Question

Assertion: Entropy of system increases for a spontaneous reaction.

Reason: Enthalpy of reaction always decreases for spontaneous reaction.

• A. If both assertion and reason are correct and the reason is a correct explanation of the assertion.
• B. If both assertion and reason are correct but reason is not a correct explanation of the assertion.
• C. If assertion is correct but reason is incorrect.
• D. If both assertion and reason are false.

Answer 1

Answer: (D)

If both assertion and reason are false.

Answer 2

The question asks us to evaluate an assertion and a reason statement regarding spontaneous reactions and their thermodynamic parameters.

Assertion Analysis: "Entropy of system increases for a spontaneous reaction." This statement is false. According to the second law of thermodynamics, for a spontaneous process, the total entropy of the universe must increase (ΔS_universe > 0). This means ΔS_system + ΔS_surroundings > 0. However, it is not necessary for the entropy of the system (ΔS_system) to increase for a reaction to be spontaneous. A spontaneous reaction can occur even if the entropy of the system decreases (ΔS_system < 0), provided that the entropy of the surroundings increases by a greater magnitude (ΔS_surroundings > |ΔS_system|). A classic example is the freezing of water below 0°C:

H₂O(l) → H₂O(s)

This process is spontaneous at temperatures below 0°C, but the entropy of the system (water turning into a more ordered solid) decreases (ΔS_system < 0). The spontaneity arises because the process is exothermic (ΔH < 0), releasing heat to the surroundings and thus increasing the entropy of the surroundings sufficiently to make ΔS_universe positive.

Reason Analysis: "Enthalpy of reaction always decreases for spontaneous reaction." This statement is also false. While exothermic reactions (ΔH < 0) are often spontaneous, it is not a universal rule. Many spontaneous reactions are endothermic (ΔH > 0), meaning they absorb heat from the surroundings. These reactions are spontaneous because they lead to a significant increase in the entropy of the system (ΔS_system > 0). According to the Gibbs free energy equation:

ΔG = ΔH - TΔS_system

For a reaction to be spontaneous, ΔG must be negative (ΔG < 0). If ΔH > 0 (endothermic), the reaction can still be spontaneous if TΔS_system is sufficiently positive and larger than ΔH, making ΔG negative. Examples of spontaneous endothermic reactions include:

1. Melting of ice above 0°C: H₂O(s) → H₂O(l) (ΔH > 0, ΔS_system > 0, spontaneous at T > 0°C).
2. Dissolution of ammonium nitrate in water (used in instant cold packs): NH₄NO₃(s) + H₂O(l) → NH₄⁺(aq) + NO₃⁻(aq) (ΔH > 0, ΔS_system > 0, spontaneous at room temperature).

Conclusion: Both the Assertion and the Reason statements are false.

Spontaneity of a reaction is determined by the change in Gibbs free energy (ΔG), where ΔG = ΔH - TΔS_system. For a spontaneous reaction, ΔG must be negative.

1. Assertion: "Entropy of system increases for a spontaneous reaction." This is false. A spontaneous reaction can have a decrease in system entropy (ΔS_system < 0), as long as the entropy increase in the surroundings (ΔS_surroundings) is larger, ensuring ΔS_universe > 0. Example: Freezing of water below 0°C.
2. Reason: "Enthalpy of reaction always decreases for spontaneous reaction." This is false. Many spontaneous reactions are endothermic (ΔH > 0), provided there is a sufficient increase in system entropy (ΔS_system > 0) to make ΔG negative. Example: Melting of ice above 0°C or dissolution of ammonium nitrate.