NEET 2025 MCQ with Answer Explanation : “Enthalpy, Entropy and Gibbs free energy”.

11. Which of the following processes results in a decrease in entropy?
    a) Melting of ice
    b) Dissolution of salt in water
    c) Freezing of water
    d) Evaporation of alcohol
    Answer: c) Freezing of water

Explanation: Freezing causes water molecules to become more ordered as they transition from liquid to solid, resulting in a decrease in entropy.

12. The term ‘spontaneous process’ refers to a process that:
    a) Occurs rapidly
    b) Requires external energy input
    c) Occurs without external intervention
    d) Always increases enthalpy
    Answer: c) Occurs without external intervention

Explanation: A spontaneous process occurs naturally without the need for continuous external energy input.

13. At equilibrium, the value of Gibbs free energy (ΔG) is:
    a) Positive
    b) Negative
    c) Zero
    d) Undefined
    Answer: c) Zero

Explanation: At equilibrium, the Gibbs free energy change (ΔG) is zero, indicating no net change in the system’s energy.

14. Which of the following statements is true about Gibbs free energy (ΔG)?
    a) It is always positive for spontaneous reactions
    b) It is always negative for non-spontaneous reactions
    c) It is zero at equilibrium
    d) It is independent of temperature
    Answer: c) It is zero at equilibrium

Explanation: ΔG = 0 at equilibrium, indicating that there is no driving force for the reaction to proceed in either direction.

15. Entropy of a perfectly crystalline substance at absolute zero is:
    a) Zero
    b) Positive
    c) Negative
    d) Infinite
    Answer: a) Zero

Explanation: According to the third law of thermodynamics, the entropy of a perfect crystal at absolute zero is zero.

16. Which statement is true for an exothermic process?
    a) Heat is absorbed from the surroundings
    b) Heat is released to the surroundings
    c) Entropy always decreases
    d) Gibbs free energy is always positive
    Answer: b) Heat is released to the surroundings

Explanation: In exothermic processes, the system releases heat to the surroundings, making ΔH negative.

17. A reaction is spontaneous if:
    a) ΔH is positive and ΔS is negative
    b) ΔH is negative and ΔS is positive
    c) ΔH is negative and ΔS is negative
    d) ΔH is positive and ΔS is positive
    Answer: b) ΔH is negative and ΔS is positive

Explanation: A reaction is most likely to be spontaneous when enthalpy decreases (ΔH < 0) and entropy increases (ΔS > 0).

18. The entropy change of a system when ice melts is:
    a) Positive
    b) Negative
    c) Zero
    d) Constant
    Answer: a) Positive

Explanation: Melting increases the disorder of the system, so entropy increases.

19. For a reaction to be non-spontaneous at all temperatures, the correct condition is:
    a) ΔH > 0, ΔS > 0
    b) ΔH > 0, ΔS < 0 c) ΔH < 0, ΔS > 0
    d) ΔH < 0, ΔS < 0 Answer: b) ΔH > 0, ΔS < 0

Explanation: If ΔH is positive and ΔS is negative, ΔG will always be positive, making the process non-spontaneous at all temperatures.

20. Which thermodynamic quantity remains constant in an isothermal process?
    a) Temperature
    b) Pressure
    c) Volume
    d) Enthalpy
    Answer: a) Temperature

Explanation: In an isothermal process, the temperature of the system remains constant.