ANS:C - an irreversible phenomenon.

Chemisorption, or chemical adsorption, is characterized by being an irreversible phenomenon. Explanation:

1. Chemisorption vs. Physisorption:
   • Chemisorption differs from physisorption (Van der Waals adsorption) in the nature of the bonding between the adsorbate (substance being adsorbed) and the adsorbent (surface on which adsorption occurs).
   • In chemisorption, strong chemical bonds are formed between the adsorbate molecules and the surface of the adsorbent. These bonds are typically covalent or ionic in nature and are much stronger than the weaker Van der Waals forces involved in physisorption.
2. Characteristics of Chemisorption:
   • Chemisorption involves the formation of new chemical bonds between the adsorbate and the adsorbent.
   • It often results in significant changes in the chemical composition and structure of the adsorbate and/or adsorbent surfaces.
   • Chemisorption is usually associated with a noticeable release or absorption of heat, which is often referred to as the heat of adsorption. This heat change is typically greater than that observed in physisorption due to the formation of chemical bonds.
3. Irreversible Phenomenon:
   • Chemisorption is considered an irreversible phenomenon because the chemical bonds formed between the adsorbate and the adsorbent are relatively strong and stable.
   • Once the chemical bonds are formed, they are not easily broken by physical means such as changes in temperature or pressure.
   • Reversing chemisorption typically requires breaking the chemical bonds, which often requires significant energy input or chemical reactions.
4. Applications:
   • Chemisorption plays a crucial role in various industrial processes, including catalysis, gas purification, heterogeneous catalytic reactions, and surface modification of materials.
   • Examples of chemisorption processes include the adsorption of gases on catalyst surfaces in chemical reactions and the adsorption of contaminants on activated carbon surfaces in water purification.

In summary, chemisorption is characterized by the irreversible formation of chemical bonds between the adsorbate and the adsorbent, resulting in significant changes in the chemical composition and structure of the surfaces involved. It is distinct from physisorption and is often associated with the release or absorption of heat.