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

11. The change in Gibbs free energy for a reaction is given by the equation:
    a) ΔG = ΔH – TΔS
    b) ΔG = ΔS – TΔH
    c) ΔG = ΔH + TΔS
    d) ΔG = ΔS + TΔH
    Answer: a) ΔG = ΔH – TΔS

Explanation: The Gibbs free energy equation is given by ΔG = ΔH – TΔS, where ΔG represents the Gibbs free energy change, ΔH is the enthalpy change, T is the temperature, and ΔS is the entropy change.

12. For a process to be spontaneous at high temperature, 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: c) ΔH > 0, ΔS > 0

Explanation: A positive ΔH and positive ΔS can lead to spontaneity at high temperatures because the TΔS term may become larger than ΔH, making ΔG negative.

13. Which of the following is a characteristic of an exergonic reaction?
    a) ΔG > 0
    b) ΔG = 0
    c) ΔG < 0 d) ΔH > 0
    Answer: c) ΔG < 0

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

14. If ΔG for a reaction is positive, the reaction is:
    a) Spontaneous
    b) Non-spontaneous
    c) At equilibrium
    d) Exothermic
    Answer: b) Non-spontaneous

Explanation: A positive ΔG means that the reaction requires energy to proceed and is non-spontaneous under the given conditions.

15. For an endothermic process with increasing entropy, the reaction will be:
    a) Always spontaneous
    b) Never spontaneous
    c) Spontaneous at high temperatures
    d) Spontaneous at low temperatures
    Answer: c) Spontaneous at high temperatures

Explanation: If a reaction is endothermic (ΔH > 0) and entropy increases (ΔS > 0), the process can become spontaneous at high temperatures because the TΔS term will outweigh ΔH.

16. Which of the following correctly defines Gibbs free energy?
    a) It is the total energy of a system
    b) It is the maximum amount of work a system can perform
    c) It is the energy stored in chemical bonds
    d) It is the heat energy released during a reaction
    Answer: b) It is the maximum amount of work a system can perform

Explanation: Gibbs free energy represents the maximum amount of work that can be done by a thermodynamic system during a reversible process.

17. At constant temperature and pressure, which condition will guarantee a spontaneous process?
    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 negative ΔH (exothermic) and positive ΔS (increase in disorder) is always spontaneous at constant temperature and pressure.

18. Which of the following is true for an ideal gas undergoing an isothermal process?
    a) ΔS = 0
    b) ΔG = 0
    c) ΔH = 0
    d) ΔU = 0
    Answer: d) ΔU = 0

Explanation: In an isothermal process, the temperature remains constant, and for an ideal gas, the change in internal energy (ΔU) is zero.

19. What is the value of ΔG when a system is at equilibrium?
    a) Positive
    b) Negative
    c) Zero
    d) Infinite
    Answer: c) Zero

Explanation: At equilibrium, the system has no net tendency to change, and therefore the Gibbs free energy change (ΔG) is zero.

20. The second law of thermodynamics states that:
    a) Energy is conserved in an isolated system
    b) The total entropy of an isolated system can only increase
    c) The total energy of an isolated system is constant
    d) Entropy of a perfect crystal at absolute zero is zero
    Answer: b) The total entropy of an isolated system can only increase

Explanation: The second law of thermodynamics states that the entropy (disorder) of an isolated system will either increase or remain constant, but it cannot decrease.