Understanding Chemisorption

The question asks for the correct statement regarding chemisorption. Chemisorption is a type of adsorption that involves a chemical reaction between the surface of the adsorbent and the substance being adsorbed (adsorbate). This process forms a chemical bond, typically a covalent or ionic bond.

Chemisorption vs. Physisorption

It is helpful to compare chemisorption with physisorption to understand its characteristics.

• Physisorption: Occurs due to weak van der Waals forces. It is generally non-specific and reversible. It can form multi-molecular layers, and the heat of adsorption is low.
• Chemisorption: Occurs due to the formation of chemical bonds. It is highly specific and generally irreversible. It usually forms a monolayer, and the heat of adsorption is high.

Analyzing the Statements

Let's examine each statement provided in the options:

• Statement 1: Chemisorption results in multi molecular layer adsorption.

  This statement is incorrect. Chemisorption involves the formation of chemical bonds between the adsorbate and specific sites on the adsorbent surface. Once these sites are occupied, further layers typically do not form via chemisorption. Multi-molecular layers are characteristic of physisorption, which occurs via weak van der Waals forces that can act between adsorbate molecules themselves.

• Statement 2: Chemisorption is reversible in nature.

  This statement is incorrect. Because chemisorption involves the formation of chemical bonds, it is generally irreversible or only reversible under drastic conditions (like high temperature or vacuum) which break the chemical bonds. Physisorption, driven by weak van der Waals forces, is reversible simply by changing pressure or temperature.

• Statement 3: Chemisorption has lower specificity than physisorption.

  This statement is incorrect. Chemisorption is highly specific. It only occurs if the adsorbate can form a chemical bond with the adsorbent surface. For example, oxygen chemisorbs on metals but not readily on noble gases. Physisorption, based on universal van der Waals forces, occurs on virtually any surface, making it non-specific.

• Statement 4: Chemisorption occurs due to formation of chemical bonds.

  This statement is correct. The fundamental driving force behind chemisorption is the formation of chemical bonds (covalent, ionic, or metallic) between the adsorbate molecules and the atoms of the adsorbent surface. This is the defining characteristic that distinguishes it from physisorption.

Conclusion on Chemisorption

Based on the characteristics of chemisorption, the statement that correctly describes it is that it occurs due to the formation of chemical bonds.