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

81. For which of the following reactions will ΔS be positive?
    a) N₂(g) + 3H₂(g) → 2NH₃(g)
    b) CaCO₃(s) → CaO(s) + CO₂(g)
    c) 2SO₂(g) + O₂(g) → 2SO₃(g)
    d) H₂(g) + Cl₂(g) → 2HCl(g)
    Answer: b) CaCO₃(s) → CaO(s) + CO₂(g)

Explanation: The decomposition of calcium carbonate produces a gas (CO₂), which increases the disorder of the system, leading to a positive entropy change.

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

Explanation: At equilibrium, there is no net change in the system, and ΔG is zero.

83. The entropy of a system decreases when:
    a) Water evaporates
    b) Ice melts
    c) A gas condenses
    d) A solid dissolves in a liquid
    Answer: c) A gas condenses

Explanation: Condensation involves the transition from a gas to a liquid, which is a more ordered state, resulting in a decrease in entropy.

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

Explanation: For an exothermic reaction (ΔH < 0) with an increase in entropy (ΔS > 0), the process is spontaneous at all temperatures because ΔG will always be negative.

85. If a chemical reaction has ΔH < 0 and ΔS < 0, the reaction will be spontaneous:
    a) At high temperatures
    b) At low temperatures
    c) At all temperatures
    d) At no temperatures
    Answer: b) At low temperatures

Explanation: When both ΔH and ΔS are negative, the reaction will be spontaneous at low temperatures because the enthalpy term (ΔH) dominates the TΔS term in the Gibbs free energy equation.

86. Which of the following processes will always result in a positive change in entropy?
    a) Freezing of water
    b) Melting of ice
    c) Condensation of steam
    d) Sublimation of dry ice
    Answer: d) Sublimation of dry ice

Explanation: Sublimation is the transition from a solid to a gas, which increases the disorder of the system and results in a positive entropy change.

87. In an isolated system, which of the following always increases for a spontaneous process?
    a) Enthalpy
    b) Gibbs free energy
    c) Internal energy
    d) Entropy
    Answer: d) Entropy

Explanation: According to the second law of thermodynamics, in an isolated system, the entropy of the system will always increase for a spontaneous process.

88. The dissolution of NaCl in water results in an increase in entropy because:
    a) NaCl is a solid.
    b) Water molecules become more ordered.
    c) Na⁺ and Cl⁻ ions become dispersed in the water.
    d) The solution cools down.
    Answer: c) Na⁺ and Cl⁻ ions become dispersed in the water

Explanation: When NaCl dissolves in water, the Na⁺ and Cl⁻ ions spread throughout the solution, increasing the disorder and thus increasing the entropy of the system.

89. Which of the following reactions will have a positive entropy change?
    a) 2H₂(g) + O₂(g) → 2H₂O(l)
    b) 2SO₂(g) + O₂(g) → 2SO₃(g)
    c) NH₄Cl(s) → NH₃(g) + HCl(g)
    d) N₂(g) + 3H₂(g) → 2NH₃(g)
    Answer: c) NH₄Cl(s) → NH₃(g) + HCl(g)

Explanation: The decomposition of solid ammonium chloride into two gases increases the number of gas molecules, which leads to an increase in entropy.

90. Which of the following statements about Gibbs free energy (ΔG) is correct?
    a) If ΔG > 0, the reaction is spontaneous.
    b) If ΔG < 0, the reaction is non-spontaneous.
    c) If ΔG = 0, the system is at equilibrium.
    d) If ΔG = 0, the reaction is non-spontaneous.
    Answer: c) If ΔG = 0, the system is at equilibrium

Explanation: When ΔG = 0, the system is at equilibrium, meaning no net change in the reaction occurs.