8) Of the following, the entropy of ________ is the largest.
A) B2F4 (g)
B) BF3 (g)
C) B2H6 (g)
D) BCl3 (l)
E) B2O3 (s)
9) Of the following, the entropy of gaseous ________ is the largest at 25 °C and 1 atm.
A) C3H8
B) C3H6
C) C3H4
D) C2H6
E)
10) Of the following, the entropy of gaseous ________ is the largest at 25 °C and 1 atm.
A) I2
B) F2
C) Cl2
D) Br2
E) O3
33
11) The standard Gibbs free energy of formation of ________ is zero.
(a) H2O (l)
(b) O (g)
(c) Cl2 (g)
A) (a) only
B) (b) only
C) (c) only
D) (b) and (c)
E) (a), (b), and (c)
12) The standard Gibbs free energy of formation of ________ is zero.
(a) H2O (l)
(b) Fe (s)
(c) I2 (s)
A) (a) only
B) (b) only
C) (c) only
D) (b) and (c)
E) (a), (b), and (c)
13) The standard Gibbs free energy of formation of ________ is zero.
(a) Mn (s)
(b) I2 (s)
(c) Cu (s)
A) (a) only
B) (b) only
C) (c) only
D) (b) and (c)
E) (a), (b), and (c)
14) The value of ΔG° at 261.0 °C for the formation of phosphorous trichloride from its constituent
elements,
P2 (g) + 3Cl2 (g) → 2PCl3 (g)
is ________ kJ/mol. At 25.0 °C for this reaction, ΔH° is –720.5 kJ/mol, ΔG° is -642.9 kJ/mol, and ΔS° is –
263.7 J/K.
A) -579.6
B) 6.81 × 104
C) 1.40 × 105
D) -651.7
E) -861.4
15) The value of ΔG° at 181.0 °C for the formation of calcium chloride from its constituent elements:
Ca (s) + Cl2 (g) → CaCl2 (s)
is ________ kJ/mol. At 25.0 °C for this reaction, ΔH° is -795.8 kJ/mol, ΔG° is -748.1 kJ/mol, and ΔS° is –
159.8 J/K.
A) -868.4
B) -723.2
C) 7.18 × 104
D) -766.9
E) 2.81 × 104
16) For a reaction to be spontaneous at low temperature, the signs of ΔH° and ΔS° must be ________ and
________, respectively.
A) –, –
B) +, +
C) –, +
D) +, –
E) +, 0
17) Given the following table of thermodynamic data,
complete the following sentence. The vaporization of PCl3 (l) is ________.
A) nonspontaneous at low temperature and spontaneous at high temperature
B) spontaneous at low temperature and nonspontaneous at high temperature
C) spontaneous at all temperatures
D) nonspontaneous at all temperatures
E) not enough information given to draw a conclusion
18) For the reaction
C(s) + H2O(g) → CO(g) + H2(g)
ΔH° = 133.3 kJ/mol and ΔS° = 121.6 J/K ∙ mol at 298 K. At temperatures greater than ________ °C this
reaction is spontaneous under standard conditions.
A) 273
B) 325
C) 552
D) 823
E) 1096
19) For a given reaction, ΔH = -24.2 kJ/mol and ΔS = -55.5 J/K-mol. The reaction will have ΔG = 0 at
________K. Assume that ΔH and ΔS do not vary with temperature.
A) 436
B) 2293
C) 298
D) 2.29
E) 0.436
36
20) For a given reaction, ΔH = +22.2 kJ/mol and ΔS = +81.1 J/K-mol. The reaction is spontaneous ________.
Assume that ΔH and ΔS do not vary with temperature.
A) at T < 274 K
B) at T > 274 K
C) at T > 298 K
D) at T < 298 K
E) at all temperatures
21) For a given reaction, ΔH = +74.6 kJ/mol, and the reaction is spontaneous at temperatures above the
crossover temperature, 449 K. The value of ΔS = ________ J/mol ∙ K, assuming that ΔH and ΔS do not vary
with temperature.
A) 166
B) 6020
C) -166
D) –6020
E) 3.35 × 104
22) For a given reaction, ΔS = +69.0 J/mol∙K, and the reaction is spontaneous at temperatures above the
crossover temperature, 439 K. The value of ΔH = ________ kJ/mol , assuming that ΔH and ΔS do not vary
with temperature.
