Surface Chemistry

Which is not correct regarding the adsorption of a gas on surface of a solid?

1. On increasing temperature adsorption increases continuously.
2. Enthalpy and entropy change is negative.
3. Adsorption is more for some specific substance.
4. It is a reversible reaction.

Answer

(1) On increasing temperature adsorption increases continuously.

Solution

As per NCERT, adsorption is always exothermic ( H < 0) and the gas loses freedom of movement on the surface ( S < 0), so (2) is correct.

Chemisorption is highly specific and physisorption is reversible, so (3) and (4) are also correct.

Physisorption, however, decreases with increase in temperature (Le Chatelier's principle), while chemisorption first increases and then decreases.

∴ Adsorption does not increase continuously with temperature, so (1) is the incorrect statement.

Answer: (1)