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1332 -CHAPTER 17
Sample Problems
Important Data
Compound
SO2(g)
SO3(g)
NO(g)
NO2 (g)
NOCl(g)
N2O(g)
Cr (s)
O2 (g)
Cr 2O3
CH4
H2O
CO2
H2
C3H8
ΔHf
kJ/mol
S
J/mol K
-74.87
-241.8
-393.5
0
211
240
264
220
23.77
205.138
81.2
186.1
188.8
213.7
130.7
ΔGf
kJ/mol
-301
-372
86.7
51.8
66.3
103.6
-50.81
-228.6
-394.4
0
-24.5
Predicting Entropy
1.
Which change is likely to be accompanied by the greatest increase in entropy?
(A)
(C)
N2(g) + 3H2(g) → 2NH3(g)
CO2(s) → CO2(g)
2.
Under which conditions does nitrogen have the largest entropy per mole?
(A)
(C)
N2(s) at 50 K and l atm
N2(g) at 80 K and 1 atm
3.
In which process is entropy decreased?
(A)
(C)
dissolving sugar in water
evaporating a liquid
4.
Which reaction has the largest positive entropy change per mole of product formed?
(A)
(C)
S(s) + 3F2(g) → SF6(g)
Fe3+(aq) + SCN-(aq) → FeSCN2+(aq)
5.
Which pair of the following has the member with the greater molar entropy listed first?
(A) CO(g), CO2(g)
(B)
(B)
(D)
Ag+(aq) + Cl–(aq) → AgCl(s)
H2O(g) → H2O(l)
(B)
(D)
(B)
(D)
NaCl(s), NaCl(aq)
N2(l) at 70 K and l atm
N2(g) at 80 K and 0.5 atm
expanding a gas
freezing water
(B)
(D)
(C)
1
SO2(g) + Na2O(s) → Na2SO3(s)
H2O(l) → H2O(g)
H2S(g), H2S(aq)
(D)
Li(s), Pb(s)
6.
For each pair of substances, at the same temperature, choose the one with the HIGHER entropy
I.
III.
(i) I2(l)
(i) MgF2(s)
(A)
(D)
(ii) (ii) (ii) (i)
(i) (ii) (i) (ii)
7.
Dissolving crystalline ammonium chloride in water lowers the temperature of the solution, so for
this process:
(A)
(C)
(E)
ΔH is negative and ΔS is positive
ΔH is positive and ΔS is positive
cannot be determined
(ii) I2(g)
(ii) BaI2(s)
(B)
(E)
II.
IV.
(i) PCl3(g)
(ii) PCl5(g)
(i) 2 moles of NO2(g) (ii) 1 mole of N2O4(g)
(i) (ii) (ii) (ii)
(ii) (i) (ii) (i)
(B)
(D)
(C)
(ii) (ii) (ii) (ii)
ΔH is positive and ΔS is negative
ΔH is negative and ΔS is negative
Entropy Calculations
8.
The value of ΔS for the following reaction 2 NO(g) + Cl2(g) → 2 NOCl(g) is –117 J/K at 25 °C.
What is the entropy of Cl2 at this temperature?
(A)
106
9.
Calculate ΔS (in J/K) for the reaction, 4 Cr + 3 O2 → 2 Cr2O3
(A)
-548.1
(B)
(B)
11
-147.7
(C)
(C)
223
147.7
(D)
(D)
-223
548.1
General ΔG
10.
Vaporization of a liquid is an example of a process for which
(A)
(C)
ΔH, ΔS, and ΔG are positive at all temperatures.
(B)
ΔG is negative at low temperatures, positive at high temperatures. (D)
11.
A chemical reaction has a negative ΔH and negative ΔS. Which statement is correct?
(A)
(B)
(C)
(D)
The reaction is spontaneous at all temperatures.
The reaction is nonspontaneous at all temperatures.
The reaction becomes spontaneous as temperature increases.
The reaction becomes spontaneous as temperature decreases.
12.
A reaction is spontaneous at all temperatures if
(A)
(C)
ΔH and ΔS are both positive.
ΔH is positive and ΔS is negative.
13.
Which one of the following statements is true about the equilibrium constant for a reaction if ΔG0
for the reaction is negative?
(A)
K=0
(B)
K=1
(B)
(D)
(C)
ΔH and ΔS are positive.
ΔH = ΔS
ΔH and ΔS are both negative.
ΔH is negative and ΔS is positive.
K<1
2
(D)
K>1
14.
If K is much less than 1 for a given reaction then:
(A)
(C)
(E)
ΔG will have the same number as K
ΔG is a large positive number
ΔG is a small positive number
15.
Which of the following is NOT true about ΔG0?
(A)
(B)
(C)
(D)
(E)
If it is negative for a reaction, the reaction is spontaneous.
If negative, it represents the maximum work available from a reaction
It cannot be measured directly in the lab.
If it is large and negative, K for the reaction is large and positive
If it is negative, ΔH has to be negative also.
16.
Which of the following is false?
(A)
(B)
(C)
(D)
(E)
The first law of thermodynamics relates to the conservation of energy
The second law of thermodynamics states that a spontaneous process MUST be accompanied by
an increase in the entropy of the universe.
The third law of thermodynamics states that only a perfect crystal at 0 K can have zero entropy
The standard state of a species is a defined quantity
Complex processes violate the laws of thermodynamics
17.
Which of the following statements are TRUE?
I.
III.
IV.
ΔG is less than zero for a spontaneous process
II.
ΔG is equal to ΔH - TΔS
ΔG is the maximum amount of work that can be obtained from a reaction
There are no instruments that can measure ΔG directly.
(A)
III
18.
