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

71. Which of the following processes will have a positive entropy change?
    a) Freezing of water
    b) Condensation of steam
    c) Evaporation of water
    d) Formation of ice from water
    Answer: c) Evaporation of water

Explanation: Evaporation involves the transition from liquid to gas, which increases the disorder of the system, resulting in a positive change in entropy.

72. Which statement is true regarding an endothermic process at equilibrium when the temperature is increased?
    a) The equilibrium shifts towards the products.
    b) The equilibrium shifts towards the reactants.
    c) The equilibrium does not change.
    d) The entropy of the system decreases.
    Answer: a) The equilibrium shifts towards the products

Explanation: According to Le Chatelier’s principle, increasing the temperature in an endothermic process shifts the equilibrium towards the products to absorb the added heat.

73. Which of the following thermodynamic quantities is a state function?
    a) Work
    b) Heat
    c) Entropy
    d) Distance
    Answer: c) Entropy

Explanation: Entropy (S) is a state function, meaning its value depends only on the current state of the system, not on how the system reached that state. Work and heat are path functions.

74. In a reaction where 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 process is spontaneous at low temperatures where the enthalpy term dominates the Gibbs free energy equation.

75. For which process does the entropy of the system decrease?
    a) Expansion of a gas
    b) Melting of ice
    c) Freezing of water
    d) Boiling of water
    Answer: c) Freezing of water

Explanation: Freezing of water involves the transition from liquid to solid, which is a more ordered state, resulting in a decrease in entropy.

76. The enthalpy change for a reaction is positive, and the entropy change is negative. This reaction will be:
    a) Spontaneous at high temperatures
    b) Spontaneous at low temperatures
    c) Non-spontaneous at all temperatures
    d) Spontaneous at all temperatures
    Answer: c) Non-spontaneous at all temperatures

Explanation: When ΔH is positive and ΔS is negative, the Gibbs free energy will always be positive, making the reaction non-spontaneous at all temperatures.

77. Which of the following will increase the entropy of the system?
    a) Freezing of water
    b) Dissolution of sugar in water
    c) Condensation of steam
    d) Compression of a gas
    Answer: b) Dissolution of sugar in water

Explanation: Dissolution increases the disorder in the system as the sugar molecules disperse in the water, leading to an increase in entropy.

78. If ΔH > 0 and ΔS > 0 for a reaction, the reaction will be spontaneous at:
    a) Low temperatures only
    b) High temperatures only
    c) All temperatures
    d) No temperatures
    Answer: b) High temperatures only

Explanation: When both ΔH and ΔS are positive, the reaction is spontaneous at high temperatures because the TΔS term dominates and makes ΔG negative.

79. For a reaction to be spontaneous at all temperatures, which condition must be met?
    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: A negative enthalpy change (exothermic process) and a positive entropy change make the reaction spontaneous at all temperatures.

80. At constant pressure, the heat absorbed by a system is equal to:
    a) The change in Gibbs free energy
    b) The change in internal energy
    c) The change in enthalpy
    d) The change in entropy
    Answer: c) The change in enthalpy

Explanation: At constant pressure, the heat absorbed by the system is equal to the change in enthalpy (ΔH).