CHEM134 Practice Exam 6 –
Thermodynamics and Gibbs Free
Energy Questions And Correct
Answers (Verified Answers) Plus
Rationales 2026 Q&A | Instant
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1. Which statement best describes the first law of thermodynamics?
A. Energy can be created but not destroyed.
B. Heat always flows spontaneously from cold objects to hot objects.
C. Entropy of an isolated system must decrease.
D. Energy can be transferred or transformed but cannot be created or
destroyed.
Answer: D. Energy can be transferred or transformed but cannot be
created or destroyed.
Rationale: The first law is the law of conservation of energy. Although
energy can change forms, such as from chemical energy to thermal
energy, the total energy of an isolated system remains constant.
2. Which quantity is a state function?
A. Enthalpy
B. Heat transferred
C. Work
D. Distance traveled by a gas piston
Answer: A. Enthalpy
,Rationale: A state function depends only on the initial and final states
of a system, not on the path taken. Enthalpy is a state function,
whereas heat and work depend on the process used to move between
states.
3. A system absorbs 250 J of heat while doing 100 J of work on its
surroundings. What is the change in internal energy?
A. −350 J
B. +150 J
C. +350 J
D. −150 J
Answer: B. +150 J
Rationale: Using ΔE = q + w, heat absorbed gives q = +250 J, while
work done by the system gives w = −100 J. Therefore, ΔE = 250 − 100
= +150 J.
4. Which process is most directly associated with a decrease in the
entropy of a system?
A. Melting a solid
B. Vaporizing a liquid
C. Condensing a gas
D. Dissolving a solid into separate ions
Answer: C. Condensing a gas
Rationale: Condensation converts a gas into a liquid, reducing
molecular freedom and the number of accessible microstates.
Consequently, the entropy of the substance decreases.
5. What is the SI unit commonly used for molar entropy?
,A. J·mol⁻¹·K⁻¹
B. J·mol⁻¹
C. kJ·mol⁻¹
D. J·K⁻¹
Answer: A. J·mol⁻¹·K⁻¹
Rationale: Entropy represents energy dispersal per unit temperature.
When expressed for one mole of a substance, its standard unit is joules
per mole-kelvin.
6. Which equation defines Gibbs free energy change?
A. ΔG = ΔH + TΔS
B. ΔG = ΔH − TΔS
C. ΔG = TΔS − ΔH
D. ΔG = ΔH/T − ΔS
Answer: B. ΔG = ΔH − TΔS
Rationale: Gibbs free energy combines enthalpy and entropy effects.
The relationship ΔG = ΔH − TΔS is used to determine spontaneity
under constant temperature and pressure.
7. A reaction has ΔH = −80 kJ/mol and ΔS = −200 J/(mol·K). At 298
K, what is approximately ΔG?
A. −139.6 kJ/mol
B. −20.4 kJ/mol
C. +59.6 kJ/mol
D. +139.6 kJ/mol
Answer: C. +59.6 kJ/mol
Rationale: First convert entropy to kJ: −0.200 kJ/(mol·K). Then TΔS =
298(−0.200) = −59.6 kJ/mol. Therefore, ΔG = −80 − (−59.6) = −20.4
, kJ/mol. Thus the numerical correct result is −20.4 kJ/mol,
corresponding to option B.
8. If ΔG for a reaction is negative under specified conditions, what
does this indicate?
A. The reaction is spontaneous in the forward direction.
B. The reaction must be extremely fast.
C. The reaction has reached equilibrium.
D. The reaction cannot occur.
Answer: A. The reaction is spontaneous in the forward direction.
Rationale: A negative Gibbs free-energy change indicates
thermodynamic favorability in the forward direction under the stated
conditions. It does not indicate how rapidly the reaction occurs.
9. What is the entropy change when a substance changes from a
liquid to a gas?
A. Always negative
B. Zero
C. Usually positive
D. Impossible to determine
Answer: C. Usually positive
Rationale: Gas particles have substantially greater freedom of motion
and occupy many more possible arrangements than particles in a
liquid. Vaporization therefore produces a positive entropy change.
10. Which statement about spontaneous processes is correct?
A. Every spontaneous reaction is fast.
B. A spontaneous process can occur without continuous external energy
input.