A spontaneous reaction is
(A)
(C)
(E)
one that starts right away
(B)
always very fast
(D)
one that proceeds without continuous input of energy
19.
Consider the endothermic reaction C2H4(g) + H2O(g) → C2H5OH(g). When would you expect
this reaction to be spontaneous?
(B)
I and II
(A) At all temperatures
(D) At no temperature
(C)
(B)
(D)
all of them
ΔG is a large negative number
ΔG is a small negative number
(D)
(B) Above a certain temperature
(E) Not enough information given
II and III
(E)
I and III
always an exothermic reaction
none of the above
(C) below a certain temperature
ΔG Calculations
20.
Calculate ΔG° for the reaction, MnO2 + 2 CO(g) ⇔ Mn(s) + 2 CO2(g) given the following values
of ΔGf° (kJ/mol): MnO2(s), -466.1; CO(g), -137.2; CO2(g), -394.4
(A) 209.1 kJ
(B) -48.3 kJ
(C) 185.5 kJ
3
(D) -185.5 kJ
(E) -346.1 kJ
21.
What is ΔG0 for this reaction 2SO2(g) + O2(g) → 2SO3(g) at 298 K?
(A)
–142 kJ
22.
Determine the value of ΔG0 (in kJ) for the reaction 2 NO(g) + O2(g) → 2 NO2(g) at 25 °C:
(A)
-69.8
23.
What is the value of ΔH0 (in kJ) for the reaction N2O + NO2 → 3 NO at 25 °C?
(A)
156.2
24.
For the combustion of one mole of liquid methanol, ΔH° = -726 kJ. Which answer is closest to
ΔG° for this combustion at 2500 °C?
(B)
(B)
(B)
–200 kJ
-34.9
(B)
(C)
5.5
Molecule
CH3OH
S° (J/K mol) 126.8
(C)
(C)
O2
205.1
(D)
53.2
(D)
CO2
213.7
H2O
188.8
-291 kJ
(D)
142 kJ
-104.7
109.0
-1161 kJ
25.
At what temperature (in K) will the reaction of methane with water to give hydrogen gas and
carbon dioxide become spontaneous?
(A)
658
26.
Calculate the temperature at which the following reaction becomes spontaneous: H2(g) + I2(g) ⇔
2 HI given that ΔS = 165.9 J/K and ΔH = 51.8 kJ.
(A)
0K
27.
Iron was originally smelted from ore by heating it with charcoal. Calculate the lowest temperature
at which this process could be carried out. 2 Fe2O3(s) + 3 C(s) → 4 Fe(s) + 3 CO2(g)
(B)
(C)
104.7
(D)
(A)
(B)
-726 kJ
–744 kJ
683
never (C)
(C)
298 K
956
(D)
(D)
312 K
291 kJ
(E)
1161 kJ
1229
(E)
3.25 K
ΔHf(CO2) = -393.5 kJ/mol; ΔHf (Fe2O3) = -825.5 kJ/mol
S° (C) = 5.7 J/K mol; S° (Fe) = 27.3 J/K mol; S°(CO2) = 213.7 J/K mol; S° (Fe2O3) = 87.4J/K mol
(A)
1061.2 °C
28.
Coupling of the two reactions below affords the overall reaction: Fe3O4(s) + 2 C → 3 Fe(s) + 2
CO2 (g). What is the value of ΔG0 for this reaction?
1.
2.
Fe3O4(s) → 3 Fe(s) + 2 O2 (g)
O2(s) + C → CO2 (g)
(A)
+788.2
(B)
(B)
842.6 °C
-788.2
(C)
2646 °C
(D)
569.5 °C
ΔG0 = 1014.2 kJ
ΔG0 = -394.4 kJ
(C)
+225.4
4
(D)
+619.8
(E)
0.84 °C
29.
Given that ΔG0 for Ba2+(aq) is –560.7, for CO32-(aq) is –528.1 and for BaCO3(s) is –1139 (all in
kJ), calculate Ksp for BaCO3 at 25 °C.
(A)
5.86
30.
Calculate the value of ΔG° for a reaction for which Kc = 5.0 x 108 at 25 °C:
(A)
-50 kJ
31.
Given that ΔGf for IBr is 1.3 kJ/mol, calculate K for the reaction at 25 °C: I2 + Br2 ⇔ 2 IBr
(A)
1.00
32.
A particular reaction has an equilibrium constant of 0.48 at 25 °C. What is the value of ΔG0 (in
kJ)?
(A)
1.8
33.
Calculate ΔG0 (in kJ) for the combustion of propane (C3H8).
(A)
-598.5
34.
ΔH0 = 131.4 kJ and ΔG0= 91.2 kJ for the reaction C(s) + H2O(g) → CO2(g) + H2(g). Calculate
ΔS0 in J/K for this reaction at 25 °C.
(A)
-135
(B)
(B)
(B)
0.59
(B)
(B)
(B)
6.3 x 108
22 kJ
(C)
-4.2
598.5
1.6
(C)
(C)
1.3
(D)
(C)
(C)
(C)
1.59 x 10-9
-4.2 kJ
3.7 x 10-6
150
-2073.1
-1.6
(D)
(D)
(E)
(D)
(D)
(D)
2.18 x 10-27
-25 kJ (E)
49,500 kJ
0.35
4.2
2073.1
135
ANSWERS:
1. C 2. D 3. D 4. D 5. C 6. A 7. C 8. C 9. A 10. B 11. D 12. D 13. D 14. C 15. E 16. E 17. C 18. E 19. D
20 B 21. A 22. A 23. A 24. A 25. C 26. D 27. D 28. C 29. C 30. A 31. E 32. A 33. C 34. D
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