Q.Write any two characteristics of Chemisorption.

Chemisorption arises when the adsorbate is held to the adsorbent by chemical bonds (formation of a surface compound), not merely by weak van der Waals forces. Two consequences follow directly: it occurs only between chemically compatible species (high specificity) and the strong bonds make it essentially irreversible.

Step 1 – Nature of the bond. In chemisorption the gas molecules are linked to the solid surface by chemical bonds (of the same order of strength as ordinary covalent/ionic bonds). This is fundamentally different from physisorption, where only weak van der Waals forces operate.

Step 2 – Derive the characteristics from the bonding.

• Because a genuine chemical bond/surface compound must form, chemisorption is highly specific: e.g. hydrogen is chemisorbed on transition metals but not on all surfaces.
• The strong bonds give a high enthalpy of adsorption, in the range $80\text{–}240\ \text{kJ mol}^{-1}$, and make the process irreversible.
• It forms only a monomolecular layer (once the surface valencies are satisfied, no further chemisorption occurs).
• It generally increases with temperature at first (it needs activation energy), which is why it is also called activated adsorption.

Step 3 – Pick any two as the answer.

Identify the distinguishing feature of chemisorption — a real chemical bond forms between adsorbate and surface — and read off the consequences of that strong bonding (specificity, irreversibility, high enthalpy, monolayer).

• Quoting weak van der Waals forces or reversibility — those belong to physisorption, not chemisorption.
• Saying chemisorption forms multilayers; it forms only a monolayer.
• Claiming it always decreases with temperature; chemisorption first increases with temperature (activated adsorption).