Question:
Assertion (A): Adsorption is an exothermic process.
Reason (R): Physisorption of a gas on a solid surface is normally reversible.
Assertion (A): Adsorption is an exothermic process.
Reason (R): Physisorption of a gas on a solid surface is normally reversible.
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Adsorption is always accompanied by a decrease in surface energy, making it an exothermic process. Physisorption is usually reversible because it involves weak van der Waals forces.
Updated On: Jun 18, 2026
- Both (A) and (R) are correct and (R) is the correct explanation of (A)
- Both (A) and (R) are correct but (R) is not the correct explanation of (A)
- (A) is correct but (R) is incorrect
- (A) is incorrect but (R) is correct
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The Correct Option is B
Solution and Explanation
Step 1: Examine Assertion (A).
Adsorption is the accumulation of molecules on the surface of a solid or liquid.
During adsorption, the adsorbate particles experience attractive forces from the surface of the adsorbent.
As a result, the surface energy decreases and energy is released.
Therefore, adsorption is generally an exothermic process. \[ \Delta H < 0 \] Hence, Assertion (A) is correct.
Step 2: Examine Reason (R).
Physisorption occurs due to weak van der Waals forces between the adsorbate and the adsorbent.
Since these forces are weak, the adsorbed molecules can be removed easily by decreasing pressure or increasing temperature.
Therefore, physisorption is generally reversible in nature.
Hence, Reason (R) is also correct.
Step 3: Check whether (R) explains (A).
The exothermic nature of adsorption is due to the release of energy when attractive forces develop between the surface and the adsorbate particles.
The reversibility of physisorption is a consequence of weak intermolecular forces and does not explain why adsorption is exothermic.
Thus, both statements are true, but the reason is not the correct explanation of the assertion.
Step 4: Final conclusion.
Therefore, \[ \boxed{\text{Both (A) and (R) are correct, but (R) is not the correct explanation of (A)}} \] Hence, the correct option is (2).
Adsorption is the accumulation of molecules on the surface of a solid or liquid.
During adsorption, the adsorbate particles experience attractive forces from the surface of the adsorbent.
As a result, the surface energy decreases and energy is released.
Therefore, adsorption is generally an exothermic process. \[ \Delta H < 0 \] Hence, Assertion (A) is correct.
Step 2: Examine Reason (R).
Physisorption occurs due to weak van der Waals forces between the adsorbate and the adsorbent.
Since these forces are weak, the adsorbed molecules can be removed easily by decreasing pressure or increasing temperature.
Therefore, physisorption is generally reversible in nature.
Hence, Reason (R) is also correct.
Step 3: Check whether (R) explains (A).
The exothermic nature of adsorption is due to the release of energy when attractive forces develop between the surface and the adsorbate particles.
The reversibility of physisorption is a consequence of weak intermolecular forces and does not explain why adsorption is exothermic.
Thus, both statements are true, but the reason is not the correct explanation of the assertion.
Step 4: Final conclusion.
Therefore, \[ \boxed{\text{Both (A) and (R) are correct, but (R) is not the correct explanation of (A)}} \] Hence, the correct option is (2).
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