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

Question 1

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)

Answer 1

N₂O₄(g)

Question 2

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.

Answer 2

.S°  0

Question 3

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.

Answer 3

CO₂(s)  CO₂(g) at 0°C

Question 4

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

A) .H_sys < 0
B) .S_sys > 0
C) .S_surr< 0
D) .S_univ > 0
E) .G_sys = 0

Question 5

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

A) q _sys > 0, S_surr < 0
B) q _sys < 0, S_surr > 0
C) q _sys < 0, S_surr < 0
D) q _sys > 0, S_surr > 0
E) w < 0

Question 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.

Question 7

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.

Question 8

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.

Question 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

Question 10

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.

Question 11

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)

Question 12

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.

Question 13

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

A) q _sys > 0, S_surr < 0
B) q _sys < 0, S_surr > 0
C) q _sys < 0, S_surr < 0
D) q _sys > 0, S_surr > 0
E) w < 0

Question 14

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

Question 15

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

Question 16

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

Question 17

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.

Question 18

A certain process has S_univ > 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

Question 19

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.

Question 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.

Question 21

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.

Question 22

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

Question 23

For a chemical reaction to be spontaneous only at high 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) .G° > 0

Question 24

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.

Question 25

Calculate S° for the combustion of propane.

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

Question 26

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

A) condensation
B) cooling a gas
C) freezing
D) crystallization
E) sublimation

Question 27

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.

Question 28

Elemental boron can be formed by reaction of boron trichloride with hydrogen. If S° = 80.3 J/K for the reaction above, what is S° for BCl₃(g)?

A) -18.2 J/Kmol
B) 18.2 J/Kmol
C) 290.1 J/Kmol
D) 355.4 J/Kmol
E) 450.6 J/Kmol

Question 29

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.

Question 30

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.

Question 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
B) from 275 K to 280 K
C) from 360 K to 365 K
D) 370 K to 375 K
E) from 395 K to 400 K

Question 32

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
E) More information is needed to make reasonable predictions.

Question 33

For a chemical reaction to be spontaneous only at low 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) .G° > 0

Question 34

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)
B) NaCl(s), NaCl(aq)
C) H₂S(g), H₂S(aq)
D) Li(s), Pb(s)
E) H₂(g), H₂O(g)

Question 35

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.

Question 36

Calculate S° for the reaction

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

Question 37

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)

Question 38

Calculate S° for the reaction

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

Question 39

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

Question 40

Calculate S° for the reaction

A) -254.96 J/K
B) -198.02 J/K
C) 198.02 J/K
D) 254.96 J/K
E) 471.86 J/K

Question 41

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

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

Question 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

Question 43

The temperature at which the following process reaches equilibrium at 1.0 atm is the normal boiling point of hydrogen peroxide.

Use the following thermodynamic information at 298 K to determine this temperature.

A) 120°C
B) 144°C
C) 196°C
D) 418°C
E) 585°C

Question 44

Sulfuryl dichloride is formed when sulfur dioxide reacts with chlorine. The data refer to 298 K. SO₂(g) + Cl₂(g)  SO₂Cl₂(g) What is the value of G° for this reaction at 600 K?

A) -162.8 kJ
B) -40.1 kJ
C) -28.4 kJ
D) 28.4 kJ
E) 162.8 kJ

Question 45

Hydrogen sulfide decomposes according to the following reaction
2H₂S(g)  2H₂(g) + S₂(g)
For this reaction at 298K S° = 78.1 J/K, H° = 169.4 kJ, and G° = 146.1 kJ. What is the value of G° at 900 K?

A) -69881 kJ
B) 48.4 kJ
C) 99.1 kJ
D) 240 kJ
E) 441 kJ

Question 46

The temperature at which the following process reaches equilibrium at 1.0 atm is the normal melting point for phosphoric acid.
Use the following thermodynamic data at 298 K to determine this temperature.

A) 286 K
B) 305 K
C) 315 K
D) 347 K
E) 3170 K

Question 47

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 S_surr > 0.

Question 48

"A diamond is forever" is one of the most successful advertising slogans of all time. But is it true? For the reaction shown below, calculate the standard free energy change at 298K and determine whether or not a diamond is "forever". C(diamond)  C(graphite)
Data: H_f°(diamond) = 1.895 kJ/mol; S°(diamond) = 2.337 J mol^-1 K^-1;
S°(graphite) = 5.740 J mol^-1K^-1.

A) .G° = 2.19 kJ; forever
B) .G° = -1.90 kJ; not forever
C) .G° = -2.90 kJ; not forever
D) .G° = 1.90 kJ; forever
E) .G° = < -1000 kJ; not forever

Question 49

Nitric oxide reacts with chlorine to form NOCl. The data refer to 298 K. What is the value of G° for this reaction at 550 K?

A) -143.76 kJ
B) -78.78 kJ
C) -22.24 kJ
D) -10.56 kJ
E) 66600 kJ

Question 50

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.

Question 51

Given: H₂O(l)  H₂O(s) H° = -6.02 kJ at 273K Calculate the entropy change of the surroundings (S_surr) 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

Question 52

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

Question 53

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

A) -293.4 kJ
B) 293.4 kJ
C) -102.8 kJ
D) 102.8 kJ
E) none of the above

Question 54

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) .S_sys > 0
D) .S_surr > S_sys
E) .S > H/T

Question 55

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.

