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

41. The term ‘enthalpy of formation’ refers to:
    a) The heat change when a compound is formed from its elements in their standard states
    b) The heat change when a compound is formed from its gaseous ions
    c) The heat change when a compound is formed from its atoms in the ground state
    d) The heat change when a compound is formed from another compound
    Answer: a) The heat change when a compound is formed from its elements in their standard states

Explanation: The enthalpy of formation (ΔHf°) is the heat change when one mole of a compound is formed from its constituent elements in their standard states.

42. A reaction at constant temperature and pressure is spontaneous if:
    a) ΔH < 0, ΔS < 0 b) ΔH > 0, ΔS > 0
    c) ΔG < 0
    d) ΔH = 0
    Answer: c) ΔG < 0

Explanation: At constant temperature and pressure, a reaction is spontaneous if the change in Gibbs free energy (ΔG) is negative.

43. At equilibrium, the entropy of the universe:
    a) Increases
    b) Decreases
    c) Remains constant
    d) Becomes zero
    Answer: c) Remains constant

Explanation: At equilibrium, there is no net change in entropy, and the total entropy of the universe remains constant.

44. Which of the following is correct about entropy (S)?
    a) It is a measure of work done
    b) It is a measure of temperature
    c) It is a measure of disorder or randomness
    d) It is a measure of internal energy
    Answer: c) It is a measure of disorder or randomness

Explanation: Entropy (S) is a thermodynamic quantity that measures the disorder or randomness of a system.

45. In a process where entropy decreases and enthalpy increases, the process will be:
    a) Spontaneous at all temperatures
    b) Spontaneous at low temperatures
    c) Non-spontaneous at all temperatures
    d) Spontaneous at high temperatures
    Answer: c) Non-spontaneous at all temperatures

Explanation: When entropy decreases (ΔS < 0) and enthalpy increases (ΔH > 0), the process is non-spontaneous because both factors oppose spontaneity.

46. A spontaneous process at high temperature is most likely to have:
    a) ΔH < 0 and ΔS > 0
    b) ΔH > 0 and ΔS > 0
    c) ΔH > 0 and ΔS < 0 d) ΔH < 0 and ΔS < 0 Answer: b) ΔH > 0 and ΔS > 0

Explanation: A process can be spontaneous at high temperatures if the entropy change (ΔS) is positive, even if the enthalpy change (ΔH) is positive.

47. Which of the following is a correct representation of the second law of thermodynamics?
    a) ΔH = TΔS
    b) ΔS ≥ 0
    c) ΔG = 0 at equilibrium
    d) ΔG < 0 for spontaneous processes
    Answer: b) ΔS ≥ 0

Explanation: The second law of thermodynamics states that the total entropy of an isolated system always increases (ΔS ≥ 0) in spontaneous processes.

48. Which of the following reactions will have the greatest increase in entropy?
    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 CaCO₃ results in the formation of a gas (CO₂), leading to a significant

49. The change in Gibbs free energy (ΔG) is related to the equilibrium constant (K) of a reaction by the equation:
    a) ΔG = RT ln K
    b) ΔG° = RT ln K
    c) ΔG = -RT ln K
    d) ΔG° = -RT ln K
    Answer: d) ΔG° = -RT ln K

Explanation: The relationship between standard Gibbs free energy change (ΔG°) and the equilibrium constant (K) is given by ΔG° = -RT ln K, where R is the gas constant and T is the temperature.

50. For which of the following reactions is entropy (ΔS) likely to decrease?
    a) Liquid water freezing into ice
    b) Dissolution of salt in water
    c) Evaporation of alcohol
    d) Decomposition of hydrogen peroxide
    Answer: a) Liquid water freezing into ice

Explanation: Freezing causes the molecules of water to become more ordered, which results in a decrease in entropy.