Question 1

For a chemical reaction to be spontaneous at all temperatures, which of the following conditions must be met?

Accepted Answer

A)  ΔS° > 0, ΔH° > 0 
B)  ΔS° > 0, ΔH°  0 
E)  It is not possible for a reaction to be spontaneous at all temperatures. 
A)  ΔS° > 0, ΔH° > 0 
B)  ΔS° > 0, ΔH°  0 
E)  It is not possible for a reaction to be spontaneous at all temperatures.

Question 2

Consider the reaction CuI(s)   Cu+(aq) + I-(aq)
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 ΔG° for the reaction?

Accepted Answer

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

Question 3

The free energy of a perfect crystal at absolute zero, is zero.

Accepted Answer

A) True 
 B)False

Question 4

Calculate the equilibrium constant at 25°C for the reaction of methane with water to form carbon dioxide and hydrogen. The data refer to 25°C. CH4(g) + 2H2O(g)   CO2(g) + 4H2(g)

Accepted Answer

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

Question 5

In 1774 Joseph Priestley prepared the element oxygen by heating mercury(II) oxide: HgO(s) → Hg(l) + ½O2(g)
For this reaction, ΔH° = 90.84 kJ and ΔS° = 108 J/K. Which of the following statements is true?

Accepted Answer

A)  The reaction is only spontaneous at low temperatures. 
B)  The reaction is spontaneous at all temperatures. 
C)  ΔG° becomes less favorable as the temperature is raised. 
D)  The reaction is spontaneous only at high temperatures. 
E)  The reaction is spontaneous under standard conditions at 25°C. 
A)  The reaction is only spontaneous at low temperatures. 
B)  The reaction is spontaneous at all temperatures. 
C)  ΔG° becomes less favorable as the temperature is raised. 
D)  The reaction is spontaneous only at high temperatures. 
E)  The reaction is spontaneous under standard conditions at 25°C.

Question 6

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

Accepted Answer

A)  ΔS°sys = ΔS°surr 
B)  ΔS°sys = -ΔS°surr 
C)  ΔS°sys = ΔS°surr = 0 
D)  ΔS°univ > 0 
E)  None of these choices are correct. 
A)  ΔS°sys = ΔS°surr 
B)  ΔS°sys = -ΔS°surr 
C)  ΔS°sys = ΔS°surr = 0 
D)  ΔS°univ > 0 
E)  None of these choices are correct.

Question 7

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

Accepted Answer

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 these choices are correct. 
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 these choices are correct.

Question 8

Which relationship best describes ΔS° for the following reaction? 8H2(g) + S8(s) → 8H2S(g)

Accepted Answer

A)  ΔS° = ΔH° 
B)  ΔS° = ΔH°/T 
C)  ΔS° ≈ 0 
D)  ΔS°  0 
A)  ΔS° = ΔH° 
B)  ΔS° = ΔH°/T 
C)  ΔS° ≈ 0 
D)  ΔS°  0

Question 9

Elemental boron can be formed by reaction of boron trichloride with hydrogen. BCl3(g) + 1.5H2(g) → B(s) + 3HCl(g)
Calculate ΔG° for the reaction.
Substance: BCl3(g) H2(g) B(s) HCl(g)
ΔG°f (kJ/mol): -388.7 0 0 -95.3

Accepted Answer

A)  -293.4 kJ 
B)  293.4 kJ 
C)  -102.8 kJ 
D)  102.8 kJ 
E)  None of these choices are correct. 
A)  -293.4 kJ 
B)  293.4 kJ 
C)  -102.8 kJ 
D)  102.8 kJ 
E)  None of these choices are correct.

Question 10

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

Accepted Answer

A)  Melting 
B)  Heating a gas 
C)  Vaporization 
D)  Condensation 
E)  Sublimation 
A)  Melting 
B)  Heating a gas 
C)  Vaporization 
D)  Condensation 
E)  Sublimation

Question 11

Which of the following pairs has the member with the greater molar entropy listed first? All systems are at 25°C.

Accepted Answer

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) 
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 12

Which relationship or statement best describes ΔS° for the following reaction? HgS(s) + O2(g) → Hg(l) + SO2(g)

Accepted Answer

A)  ΔS° ≈ 0 
B)  ΔS°  0 
D)  ΔS° = ΔH°/T 
E)  More information is needed to make a reasonable prediction. 
A)  ΔS° ≈ 0 
B)  ΔS°  0 
D)  ΔS° = ΔH°/T 
E)  More information is needed to make a reasonable prediction.

Question 13

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

Accepted Answer

A)  ΔH°f = 0 for Al(s) at 298 K 
B)  ΔG°f = 0 for H2(g) at 298 K 
C)  S° = 51.446 J/(mol·K) for Na(s) at 298 K 
D)  qsys 2O(l) → H2O(s) at 0°C 
E)  None of these choices are correct. 
A)  ΔH°f = 0 for Al(s) at 298 K 
B)  ΔG°f = 0 for H2(g) at 298 K 
C)  S° = 51.446 J/(mol·K) for Na(s) at 298 K 
D)  qsys 2O(l) → H2O(s) at 0°C 
E)  None of these choices are correct.

Question 14

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

Accepted Answer

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

Question 15

Calculate ΔG° for the reaction SiCl4(g) + 2Mg(s) → 2MgCl2(s) + Si(s)
Substance: SiCl4(g) Mg(s) MgCl2(s) Si(s)
ΔG°f (kJ/mol): -616.98 0 -591.79 0

Accepted Answer

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

Question 16

For any reaction, if ΔG° > 0, then K < 1.

