Questions on Spontaneous and Nonspontaneous Reactions
Multiple-Choice Questions: Spontaneous and Nonspontaneous Reactions
1. What defines a spontaneous reaction?
A) A reaction that requires continuous energy input
B) A reaction that occurs without any energy change
C) A reaction that proceeds on its own once started
D) A reaction that occurs only under high pressure
E) A reaction that produces light
2. Which of the following is an example of a spontaneous process?
A) Electrolysis of water
B) Melting of ice at 25 °C
C) Photosynthesis
D) Charging a battery
E) Boiling water at 80 °C
3. What is true about a nonspontaneous reaction?
A) It happens without outside help
B) It never happens at any conditions
C) It requires continuous energy input
D) It only happens at low temperatures
E) It always releases energy
4. A reaction is spontaneous when:
A) ΔG > 0
B) ΔH = 0
C) ΔG = 0
D) ΔG < 0
E) ΔH > 0
5. Which of the following changes could make a nonspontaneous reaction become spontaneous?
A) Lowering temperature when ΔH < 0 and ΔS < 0
B) Increasing pressure in a solid system
C) Adding a catalyst
D) Adding more products
E) Increasing temperature when ΔH > 0 and ΔS > 0
6. Spontaneous reactions always:
A) Occur rapidly
B) Require a catalyst
C) Are exothermic
D) Increase entropy of the universe
E) Happen at 0 K
7. Which combination of enthalpy and entropy favors spontaneity at all temperatures?
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
8. At what value of Gibbs free energy (ΔG) is a system at equilibrium?
A) ΔG < 0
B) ΔG = 0
C) ΔG > 0
D) ΔG = 1
E) ΔG = ΔH – TΔS
9. Which of the following is NOT a criterion for spontaneity?
A) Entropy of the universe increases
B) Gibbs free energy decreases
C) Enthalpy of the system increases
D) The process proceeds without continuous energy
E) The reaction has a negative ΔG
10. Which reaction is spontaneous at low temperatures?
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
11. Which of the following statements is true about spontaneous reactions?
A) They always release heat
B) They always increase entropy of the system
C) They are always fast
D) They may occur slowly
E) They occur only with a catalyst
12. A reaction has ΔH = +100 kJ and ΔS = +250 J/K. At what temperature will it become spontaneous?
A) 100 K
B) 200 K
C) 300 K
D) 400 K
E) 500 K
13. Which of the following would most likely make a nonspontaneous endothermic reaction spontaneous?
A) Decrease temperature
B) Increase entropy significantly
C) Add energy continuously
D) Lower activation energy
E) Remove reactants
14. The decomposition of hydrogen peroxide (H₂O₂) is spontaneous. Why does it require a catalyst?
A) The reaction is nonspontaneous
B) Entropy decreases
C) The activation energy is high
D) It’s an endothermic reaction
E) The products are unstable
15. Spontaneous reactions can be identified using:
A) ΔH only
B) ΔS only
C) ΔG = ΔH – TΔS
D) Le Châtelier’s Principle
E) Equilibrium constant alone
16. Which process is spontaneous under standard conditions?
A) Freezing of water at 25 °C
B) Rusting of iron
C) Electroplating of silver
D) Electrolysis of molten NaCl
E) Compression of a gas into a smaller volume
17. If ΔH is positive and ΔS is negative, the reaction will be:
A) Always spontaneous
B) Spontaneous only at low temperatures
C) Spontaneous only at high temperatures
D) Nonspontaneous at all temperatures
E) At equilibrium
18. What does a negative ΔG indicate?
A) The reaction is nonspontaneous
B) Energy must be added
C) The reaction is spontaneous
D) Temperature must be lowered
E) The system is at equilibrium
19. A catalyst affects spontaneity by:
A) Making a reaction more spontaneous
B) Changing ΔG
C) Lowering activation energy but not ΔG
D) Increasing product yield
E) Making ΔS positive
20. Which factor does NOT influence spontaneity?
A) Temperature
B) Entropy
C) Enthalpy
D) Activation energy
E) Free energy
Answer Key with Explanations
1. C – A spontaneous reaction proceeds on its own once initiated.
2. B – Ice melts spontaneously at temperatures above 0 °C.
3. C – Nonspontaneous reactions need continuous energy input.
4. D – Spontaneity is indicated by ΔG < 0.
5. E – For ΔH > 0 and ΔS > 0, high temperatures favor spontaneity.
6. D – Even if the system’s entropy decreases, the universe’s entropy must increase.
7. C – Exothermic (ΔH < 0) and entropy-increasing (ΔS > 0) always gives ΔG < 0.
8. B – ΔG = 0 indicates equilibrium.
9. C – Increasing enthalpy does not favor spontaneity.
10. A – For ΔH < 0 and ΔS < 0, low temperatures can make ΔG negative.
11. D – Spontaneous reactions can be slow without a catalyst.
12. D – ΔG = ΔH – TΔS → Set ΔG = 0 → 100,000 J – T(250 J/K) = 0 → T = 400 K
13. B – Large positive ΔS can help make ΔG negative despite ΔH > 0.
14. C – Despite being spontaneous, high activation energy can slow it down.
15. C – ΔG = ΔH – TΔS determines spontaneity.
16. B – Rusting occurs spontaneously due to thermodynamic favorability.
17. D – ΔH > 0 and ΔS < 0 makes ΔG always positive.
18. C – ΔG < 0 = spontaneous reaction.
19. C – Catalysts only affect activation energy, not spontaneity (ΔG).
20. D – Activation energy affects reaction rate, not spontaneity.
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