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

91. A reaction that has a negative ΔH and a positive ΔS will be:
    a) Spontaneous at all temperatures
    b) Non-spontaneous at all temperatures
    c) Spontaneous at low temperatures only
    d) Spontaneous at high temperatures only
    Answer: a) Spontaneous at all temperatures

Explanation: A reaction with a negative ΔH (exothermic) and a positive ΔS (increasing disorder) will always have a negative ΔG, making it spontaneous at all temperatures.

92. The entropy change for the reaction 2H₂(g) + O₂(g) → 2H₂O(l) is likely to be:
    a) Positive
    b) Negative
    c) Zero
    d) Infinite
    Answer: b) Negative

Explanation: The reaction results in the formation of a liquid from gases, which leads to a more ordered system and a decrease in entropy.

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

Explanation: For an exothermic reaction (ΔH < 0) with a decrease in entropy (ΔS < 0), the reaction is spontaneous at low temperatures where the enthalpy term dominates.

94. Which of the following is true for a reaction where ΔG < 0?
    a) The reaction is at equilibrium.
    b) The reaction is spontaneous.
    c) The reaction is non-spontaneous.
    d) The reaction will not proceed.
    Answer: b) The reaction is spontaneous

Explanation: A negative ΔG indicates that the reaction is spontaneous under the given conditions.

95. Which of the following conditions indicates a non-spontaneous process?
    a) ΔG = 0
    b) ΔG < 0 c) ΔG > 0
    d) ΔH < 0 and ΔS > 0
    Answer: c) ΔG > 0

Explanation: A positive ΔG means that the process is non-spontaneous under the given conditions.

96. The entropy change for the process of freezing water is:
    a) Positive
    b) Negative
    c) Zero
    d) Infinite
    Answer: b) Negative

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

97. For a reaction where ΔH > 0 and ΔS < 0, the 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, ΔG will always be positive, making the reaction non-spontaneous at all temperatures.

98. A spontaneous reaction is one where:
    a) ΔG is positive
    b) ΔG is negative
    c) ΔH is positive
    d) ΔS is negative
    Answer: b) ΔG is negative

Explanation: A spontaneous reaction occurs when the Gibbs free energy (ΔG) is negative.

99. The change in entropy when a solid dissolves in a solvent is usually:
    a) Positive
    b) Negative
    c) Zero
    d) Infinite
    Answer: a) Positive

Explanation: Dissolution usually increases disorder in the system, resulting in a positive entropy change.

100. The second law of thermodynamics states that:
     a) Energy can be created or destroyed.
     b) The entropy of the universe is always increasing.
     c) The entropy of a system decreases in a spontaneous process.
     d) Enthalpy and entropy are independent of each other.
     Answer: b) The entropy of the universe is always increasing

Explanation: The second law of thermodynamics states that in any spontaneous process, the total entropy of the universe increases.