ

E)     none of these

47. From these data, which of the following statements is true?
    1. The reaction is exothermic.
    2. The reaction is endothermic.
    3. The reaction rate is high
    4. The reaction is not spontaneous.
    5. None of these.

48. The value of DH for this dissociation reaction is: A) –137.3 kJ

B)     137.3 kJ

C)     1.986 kJ

D)   –1.986 kJ

E)     none of these

49. For a certain process, at 300. K, DG = –10.4 kJ and DH = –7.0 kJ. If the process is carried out reversibly, the amount of useful work that can be performed is

A) –17.4 kJ

2. –7.0 kJ
3. –3.4 kJ

D) –10.4 kJ

E)     3.4 kJ

50. For a certain process, at 300. K, DG = –12.2 kJ and DH = –7.0 kJ. If the process is carried out so that no useful work is performed, DG is
    1. 12.2 kJ
    2. 7.0 kJ
    3. 0
    4. –7.0 kJ

E)     –12.2 kJ

51. Which statement is true?
    1. All real processes are irreversible.
    2. A thermodynamically reversible process takes place infinitely fast.
    3. In a reversible process, the state functions of the system are always much greater than those of the surroundings.
    4. There is always more heat given off to the surroundings in a reversible process than in an unharnessed one.
    5. All statements (A–D) are true.

Would you predict an increase or decrease in entropy for each of the following?

52. The freezing of water

 ANS:

decrease in entropy

Positional entropy relates to the phase of the substance: Ssolid < Sliquid < Sgas. See Sec. 17.1, Zumdahl, Chemistry.

53. He(g) at 3 atm ® He(g) at 1 atm

 ANS:

increase in entropy

At constant temperature, a decrease in pressure corresponds to an increase in volume, which imparts greater positional entropy.

See Sec. 17.1, Zumdahl, Chemistry.

54.  2H2(g) + O2(g) ® 2H2O(g)

ANS:

decrease in entropy

The change in positional entropy is dominated by the relative numbers of molecules in the gas phase. See Sec. 17.5, Zumdahl, Chemistry.

55.  2KClO3(s) ® 2KCl(s) + 3O2(g) ANS:

increase in entropy

The change in positional entropy is dominated by the relative numbers of molecules in the gas phase. See Sec. 17.5, Zumdahl, Chemistry.

 | second law of thermodynamics | entropy change for a reaction              

56. Which of the following statements is FALSE?
    1. Increasing the temperature of a system increases the entropy.
    2. 2 moles of a gas at one atmosphere pressure has a greater entropy than 2 moles of the same gas at 1000 mmHg pressure.
    3. The reaction of O3(g) to produce O2(g) has a negative entropy change.
    4. The S° of methane (CH4) is less than that of ethane (C2H6).
    5. A substance can only have zero entropy at absolute zero in a perfect crystal.

57. Which item (a, b or c) in each of the three groups below has the lowest entropy?

1. (a) 10 g ice           (b) 10 g water vapor       (c) 10 g liquid water
2. (a) 1 mole NaCl solid       (b) 1 mol NaCl in 1 M aqueous solution (c) 1 mol molten NaCl
3. (a) 1 mole C2H6(g), (b) 1 mole CH4(g), (c) 1 mol C3H8(g) all at 25 °C and 1 atm

1. a, a, b
2. c, b, c
3. b, b, c
4. a, b, c
5. a, a, c

58. In which of the following pairs is the substance with the HIGHER entropy listed first?

1. NaCl(s), NaCl(aq)
2. CO(g), CO2(g)
3. 1 mole of H2(g) at 25°C; 1 mole of H2(g) at 50°C

1. I only
2. II only
3. III only
4. all of them

5. none of them

59. Predict the sign of DS° for each of the following processes:

1. 2 K(s) + Cl2(g) " 2 KCl(s)
2. CH4(g) " C(s) + 2 H2(g)
3. CaCO3(s) " CaO(s) + CO2(g)

1. negative, negative, positive
2. negative, negative, negative
3. positive, negative, negative
4. negative, positive, positive
5. positive, positive, positive

60. Which of the following reactions will have a positive value of DS°?

I. Pb(s) + Cl2(g)   "  PbCl2(s)

II. 2H2S(g)   + 3O2(g) "  2H2O(g) + 2SO2(g)

1. I only
2. II only
3. III only
4. I and II
5. II and III

61. Which of the following processes should show the greatest increase in entropy? A) C6H6(l) + 15/2 O2(g) " 6 CO2(g) + 3 H2O(g)

B)     2 NO2(g) " N2O4(g)

C)     C2H4(g) + H2(g) " C2H6(g)

4. BaS(s) + 2 NaNO3(s) "  Ba(NO3)2(s) + Na2S(s)
5. This cannot be predicted without additional information.

62. In which of the following reactions do you expect to have the largest increase in entropy? A) I2(s) " I2(g)

B)     2IF(g) " I2(g) + F2(g)

3. Mn(s) + O2(g) " MnO2(s)
4. Hg(l) + S(s) " HgS(s)
5.  

63. The exothermic reaction, 2 Cu(s) + O2(g) " 2 CuO(s), is spontaneous
    1. At high temperatures
    2. At low temperatures
    3. At all temperatures
    4. The reaction is nonspontaneous at all temperatures