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

31. Which of the following is true for a reaction in which ΔH is negative and ΔS is negative?
    a) The reaction is always spontaneous
    b) The reaction is never spontaneous
    c) The reaction is spontaneous at low temperatures
    d) The reaction is spontaneous at high temperatures
    Answer: c) The reaction is spontaneous at low temperatures

Explanation: If both ΔH and ΔS are negative, the reaction will be spontaneous at low temperatures when the TΔS term is small.

32. Which of the following laws is used to determine the direction of spontaneous change?
    a) First law of thermodynamics
    b) Second law of thermodynamics
    c) Third law of thermodynamics
    d) Zeroth law of thermodynamics
    Answer: b) Second law of thermodynamics

Explanation: The second law of thermodynamics states that spontaneous processes occur in a direction that increases the overall entropy of the universe.

33. If a process is spontaneous, what can be said about its entropy change?
    a) ΔS = 0
    b) ΔS > 0
    c) ΔS < 0 d) ΔS is positive or negative depending on temperature Answer: b) ΔS > 0

Explanation: A spontaneous process typically increases the entropy of the system and its surroundings, resulting in a positive overall entropy change.

34. For a reaction to be non-spontaneous at any temperature, 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: b) ΔH > 0, ΔS < 0

Explanation: If ΔH is positive (endothermic) and ΔS is negative (decrease in entropy), the reaction will always be non-spontaneous since ΔG will always be positive.

35. Which of the following is true regarding a reversible process?
    a) It increases the entropy of the universe
    b) It decreases the entropy of the system
    c) It has no effect on the entropy of the universe
    d) It increases the entropy of the surroundings
    Answer: c) It has no effect on the entropy of the universe

Explanation: A reversible process occurs in such a way that the system and surroundings are always in equilibrium, resulting in no net change in the entropy of the universe.

36. In a chemical reaction, if ΔG is positive, the reaction:
    a) Will occur spontaneously
    b) Will not occur spontaneously
    c) Is at equilibrium
    d) Releases heat
    Answer: b) Will not occur spontaneously

Explanation: A positive Gibbs free energy (ΔG) indicates that the reaction is non-spontaneous and requires external energy to proceed.

37. Which of the following processes has a negative Gibbs free energy?
    a) Rusting of iron
    b) Decomposition of water into hydrogen and oxygen
    c) Electrolysis of NaCl
    d) Freezing of water
    Answer: a) Rusting of iron

Explanation: Rusting of iron is a spontaneous process, which means that its Gibbs free energy (ΔG) is negative.