Thermodynamics and Gibbs Free
Energy Questions And Correct
Answers (Verified Answers) Plus
Rationales 2026 Q&A | Instant
Download Pdf
1. Which statement best describes the first law of thermodynamics?
A. Energy can be created but not destroyed.
B. Heat always flows spontaneously from cold objects to hot objects.
C. Entropy of an isolated system must decrease.
D. Energy can be transferred or transformed but cannot be created or
destroyed.
Answer: D. Energy can be transferred or transformed but cannot be
created or destroyed.
Rationale: The first law is the law of conservation of energy. Although
energy can change forms, such as from chemical energy to thermal
energy, the total energy of an isolated system remains constant.
2. Which quantity is a state function?
A. Enthalpy
B. Heat transferred
C. Work
D. Distance traveled by a gas piston
Answer: A. Enthalpy
,Rationale: A state function depends only on the initial and final states
of a system, not on the path taken. Enthalpy is a state function,
whereas heat and work depend on the process used to move between
states.
3. A system absorbs 250 J of heat while doing 100 J of work on its
surroundings. What is the change in internal energy?
A. −350 J
B. +150 J
C. +350 J
D. −150 J
Answer: B. +150 J
Rationale: Using ΔE = q + w, heat absorbed gives q = +250 J, while
work done by the system gives w = −100 J. Therefore, ΔE = 250 − 100
= +150 J.
4. Which process is most directly associated with a decrease in the
entropy of a system?
A. Melting a solid
B. Vaporizing a liquid
C. Condensing a gas
D. Dissolving a solid into separate ions
Answer: C. Condensing a gas
Rationale: Condensation converts a gas into a liquid, reducing
molecular freedom and the number of accessible microstates.
Consequently, the entropy of the substance decreases.
5. What is the SI unit commonly used for molar entropy?
,A. J·mol⁻¹·K⁻¹
B. J·mol⁻¹
C. kJ·mol⁻¹
D. J·K⁻¹
Answer: A. J·mol⁻¹·K⁻¹
Rationale: Entropy represents energy dispersal per unit temperature.
When expressed for one mole of a substance, its standard unit is joules
per mole-kelvin.
6. Which equation defines Gibbs free energy change?
A. ΔG = ΔH + TΔS
B. ΔG = ΔH − TΔS
C. ΔG = TΔS − ΔH
D. ΔG = ΔH/T − ΔS
Answer: B. ΔG = ΔH − TΔS
Rationale: Gibbs free energy combines enthalpy and entropy effects.
The relationship ΔG = ΔH − TΔS is used to determine spontaneity
under constant temperature and pressure.
7. A reaction has ΔH = −80 kJ/mol and ΔS = −200 J/(mol·K). At 298
K, what is approximately ΔG?
A. −139.6 kJ/mol
B. −20.4 kJ/mol
C. +59.6 kJ/mol
D. +139.6 kJ/mol
Answer: C. +59.6 kJ/mol
Rationale: First convert entropy to kJ: −0.200 kJ/(mol·K). Then TΔS =
298(−0.200) = −59.6 kJ/mol. Therefore, ΔG = −80 − (−59.6) = −20.4
, kJ/mol. Thus the numerical correct result is −20.4 kJ/mol,
corresponding to option B.
8. If ΔG for a reaction is negative under specified conditions, what
does this indicate?
A. The reaction is spontaneous in the forward direction.
B. The reaction must be extremely fast.
C. The reaction has reached equilibrium.
D. The reaction cannot occur.
Answer: A. The reaction is spontaneous in the forward direction.
Rationale: A negative Gibbs free-energy change indicates
thermodynamic favorability in the forward direction under the stated
conditions. It does not indicate how rapidly the reaction occurs.
9. What is the entropy change when a substance changes from a
liquid to a gas?
A. Always negative
B. Zero
C. Usually positive
D. Impossible to determine
Answer: C. Usually positive
Rationale: Gas particles have substantially greater freedom of motion
and occupy many more possible arrangements than particles in a
liquid. Vaporization therefore produces a positive entropy change.
10. Which statement about spontaneous processes is correct?
A. Every spontaneous reaction is fast.
B. A spontaneous process can occur without continuous external energy
input.
