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

21. When water freezes, what happens to its enthalpy and entropy?
    a) Both increase
    b) Both decrease
    c) Enthalpy increases, entropy decreases
    d) Enthalpy decreases, entropy increases
    Answer: b) Both decrease

Explanation: Freezing releases heat (enthalpy decreases) and decreases randomness (entropy decreases).

22. A process is spontaneous if:
    a) ΔG > 0
    b) ΔG < 0 c) ΔH > 0
    d) ΔS < 0
    Answer: b) ΔG < 0

Explanation: For a process to be spontaneous, the Gibbs free energy change (ΔG) must be negative.

23. The change in entropy (ΔS) for the dissolution of a solid in water is generally:
    a) Zero
    b) Positive
    c) Negative
    d) Unpredictable
    Answer: b) Positive

Explanation: Dissolution increases the randomness or disorder of the system, leading to a positive change in entropy.

24. The third law of thermodynamics states that:
    a) The entropy of the system is zero at absolute zero
    b) The enthalpy of the system is zero at absolute zero
    c) The energy of the system is zero at absolute zero
    d) The temperature of the system is zero at absolute zero
    Answer: a) The entropy of the system is zero at absolute zero

Explanation: The third law states that the entropy of a perfectly crystalline substance becomes zero as the temperature approaches absolute zero.

25. If the value of ΔG for a reaction is zero, the reaction is:
    a) At equilibrium
    b) Spontaneous
    c) Non-spontaneous
    d) Impossible
    Answer: a) At equilibrium

Explanation: A ΔG of zero indicates that the system is at equilibrium, meaning no net change is occurring.

26. In a spontaneous process, the total entropy of the system and surroundings:
    a) Decreases
    b) Increases
    c) Remains constant
    d) None of these
    Answer: b) Increases

Explanation: According to the second law of thermodynamics, the total entropy of the system and surroundings increases in a spontaneous process.

27. For a reaction to be spontaneous at low temperatures, which condition must be satisfied?
    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: If both ΔH and ΔS are negative, the reaction will be spontaneous at low temperatures since ΔG = ΔH – TΔS.

28. The enthalpy change (ΔH) for a chemical reaction is the difference between:
    a) The energy of products and reactants
    b) The internal energy of products and reactants
    c) The total heat absorbed and released
    d) The heat content of products and reactants
    Answer: d) The heat content of products and reactants

Explanation: Enthalpy change (ΔH) is the difference between the enthalpy of the products and the reactants.

29. Entropy of the surroundings decreases in which of the following processes?
    a) Exothermic reaction
    b) Endothermic reaction
    c) Reversible process
    d) Isothermal expansion
    Answer: b) Endothermic reaction

Explanation: In an endothermic reaction, heat is absorbed from the surroundings, decreasing their entropy.

30. At constant pressure, the heat change in a process is equal to:
    a) Change in internal energy
    b) Change in enthalpy
    c) Change in entropy
    d) Change in Gibbs free energy
    Answer: b) Change in enthalpy

Explanation: At constant pressure, the heat absorbed or released in a process is equal to the change in enthalpy (ΔH).