Question 56

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

A) 179.1 kJ
B) -179.1 kJ
C) 1539.7 kJ
D) -1539.7 kJ
E) none of the above

Question 57

Calculate G° for the reaction

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

Question 58

Calculate G° for the combustion of propane.

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

Question 59

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) .S_sys + S_surr must be greater than zero.

Question 60

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.

Question 61

Given: C₂H₂(g)  2C(graphite) + H₂(g) G° = -209 kJ
A sample of gaseous C₂H₂ (acetylene, or ethyne) was stored for one year, yet at the end of this period the sample remained unchanged and no graphite or hydrogen gas had been formed. Briefly explain why there is no inconsistency between the sign of G° and the apparent stability of the sample.

Question 62

A chemical reaction has G° = 10.0 kJ and S° = 50.0 J/K
a. Calculate H° for this reaction at 25°C.
b. Could this reaction ever be spontaneous? Explain your answer.

Question 63

What is the free energy change, G°, for the equilibrium between hydrogen iodide, hydrogen, and iodine at 453°C?

A) 6.4 kJ
B) 8.8 kJ
C) 15 kJ
D) 19 kJ
E) 24 kJ

Question 64

A reaction is proceeding toward equilibrium. At a certain stage, the concentrations of reactants and products are such that G = G°. What conclusion can reasonably be drawn about the reaction at this time?

A) K > Q
B) K < Q
C) K = Q
D) K = 1
E) Q = 1

Question 65

Consider the figure which shows G° for a chemical process plotted against absolute temperature. Which one of the following is an incorrect conclusion, based on the information in the diagram?

A) .H° > 0
B) .S° > 0
C) The reaction is spontaneous at high temperatures.
D) .S° increases with temperature while H° remains constant.
E) There exists a certain temperature at which H° = TS°.

Question 66

For each of the following pairs, predict which (A or B) will have the greater entropy, and in one sentence indicate your reasoning.

Question 67

a. Explain what is meant by a spontaneous process.
b. Is a spontaneous process necessarily a rapid one? Explain, and provide a real reaction as an example to illustrate your answer.

Question 68

A reaction has G = 10.0 kJ and G° = 15.0 kJ at a temperature of 50 °C. Calculate the value of the reaction quotient Q under these conditions.

A) 0.16
B) 9.1 × 10^-5
C) 1.1 × 10⁴
D) 6.4
E) 6.0 × 10^-6

Question 69

In the expression, S = k ln W, W is called the number of microstates. Explain clearly the meaning of the word "microstate", and why a system under a given set of conditions normally has many microstates.

Question 70

The reaction of methane with water to form carbon dioxide and hydrogen is non-spontaneous at 298 K. At what temperature will this system make the transition from non-spontaneous to spontaneous? The data refer to 298 K.

A) 658 K
B) 683 K
C) 955 K
D) 1047 K
E) 1229 K

Question 71

Consider the figure which shows G° for a chemical process plotted against absolute temperature. From this plot, it is reasonable to conclude that:

A) .H° > 0, S° > 0
B) .H° > 0, S° < 0
C) .H° < 0, S° > 0
D) .H° < 0, S° < 0
E) none of the above

Question 72

Consider the reaction
11ec6d53_97e1_49fd_9cd1_030bf8888a28_TB5833_
If the concentrations of the Cu⁺ and I^- ions in equilibrium at 298 K are both equal to 1.03 × 10^-6 M, what is the value of G° for the reaction?

A) -68 kJ
B) 68 kJ
C) -30. kJ
D) 30 kJ
E) 34 kJ

Question 73

Iron(III) oxide can be reduced by carbon monoxide.
Use the following thermodynamic data at 298 K to determine the equilibrium constant at this temperature.

A) 7.0 × 10^-6
B) 1.3 × 10^-3
C) 2.2 × 10⁴
D) 1.4 × 10⁵
E) > 2.0 × 10⁵

Question 74

For the reaction of xenon and fluorine gases to form solid XeF₄, H° = -251 kJ and G° = -121 kJ at 25°C. Calculate S° for the reaction.

Question 75

In tables of thermodynamic data provided in chemistry books, one finds H°_f, G°_f, and S° listed. Briefly, explain why the entropy data are supplied as S°, while the enthalpy and free energy data are in the form of H°_f and G°_f, respectively.

Question 76

Consider the figure which shows G° for a chemical process plotted against absolute temperature. From this plot, it is reasonable to conclude that:

A) .H° > 0, S° > 0
B) .H° > 0, S° < 0
C) .H° < 0, S° > 0
D) .H° < 0, S° < 0
E) none of the above

Question 77

The formation constant for the reaction

Is K_f = 1.7 × 10⁷ at 25°C. What is G° at this temperature?

A) -1.5 kJ
B) -3.5 kJ
C) -18 kJ
D) -23 kJ
E) -41 kJ

Question 78

A chemical reaction has H° = 42.8 kJ and S° = 92.5 J/K, at 25°C. Calculate the temperature at which G° = 0. State any approximation involved in your calculation.

Question 79

State the second and third laws of thermodynamics.

Question 80

Calculate the equilibrium constant at 25°C for the reaction of methane with water to form carbon dioxide and hydrogen. The data refer to

A) 8.2 × 10¹⁹
B) 0.96
C) 0.58
D) 1.2 × 10^-20
E) 1.4 × 10^-46