Questions on Free Energy (ΔG°), Temperature, Phase Changes, and Keq
Multiple-Choice Questions: Free Energy (ΔG°), Temperature, Phase Changes, and Keq
🧪 Free Energy Concepts
1. What does the Gibbs free energy (ΔG) determine for a chemical reaction?
A) Reaction rate
B) Heat capacity
C) Spontaneity
D) Activation energy
E) pH value
2. What is the equation for calculating Gibbs free energy?
A) ΔG = ΔH + TΔS
B) ΔG = ΔH – TΔS
C) ΔG = TΔH – ΔS
D) ΔG = ΔS – TΔH
E) ΔG = ΔH × ΔS
3. A reaction is spontaneous at constant temperature and pressure when:
A) ΔG > 0
B) ΔG = 0
C) ΔG < 0
D) ΔS < 0
E) ΔH > 0
4. What are the units of ΔG in the SI system?
A) kJ
B) cal
C) J/mol
D) J
E) J/mol·K
5. Which of the following contributes to a negative ΔG?
A) ΔH > 0 and ΔS > 0
B) ΔH < 0 and ΔS < 0
C) ΔH < 0 and ΔS > 0
D) ΔH > 0 and ΔS < 0
E) ΔH = 0 and ΔS = 0
6. At equilibrium, the value of ΔG is:
A) Positive
B) Negative
C) Zero
D) Equal to ΔH
E) Always increasing
7. What condition always leads to a spontaneous reaction at any temperature?
A) ΔH > 0 and ΔS > 0
B) ΔH < 0 and ΔS < 0
C) ΔH < 0 and ΔS > 0
D) ΔH = 0 and ΔS = 0
E) ΔH > 0 and ΔS < 0
8. When ΔG is positive, the reaction is:
A) Spontaneous
B) At equilibrium
C) Nonspontaneous
D) Exothermic
E) Reversible
🌡️ Temperature and Changes of State
9. The boiling of water at 100°C and 1 atm is a process where ΔG is:
A) Negative
B) Positive
C) Zero
D) Undefined
E) Constant
10. During a phase change at equilibrium (like melting or boiling), what is true about ΔG?
A) ΔG > 0
B) ΔG < 0
C) ΔG = 0
D) ΔG = ΔH
E) ΔG = TΔS
11. Which of the following phase transitions is associated with a positive ΔS?
A) Freezing
B) Condensation
C) Vaporization
D) Deposition
E) Crystallization
12. What happens to ΔG when temperature increases for a process with positive ΔH and ΔS?
A) Becomes more negative
B) Becomes more positive
C) Remains unchanged
D) Approaches zero
E) Cannot be determined
13. Which phase change occurs when ΔH > 0 and ΔS > 0?
A) Freezing
B) Condensation
C) Sublimation
D) Deposition
E) Solidification
14. If ΔH = 40.7 kJ/mol and ΔS = 109 J/mol·K for the vaporization of water, at what temperature (in K) is the boiling point?
