Question

The following statements related to the adsorption of gases on a solid surface. Identify the incorrect statement among them.
(A) Enthalpy of adsorption is negative.
(B) On adsorption, the residual forces on the surface are increased.
(C) On adsorption, decrease in surface energy appears as heat.
(D) Entropy of adsorption is negative.

Answer 1

Hint: Recollect the process of adsorption. Carefully think about the statements given in the options. Think whether adsorption is an endothermic or an exothermic process. Think what happens to the residual forces after adsorption. Choose the incorrect option.

Complete step by step solution:
- Let’s take a look at the given options and identify the correct statements for adsorption on a solid surface.
- Enthalpy of adsorption is negative. This statement is correct because adsorption involves formation of bonds or interactions between adsorbent and adsorbate. Therefore, adsorption is an exothermic process as energy is released.
- On adsorption, the residual forces on the surface are increased. This is an incorrect statement because during adsorption, residual forces on the surface decrease as adsorbent forms bonds with adsorbate and the surface is covered by adsorbate molecules.
- On adsorption, decrease in surface energy appears as heat. This is a correct statement because adsorption is an exothermic process and therefore, heat evolved will make the adsorbent surface hot.
- Entropy of adsorption is negative. This is also correct because, during adsorption, adsorbate molecules will be forming interactions with the adsorbent and so the entropy of the system will decrease as more and more molecules are adsorbed on the surface of the adsorbent.

Therefore, the only incorrect statement is option (B).

Note: Remember adsorption is an exothermic process and so enthalpy of adsorption will be negative. Entropy of adsorption is negative because the randomness of the system decreases due to more and more molecules getting adsorbed on the surface of the adsorbent because of which surface energy will also decrease.