JEE Chemistry Surface Chemistry Notes Set 01

Adsorption

1. Adsorption is the accumulation of a substance on the surface of a solid or liquid, while absorption is uniform penetration into the bulk. Sorption means both occur together, and occlusion is absorption of gases by metals such as palladium.
2. Physical adsorption involves van der Waals forces, is reversible, non-specific, forms multilayers, has low enthalpy (20 to 40 kJ/mol) and decreases with temperature. Chemical adsorption forms bonds, is irreversible, specific and monolayer, with enthalpy 80 to 240 kJ/mol.
3. Adsorption is exothermic with a decrease in entropy. It increases with pressure and surface area and decreases with temperature. Easily liquefiable gases such as NH3 and CO2 are adsorbed more than H2.
4. Freundlich isotherm: \( x/m=kp^{1/n} \) with \( 0 \lt 1/n \lt 1 \); plotting \( \log(x/m) \) against \( \log p \) gives a straight line with slope \( 1/n \) and intercept \( \log k \). Langmuir isotherm assumes a monolayer and equivalent sites.

Colloids

1. A colloid has particle size of 1 to 1000 nm and two phases: dispersed phase and dispersion medium. Lyophilic sols (gelatin, starch) are reversible and stable, while lyophobic sols (metal sulphides, gold) are irreversible and need stabilisers.
2. Types by phase: aerosol (cloud), foam (lather), emulsion (milk O/W, butter W/O), sol (paint, ink), gel (cheese, opal). Multimolecular colloids are held by van der Waals forces, macromolecular ones by covalent bonds, and associated colloids (micelles, such as soap above its CMC) form by aggregation.
3. Preparation: chemical methods, peptization and Bredig's arc method. Purification: dialysis, electrodialysis and ultrafiltration.

Properties of Colloids

1. Optical: the Tyndall effect. Kinetic: Brownian motion and diffusion. Electrical: charge from preferential adsorption of ions (As2S3 negative, Fe(OH)3 positive), electrophoresis and electro-osmosis.
2. Coagulation follows the Hardy-Schulze rule: the ion of opposite charge coagulates, and the greater its charge, the smaller the flocculation value. Lyophilic sols are protected from coagulation by protective colloids; the gold number (in mg) measures this, and a smaller gold number means greater protective power.
3. Colloids have low osmotic pressure. Emulsifiers such as soap and detergents stabilise emulsions.

Catalysis

1. A catalyst speeds up a reaction without being consumed by giving an alternative path of lower activation energy. It does not change the equilibrium position or start a reaction and it speeds up both directions equally.
2. Homogeneous catalysis has the catalyst in the same phase (lead chamber process). Heterogeneous catalysis has a different phase (Pt in the contact process, Pt-Rh gauze in the Ostwald process, Fe with Mo promoter in Haber's process). Other types are autocatalysis (Mn2+ in permanganate and oxalic acid), induced catalysis, promoters, poisons and inhibitors.
3. Enzymes are highly specific, efficient proteins that work best at 298 to 310 K and pH 5 to 7 by a lock-and-key mechanism. Zeolites are shape-selective catalysts whose pore size decides which molecules react.