A) 273
B) 373
C) 298
D) 400
E) 350
15. A process is nonspontaneous at low temperatures but becomes spontaneous at high temperatures. This implies:
A) ΔH < 0, ΔS < 0
B) ΔH < 0, ΔS > 0
C) ΔH > 0, ΔS < 0
D) ΔH > 0, ΔS > 0
E) ΔH = 0, ΔS = 0
🔢 Calculations of ΔG° and Keq
16. What is the relationship between ΔG° and Keq?
A) ΔG° = –RT ln Keq
B) ΔG° = RT ln Keq
C) ΔG° = –ΔH/T
D) ΔG° = –Keq × R
E) ΔG° = R ln T
17. In the equation ΔG° = –RT ln Keq, what does R represent?
A) Reaction rate
B) Gas constant
C) Enthalpy
D) Resistance
E) Pressure
18. If ΔG° is negative, then Keq is:
A) Equal to 1
B) Less than 1
C) Greater than 1
D) Negative
E) Undefined
19. If Keq = 1, what is the value of ΔG°?
A) Positive
B) Negative
C) 0
D) Depends on T
E) Undefined
20. For a reaction at 298 K with ΔG° = –28.5 kJ/mol, what can be said about the equilibrium position?
A) Lies far to the left
B) Is at equilibrium
C) Lies far to the right
D) Is not possible
E) ΔG = 0
21. At what condition is Keq > 1 and ΔG° < 0?
A) Reactants favored
B) Products favored
C) Equilibrium
D) Nonspontaneous
E) Exothermic only
22. If ΔG° = +10 kJ/mol, what can be inferred?
A) Reaction is spontaneous
B) Keq > 1
C) Keq < 1
D) System is at equilibrium
E) Temperature must be lowered
23. What is the sign of ΔG° when a reaction heavily favors the products?
A) Positive
B) Negative
C) Zero
D) Undefined
E) Temperature dependent
24. Given ΔH = –50 kJ/mol and ΔS = –150 J/mol·K, at what temperature (in K) does the reaction become nonspontaneous?
A) 100 K
B) 200 K
C) 333 K
D) 400 K
E) 500 K
25. A reaction has ΔH = +80 kJ/mol and ΔS = +200 J/mol·K. At what temperature will it become spontaneous (ΔG < 0)?
A) 100 K
B) 200 K
C) 300 K
D) 400 K
E) 500 K
26. What is the value of ΔG° at 298 K for Keq = 10? (R = 8.314 J/mol·K)
A) –2.30 kJ/mol
B) –5.71 kJ/mol
C) –12.4 kJ/mol
D) –15.2 kJ/mol
E) –20.0 kJ/mol
27. What is Keq if ΔG° = 0?
A) Keq = 0
B) Keq < 1
C) Keq = 1
D) Keq > 1
E) Cannot be determined
28. If Keq << 1, then ΔG° is likely to be:
A) Very negative
B) Slightly negative
C) Close to zero
D) Very positive
E) Always zero
29. What factor(s) can affect Keq?
A) Temperature only
B) Pressure only
C) Catalyst presence
D) Concentration of products
E) All of the above
30. Which of the following best describes the relationship between temperature and ΔG for a reaction where ΔH and ΔS are both positive?
A) ΔG becomes more negative as T increases
B) ΔG becomes more positive as T increases
C) ΔG is independent of T
D) ΔG = ΔH
E) ΔG = ΔS
Answer Key with Explanations
1. C – ΔG tells whether a reaction is thermodynamically spontaneous.
2. B – The correct equation is ΔG = ΔH – TΔS.
3. C – Spontaneity requires ΔG < 0.
4. C – ΔG is expressed in joules per mole.
5. C – Negative ΔH (exothermic) and positive ΔS favor negative ΔG.
6. C – At equilibrium, ΔG = 0.
7. C – ΔH < 0 and ΔS > 0 guarantee spontaneity at all temperatures.
8. C – ΔG > 0 means nonspontaneous.
9. C – At boiling point, the system is at equilibrium → ΔG = 0.
10. C – All phase changes at equilibrium have ΔG = 0.
11. C – Vaporization increases disorder, so ΔS > 0.
12. A – For +ΔH and +ΔS, increasing T makes –TΔS more negative.
13. C – Sublimation is endothermic and increases entropy.
14. B – T = ΔH/ΔS = 40700 J / 109 J/K = approx. 373 K.
15. D – Positive ΔH and ΔS → spontaneous at high T.
16. A – ΔG° = –RT ln Keq is the correct relationship.
17. B – R = gas constant (8.314 J/mol·K).
18. C – If ΔG° < 0, Keq > 1 (products favored).
19. C – If Keq = 1, ln(1) = 0 → ΔG° = 0.
20. C – Large negative ΔG° = products favored (right side).
21. B – Keq > 1 means more products → spontaneous (ΔG° < 0).
22. C – Positive ΔG° = nonspontaneous, so Keq < 1.
23. B – Favoring products means ΔG° is negative.
24. C – T = ΔH/ΔS = 50000 / 150 = 333 K.
25. D – T = ΔH/ΔS = 80000 / 200 = 400 K.
26. B – ΔG° = –RT ln(10) = –(8.314)(298)(2.303) ≈ –5.71 kJ/mol.
27. C – At ΔG° = 0, Keq = 1.
28. D – Keq << 1 means ΔG° is large and positive (reactants favored).
29. A – Only temperature affects Keq. Catalysts do not affect equilibrium positions.
30. A – With ΔH and ΔS both positive, higher T → more negative ΔG.
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