A) 30.3
B) -30.3
C) 1.57 × 10-4
D) -1.57 × 10-4
E) 6.36 × 10-3
23) For a given reaction with ΔH = –28.1 kJ/mol, the ΔG = 0 at 372 K. The value of ΔS must be
________ J/K-mol, assuming that ΔH and ΔS do not vary with temperature.
A) -75.5
B) 75.5
C) -7.55 × 10-5
D) 7.55 × 10-5
E) -1.32 × 10-2
24) For a given reaction with ΔS = -50.8 J/K–mol, the ΔG = 0 at 395 K. The value of ΔH must be
________ kJ/mol, assuming that ΔH and ΔS do not vary with temperature.
A) -20.1
B) 20.1
C) -1.29 × 10-4
D) 1.29 × 10-4
E) -7.78 × 10-3
25) The value of ΔG° (kJ/mol) at 25 °C is -57.5. What is the equilibrium constant for this reaction at this
temperature expressed with 2 significant digits?
A) 1.2 × 1010
B) 10
C) 1.0
D) 8.4 × 10101
E) more information is needed
26) If ΔG° for a reaction is less than zero, then ________.
A) K > 1
B) K = 1
C) K < 1
D) K = 0
E) more information is needed.
27) In the Haber process, ammonia is synthesized from nitrogen and hydrogen:
N2 (g) + 3H2 (g) → 2NH3 (g)
ΔG° at 298 K for this reaction is -33.3 kJ/mol. The value of ΔG at 298 K for a reaction mixture that consists
of 1.9 atm N2, 2.3 atm H2, and 0.85 atm NH3 is ________.
A) -1.0
B) -4.09 × 103
C) -8.62 × 103
D) -118.0
E) -41.9
28) Phosphorous and chlorine gases combine to produce phosphorous trichloride:
P2 (g) + 3Cl2 (g) → 2PCl3 (g)
ΔG° at 298 K for this reaction is –642.9 kJ/mol. The value of ΔG at 298 K for a reaction mixture that
consists of 1.9 atm P2, 1.6 atm Cl2, and 0.65 atm PCl3 is ________.
A) -34.9
B) -4.46 × 103
C) -7.86 × 103
D) -714.1
E) -650.1
29) The equilibrium constant for a reaction is 0.38 at 25 °C. What is the value of ΔG° (kJ/mol) at this
temperature?
A) 2.4
B) -4.2
C) 200
D) 4.2
E) More information is needed.
30) The equilibrium constant for the following reaction is 3.0 × 108 at 25 °C.
N2 (g) + 3H2 (g) 2NH3 (g)
The value of ΔG° for this reaction is ________ kJ/mol.
A) 22
B) -4.1
C) 4.1
D) –48
E) –22
19.4 Short Answer Questions
1) A reversible change produces the maximum amount of ________ that can be achieved by the system on
the surroundings.
2) Calculate ΔG° (in kJ/mol) for the following reaction at 1 atm and 25 °C:
C2H6 (g) + O2 (g) → CO2 (g) + H2O (l) (unbalanced)
ΔGf° C2H6 (g) = –32.89 kJ/mol; ΔGf° CO2 (g) = –394.4 kJ/mol; ΔGf° H2O (l) = -237.13 kJ/mol
3) Calculate ΔG∘ (in kJ/mol) for the following reaction at 1 atm and 25 °C:
C2H6 (g) + O2 (g) → CO2 (g) + H2O (l) (unbalanced)
ΔHf∘ C2H6 (g) = -84.7 kJ/mol; S∘ C2H6 (g) = 229.5 J/K ∙ mol;
ΔHf∘ CO2 (g) = -393.5 kJ/mol; S∘ CO2 (g) = 213.6 J/K ∙ mol;
ΔHf∘ H2O (l) = –285.8 kJ/mol; S∘ H2O (l) = 69.9 J/K ∙ mol;
S∘ O2 (g) = 205.0 J/K ∙ mol
4) Find the temperature (in K) above which a reaction with a ΔH of 123.0 kJ/mol and a ΔS of 90.00 J/K ∙
mol becomes spontaneous.
5) Find the temperature (in K) above which a reaction with a ΔH of 53.00 kJ/mol and a ΔS of 100.0 J/K ∙
mol becomes spontaneous.
6) Calculate ΔG° for the autoionization of water at 25 °C. Kw = 1.0 × 10–14
1) The melting of a substance at its melting point is an isothermal process.
2) The vaporization of a substance at its boiling point is an isothermal process.
3) The quantity of energy gained by a system equals the quantity of energy gained by its surroundings.
4) The entropy of a pure crystalline substance at 0 °C is zero.
5) The more negative ΔG° is for a given reaction, the larger the value of the corresponding equilibrium
constant, K.