Accepted Answer

A) True 
 B)False

Question 17

Which relationship or statement best describes ΔS° for the following reaction? 2H2S(g) + 3O2(g) → 2H2O(g) + 2SO2(g)

Accepted Answer

A)  ΔS° ≈ 0 
B)  ΔS°  0 
D)  ΔS° = ΔH°/T 
E)  More information is needed to make a reasonable prediction. 
A)  ΔS° ≈ 0 
B)  ΔS°  0 
D)  ΔS° = ΔH°/T 
E)  More information is needed to make a reasonable prediction.

Question 18

The temperature at which the following process reaches equilibrium at 1.0 atm is the normal boiling point of hydrogen peroxide. H2O2(l)   H2O2(g)
Use the following thermodynamic information at 298 K to determine this temperature.
Substance: H2O2(l) H2O2(g)
ΔH°f (kJ/mol): -187.7 -136.3
ΔG°f (kJ/mol): -120.4 -105.6
S°(J/K·mol): 109.6 232.7

Accepted Answer

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

Question 19

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

Accepted Answer

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) 
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 20

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

Accepted Answer

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

Exam 20: Thermodynamics Entropy, Free Energy, and Reaction Direction

For a chemical reaction to be spontaneous at all temperatures, which of the following conditions must be met?

Consider the reaction CuI(s) Cu+(aq) + I-(aq) 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 ΔG° for the reaction?

The free energy of a perfect crystal at absolute zero, is zero.

Calculate the equilibrium constant at 25°C for the reaction of methane with water to form carbon dioxide and hydrogen. The data refer to 25°C. CH4(g) + 2H2O(g) CO2(g) + 4H2(g)

In 1774 Joseph Priestley prepared the element oxygen by heating mercury(II) oxide: HgO(s) → Hg(l) + ½O2(g) For this reaction, ΔH° = 90.84 kJ and ΔS° = 108 J/K. Which of the following statements is true?

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

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

Which relationship best describes ΔS° for the following reaction? 8H2(g) + S8(s) → 8H2S(g)

Elemental boron can be formed by reaction of boron trichloride with hydrogen. BCl3(g) + 1.5H2(g) → B(s) + 3HCl(g) Calculate ΔG° for the reaction. Substance: BCl3(g) H2(g) B(s) HCl(g) ΔG°f (kJ/mol): -388.7 0 0 -95.3

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

Which of the following pairs has the member with the greater molar entropy listed first? All systems are at 25°C.

Which relationship or statement best describes ΔS° for the following reaction? HgS(s) + O2(g) → Hg(l) + SO2(g)

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

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

Calculate ΔG° for the reaction SiCl4(g) + 2Mg(s) → 2MgCl2(s) + Si(s) Substance: SiCl4(g) Mg(s) MgCl2(s) Si(s) ΔG°f (kJ/mol): -616.98 0 -591.79 0

For any reaction, if ΔG° > 0, then K < 1.

Which relationship or statement best describes ΔS° for the following reaction? 2H2S(g) + 3O2(g) → 2H2O(g) + 2SO2(g)

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

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

Keys to Studying Chemistry Definitions, Units, and Problem Solving

The Components of Matter

Stoichiometry of Formulas and Equations

Three Major Classes of Chemical Reactions

Gases and the Kinetic-Molecular Theory

Thermochemistry Energy Flow and Chemical Change

Quantum Theory and Atomic Structure

Electron Configuration and Chemical Periodicity

Models of Chemical Bonding

The Shapes of Molecules

Theories of Covalent Bonding

Intermolecular Forces Liquids, Solids, and Phase Changes

The Properties of Mixtures Solutions and Colloids

Periodic Patterns in the Main-Group Elements

Organic Compounds and the Atomic Properties of Carbon

Kinetics Rates and Mechanisms of Chemical Reactions

Equilibrium the Extent of Chemical Reactions

Acid-Base Equilibria

Ionic Equilibria in Aqueous Systems

Electrochemistry Chemical Change and Electrical Work

The Elements in Nature and Industry

Transition Elements and Their Coordination Compounds

Nuclear Reactions and Their Applications

Filters

Consider the reaction CuI(s) Cu⁺(aq) + I^-(aq) 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 ΔG° for the reaction?

Calculate the equilibrium constant at 25°C for the reaction of methane with water to form carbon dioxide and hydrogen. The data refer to 25°C. CH₄(g) + 2H₂O(g) CO₂(g) + 4H₂(g)

Which relationship best describes ΔS° for the following reaction? 8H₂(g) + S₈(s) → 8H₂S(g)

Elemental boron can be formed by reaction of boron trichloride with hydrogen. BCl₃(g) + 1.5H₂(g) → B(s) + 3HCl(g) Calculate ΔG° for the reaction. Substance: BCl₃(g) H₂(g) B(s) HCl(g) ΔG°_f (kJ/mol): -388.7 0 0 -95.3

Which relationship or statement best describes ΔS° for the following reaction? HgS(s) + O₂(g) → Hg(l) + SO₂(g)

Calculate ΔG° for the reaction SiCl₄(g) + 2Mg(s) → 2MgCl₂(s) + Si(s) Substance: SiCl₄(g) Mg(s) MgCl₂(s) Si(s) ΔG°_f (kJ/mol): -616.98 0 -591.79 0

Which relationship or statement best describes ΔS° for the following reaction? 2H₂S(g) + 3O₂(g) → 2H₂O(g) + 2SO₂(g)

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