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

61. When a gas expands in an isolated system, the entropy:
    a) Decreases
    b) Increases
    c) Remains constant
    d) Becomes zero
    Answer: b) Increases

Explanation: The expansion of a gas increases the number of accessible microstates, resulting in an increase in entropy.

62. A process with ΔH > 0 and ΔS > 0 will be:
    a) Spontaneous at high temperatures
    b) Spontaneous at low temperatures
    c) Non-spontaneous at all temperatures
    d) Non-spontaneous at high temperatures
    Answer: a) Spontaneous at high temperatures

Explanation: When both ΔH and ΔS are positive, the process is spontaneous at high temperatures because the TΔS term will dominate the ΔG = ΔH – TΔS equation.

63. If ΔH is negative and ΔS is negative, the process will be spontaneous at:
    a) High temperatures
    b) Low temperatures
    c) All temperatures
    d) No temperatures
    Answer: b) Low temperatures

Explanation: When both ΔH and ΔS are negative, the process is spontaneous at low temperatures, where the enthalpy term dominates.

64. Which statement is true for a reaction that is non-spontaneous at standard conditions but becomes spontaneous at high temperatures?
    a) ΔH < 0, ΔS < 0 b) ΔH > 0, ΔS < 0 c) ΔH < 0, ΔS > 0
    d) ΔH > 0, ΔS > 0
    Answer: d) ΔH > 0, ΔS > 0

Explanation: If both enthalpy and entropy are positive, the process becomes spontaneous at high temperatures because the TΔS term will dominate the equation for Gibbs free energy.

65. For an endothermic reaction with an increase in entropy, the reaction will be:
    a) Spontaneous at all temperatures
    b) Non-spontaneous at all temperatures
    c) Spontaneous at high temperatures
    d) Spontaneous at low temperatures
    Answer: c) Spontaneous at high temperatures

Explanation: For an endothermic process (ΔH > 0) with an increase in entropy (ΔS > 0), the process is spontaneous at high temperatures, where the TΔS term becomes large enough to make ΔG negative.

66. At absolute zero, the entropy of a perfect crystal is:
    a) Zero
    b) One
    c) Infinite
    d) Constant
    Answer: a) Zero

Explanation: According to the third law of thermodynamics, the entropy of a perfect crystal approaches zero as the temperature approaches absolute zero.

67. Which of the following is correct about a spontaneous process?
    a) It always requires energy input.
    b) It increases the entropy of the system.
    c) It increases the entropy of the universe.
    d) It decreases the entropy of the universe.
    Answer: c) It increases the entropy of the universe

Explanation: According to the second law of thermodynamics, a spontaneous process always increases the entropy of the universe, even if the system’s entropy decreases.

68. If the reaction N₂(g) + 3H₂(g) → 2NH₃(g) is exothermic, what will happen to the equilibrium constant as temperature increases?
    a) It increases
    b) It decreases
    c) It remains constant
    d) It becomes zero
    Answer: b) It decreases

Explanation: For an exothermic reaction, increasing the temperature decreases the equilibrium constant because the reaction shifts towards the reactants to counteract the added heat (Le Chatelier’s principle).

69. The third law of thermodynamics states that:
    a) The entropy of the universe is always increasing.
    b) The entropy of a perfect crystal at absolute zero is zero.
    c) The total energy of an isolated system is constant.
    d) No process can occur if ΔG is positive.
    Answer: b) The entropy of a perfect crystal at absolute zero is zero

Explanation: The third law of thermodynamics states that the entropy of a perfect crystalline substance is zero at absolute zero because there is no disorder in the crystal at that temperature.

70. Which of the following conditions will favor a reaction to be spontaneous at high temperatures?
    a) ΔH > 0, ΔS > 0
    b) ΔH < 0, ΔS < 0 c) ΔH > 0, ΔS < 0 d) ΔH < 0, ΔS > 0
    Answer: a) ΔH > 0, ΔS > 0

Explanation: When both enthalpy (ΔH) and entropy (ΔS) are positive, the process becomes spontaneous at high temperatures where the TΔS term dominates the Gibbs free energy equation.