CBSE Class 12 Chemistry: Surface Chemistry — Notes 2026

Adsorption

• Adsorption: accumulation of substance on surface; adsorbent = surface (solid); adsorbate = substance adsorbing
• Physical adsorption (physisorption): weak van der Waals forces; low ΔH; reversible; multi-layer; decreases with temperature
• Chemical adsorption (chemisorption): chemical bond; high ΔH; irreversible (often); monolayer; may increase with temperature initially

Adsorption Isotherms

• Freundlich isotherm: x/m = k P^(1/n) where x/m = amount adsorbed per gram, P = pressure; empirical
• Langmuir isotherm: monolayer adsorption; saturation at high pressure; x/m = aP/(1+bP)
• Applications: gas masks (activated charcoal), decolourisation of sugar (bone charcoal), water purification

Catalysis

• Catalyst: speeds up reaction without being consumed; lowers activation energy; specific to reaction
• Homogeneous catalysis: catalyst and reactants in same phase; SO₂ + O₂ with NO (g) catalyst
• Heterogeneous catalysis: different phases; Haber process (N₂ + H₂, Fe catalyst solid); Contact process (SO₂, Pt catalyst)

Mechanism of Heterogeneous Catalysis

• Adsorption theory: reactants adsorb on catalyst surface → intermediate complex → products desorb
• Active sites: specific surface atoms with unsatisfied valences; metal surfaces have more active sites than ionic surfaces
• Promoters: enhance catalyst activity (Mo added to Fe in Haber process); poisons: reduce activity (As poisons Pt)

Colloids

• Colloid: particles 1–1000 nm; heterogeneous mixture; Tyndall effect (scattering of light); Brownian motion
• Types: sol (solid in liquid), gel (liquid in solid), aerosol (liquid/solid in gas), foam, emulsion
• Lyophilic (solvent-loving): gum, starch, gelatin; stable; Lyophobic (solvent-hating): Fe(OH)₃ sol; unstable, need charge for stability

Emulsions and Coagulation

• Emulsion: liquid dispersed in another immiscible liquid; emulsifier stabilises it (soap reduces surface tension)
• O/W emulsion: oil drops in water (milk, vanishing cream); W/O emulsion: water in oil (butter, cold cream)
• Coagulation (flocculation): addition of electrolyte neutralises charge on colloid → particles aggregate and settle

CBSE Board Focus

• Surface Chemistry: 4–5 marks; adsorption types comparison, Tyndall effect, coagulation causes
• Catalysis: define promoter and poison; explain heterogeneous catalysis with Haber process example
• Colloid types: identify colloid type from components (sol, gel, aerosol, emulsion)