The incorrect statement(s) pertaining to the adsorption of a gas on a solid surface are:

(A) Adsorption is always exothermic
(B) Physisorption may transform into chemisorption at high temperature
(C) Physisorption increases with increase in temperature
(D) Chemisorption is more exothermic than physisorption however it is very slow due to higher energy of activation
(E) Physisorption is reversible in nature

Choose the correct answer from the options given below:

(C) Only

The correct answer is Option (2) → (C) Only

Let us break down and explain each of the statements regarding the adsorption of a gas on a solid surface in more detail.

(A) Adsorption is always exothermic

Adsorption is the process where gas or liquid molecules stick to the surface of a solid (adsorbent). During this process, the molecules are attracted to the surface and lose kinetic energy. This energy loss results in the release of heat, making the process exothermic. The system’s energy decreases, making the process spontaneous. Thus, adsorption is always exothermic.

\(\Delta H < 0\)

The exothermic nature of adsorption applies to both physisorption (involving weak Van der Waals forces) and chemisorption (involving chemical bonds).

Conclusion: This statement is true.

(B) Physisorption may transform into chemisorption at high temperature

Physisorption involves weak, non-specific Van der Waals forces, while chemisorption involves the formation of specific chemical bonds between the adsorbate and the adsorbent. At low temperatures, physisorption dominates because the weak forces are sufficient to hold the adsorbate molecules on the surface. However, as temperature increases, higher energy is available for the molecules to overcome the activation energy barrier required for chemisorption, where stronger chemical bonds are formed.

Example: For example, in the adsorption of hydrogen on metal surfaces, at low temperatures, hydrogen is adsorbed via physisorption. At higher temperatures, it dissociates into atoms and forms stronger chemical bonds with the surface, undergoing chemisorption.

Conclusion: This statement is true.

(C) Physisorption increases with increase in temperature

This statement is incorrect because physisorption decreases with an increase in temperature. Physisorption is an exothermic process (it releases heat), so according to Le Chatelier’s Principle, if the temperature increases, the system will respond by minimizing the adsorption to counteract the added heat. In other words, increased temperature provides energy that causes the adsorbed molecules to desorb, reducing the amount of adsorption.

Physisorption is more favorable at low temperatures. As the temperature rises, the weak interactions between the gas molecules and the solid surface cannot hold the molecules in place, leading to decreased adsorption.

Conclusion: This statement is false, and thus incorrect.

(D) Chemisorption is more exothermic than physisorption, however, it is very slow due to higher energy of activation

Chemisorption typically involves stronger, specific chemical bonds (like covalent or ionic bonds) between the adsorbate and adsorbent. As a result, it is generally more exothermic than physisorption, which only involves weak Van der Waals forces. However, chemisorption requires an activation energy for bond formation, making it a slower process compared to physisorption, especially at lower temperatures.

Energy of Activation: Unlike physisorption, which occurs almost instantaneously due to weak interactions, chemisorption requires overcoming an energy barrier to form chemical bonds, which is why it can be slower, even though it releases more heat when the bonds are formed.

Conclusion: This statement is true.

(E) Physisorption is reversible in nature

Physisorption is generally reversible because the forces involved are weak Van der Waals forces, which can easily be overcome by slight changes in temperature or pressure. When the external conditions change, the adsorbed molecules can be released (desorbed) from the surface without significant energy input.

Example: In gas adsorption on solid surfaces, if the temperature is increased or the pressure is decreased, the adsorbed gas molecules can desorb easily. This contrasts with chemisorption, which is typically irreversible due to the strong chemical bonds formed.

Conclusion: This statement is true.