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

21. Which of the following processes is associated with a decrease in entropy?
    a) Evaporation of alcohol
    b) Mixing of gases
    c) Crystallization of a supersaturated solution
    d) Melting of ice
    Answer: c) Crystallization of a supersaturated solution

Explanation: During crystallization, particles in a disordered state (solution) become ordered (solid crystals), which decreases entropy.

22. If a reaction is exergonic, which of the following is true?
    a) ΔG > 0
    b) ΔG = 0
    c) ΔG < 0 d) ΔH > 0
    Answer: c) ΔG < 0

Explanation: An exergonic reaction is spontaneous, meaning the Gibbs free energy (ΔG) is negative.

23. Which of the following would increase the entropy of a system?
    a) Freezing of water
    b) Formation of NaCl from Na and Cl
    c) Vaporization of liquid water
    d) Dissolution of sugar in water
    Answer: c) Vaporization of liquid water

Explanation: Vaporization increases entropy because the water molecules move from a more ordered liquid phase to a more disordered gas phase.

24. In which of the following reactions is ΔS expected to be negative?
    a) N₂(g) + 3H₂(g) → 2NH₃(g)
    b) 2H₂O₂(l) → 2H₂O(l) + O₂(g)
    c) NaCl(s) → Na⁺(aq) + Cl⁻(aq)
    d) C(s) + O₂(g) → CO₂(g)
    Answer: a) N₂(g) + 3H₂(g) → 2NH₃(g)

Explanation: In this reaction, there is a decrease in the number of gas molecules (from 4 moles to 2 moles), leading to a decrease in entropy.

25. Which factor affects the Gibbs free energy change of a reaction?
    a) Only temperature
    b) Only pressure
    c) Both temperature and entropy
    d) Only the reaction enthalpy
    Answer: c) Both temperature and entropy

Explanation: The Gibbs free energy change (ΔG) depends on both temperature (T) and entropy (ΔS) in the equation ΔG = ΔH – TΔS.

26. For a reaction with ΔH > 0 and ΔS < 0, the reaction will be:
    a) Always spontaneous
    b) Always non-spontaneous
    c) Spontaneous at high temperatures
    d) Spontaneous at low temperatures
    Answer: b) Always non-spontaneous

Explanation: If ΔH is positive (endothermic) and ΔS is negative (decreasing entropy), the reaction will never be spontaneous because ΔG will always be positive.

27. For a process to be spontaneous at low temperatures, which of the following must be true?
    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: A process with a negative ΔH (exothermic) and a negative ΔS (decreasing entropy) will be spontaneous at low temperatures since the TΔS term is minimized.

28. Which of the following is correct for the enthalpy change of a reaction (ΔH)?
    a) It is negative for endothermic reactions
    b) It is zero for all reactions
    c) It is positive for exothermic reactions
    d) It is negative for exothermic reactions
    Answer: d) It is negative for exothermic reactions

Explanation: In exothermic reactions, heat is released, so the change in enthalpy (ΔH) is negative.

29. At constant temperature, the spontaneity of a process is governed by:
    a) ΔS only
    b) ΔH only
    c) ΔG
    d) TΔS only
    Answer: c) ΔG

Explanation: At constant temperature, the Gibbs free energy (ΔG) determines the spontaneity of a process. If ΔG < 0, the process is spontaneous.

30. The dissolution of ammonium chloride in water is endothermic, but it is spontaneous. This implies that:
    a) ΔH > 0 and ΔS > 0
    b) ΔH < 0 and ΔS > 0
    c) ΔH > 0 and ΔS < 0 d) ΔH < 0 and ΔS < 0 Answer: a) ΔH > 0 and ΔS > 0

Explanation: The dissolution of ammonium chloride is endothermic (ΔH > 0), but the increase in entropy (ΔS > 0) makes the process spontaneous, especially at room temperature.