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

31. Which of the following correctly describes a spontaneous reaction at all temperatures?
    a) ΔH > 0, ΔS > 0
    b) ΔH < 0, ΔS < 0 c) ΔH < 0, ΔS > 0
    d) ΔH > 0, ΔS < 0 Answer: c) ΔH < 0, ΔS > 0

Explanation: A reaction is spontaneous at all temperatures if it releases heat (ΔH < 0) and increases entropy (ΔS > 0).

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

33. The term ‘enthalpy’ refers to:
    a) The sum of internal energy and pressure
    b) The sum of internal energy and volume
    c) The sum of internal energy and the product of pressure and volume
    d) The sum of internal energy and temperature
    Answer: c) The sum of internal energy and the product of pressure and volume

Explanation: Enthalpy (H) is defined as the sum of the internal energy (U) of the system and the product of pressure (P) and volume (V), i.e., H = U + PV.

34. A reaction is spontaneous when:
    a) ΔG < 0 b) ΔG = 0 c) ΔG > 0
    d) ΔH > 0
    Answer: a) ΔG < 0

Explanation: A reaction is spontaneous if the change in Gibbs free energy (ΔG) is negative, which indicates that the process can occur without external energy input.

35. The entropy of an isolated system always:
    a) Decreases in a spontaneous process
    b) Increases in a spontaneous process
    c) Remains constant in a spontaneous process
    d) Becomes zero in a spontaneous process
    Answer: b) Increases in a spontaneous process

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

36. In which of the following processes is entropy likely to decrease?
    a) Sublimation of dry ice
    b) Condensation of steam
    c) Melting of ice
    d) Evaporation of alcohol
    Answer: b) Condensation of steam

Explanation: Condensation results in a transition from the gaseous phase (more disordered) to the liquid phase (more ordered), thus decreasing entropy.

37. In an exothermic reaction, which of the following is true?
    a) ΔH is positive, ΔS is positive
    b) ΔH is positive, ΔS is negative
    c) ΔH is negative, ΔS is positive
    d) ΔH is negative, ΔS is negative
    Answer: d) ΔH is negative, ΔS is negative

Explanation: In an exothermic reaction, heat is released (ΔH < 0), and if the reaction leads to a more ordered system, entropy decreases (ΔS < 0).

38. In which condition will a reaction always be spontaneous?
    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: A reaction is always spontaneous if enthalpy decreases (ΔH < 0) and entropy increases (ΔS > 0).

39. The entropy change when water vapor condenses into liquid water is:
    a) Positive
    b) Negative
    c) Zero
    d) Undefined
    Answer: b) Negative

Explanation: Condensation causes water vapor molecules (high disorder) to form a liquid (lower disorder), resulting in a decrease in entropy.

40. A process is non-spontaneous if:
    a) ΔG > 0
    b) ΔH < 0 c) ΔS > 0
    d) ΔG = 0
    Answer: a) ΔG > 0

Explanation: If the Gibbs free energy change (ΔG) is positive, the process is non-spontaneous under the given conditions.