Why does chemisorption initially increase with temperature while physisorption decreases?

Physisorption is exothermic, governed by van der Waals forces. Higher temperature provides energy to overcome these weak interactions, so adsorption decreases with temperature (Le Chatelier's principle). Chemisorption involves chemical bond formation and has a higher activation energy. Initially, increasing temperature provides the necessary activation energy to form chemical bonds → adsorption increases. Beyond the optimum temperature, the increased kinetic energy breaks the chemical bonds → desorption increases → adsorption decreases.

Why does adding electrolyte coagulate a lyophobic colloid?

Lyophobic colloidal particles carry a surface charge (e.g., As₂S₃ sol is negatively charged). This charge creates a repulsive barrier preventing particles from coming close enough to aggregate. When an electrolyte is added, oppositely charged ions (counter-ions) are attracted to the colloidal particles, neutralising their surface charge. Once the charge is neutralised, there is no repulsive barrier, and van der Waals attraction causes particles to aggregate and precipitate. Higher valency counter-ions are more effective (Hardy-Schulze law).

Adsorption, Catalysis, and Colloids

Surface Chemistry • Class 12 Chemistry • NCERT • CBSE

Adsorption: accumulation of substance on surface. Adsorbent (surface) vs adsorbate (substance). Physisorption: weak van der Waals forces, reversible. Chemisorption: strong chemical bonds, irreversible. Tyndall effect distinguishes colloids.

Key Formulas

Freundlich isotherm: x/m = kP^(1/n)
Log form: log(x/m) = log k + (1/n)log P
Colloidal size range: 1–1000 nm (10⁻⁹ to 10⁻⁶ m)

Frequently Asked Questions

Why does chemisorption initially increase with temperature while physisorption decreases?: Physisorption is exothermic, governed by van der Waals forces. Higher temperature provides energy to overcome these weak interactions, so adsorption decreases with temperature (Le Chatelier's principle). Chemisorption involves chemical bond formation and has a higher activation energy. Initially, increasing temperature provides the necessary activation energy to form chemical bonds → adsorption increases. Beyond the optimum temperature, the increased kinetic energy breaks the chemical bonds → desorption increases → adsorption decreases.
Why does adding electrolyte coagulate a lyophobic colloid?: Lyophobic colloidal particles carry a surface charge (e.g., As₂S₃ sol is negatively charged). This charge creates a repulsive barrier preventing particles from coming close enough to aggregate. When an electrolyte is added, oppositely charged ions (counter-ions) are attracted to the colloidal particles, neutralising their surface charge. Once the charge is neutralised, there is no repulsive barrier, and van der Waals attraction causes particles to aggregate and precipitate. Higher valency counter-ions are more effective (Hardy-Schulze law).

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