Surface Chemistry
What this domain tests
Adsorption types and isotherms, catalysis, colloids and their properties, coagulation, emulsions and applications such as cleansing action.
Core rules
- Physisorption: weak van der Waals forces, reversible, multilayer, enthalpy about 20–40 kJ mol⁻¹. Chemisorption: chemical bonds, irreversible, monolayer, enthalpy roughly 80–240 kJ mol⁻¹. Both are exothermic.
- Freundlich isotherm: x/m = k·p^(1/n). A plot of log(x/m) against log p is linear with slope 1/n, where 0 < 1/n < 1; it fails at high pressure.
- Homogeneous catalysis has catalyst and reactants in one phase; heterogeneous catalysis occurs at a surface. Promoters increase activity; poisons destroy it.
- Colloidal particles are 1–1000 nm and are classified as lyophilic or lyophobic, and as multimolecular, macromolecular or associated.
- Colligative properties of sols are small because the number of particles is small.
- Coagulation follows the Hardy–Schulze rule: the ion with the charge opposite to the sol is effective, and effectiveness rises steeply with charge.
- Emulsions are oil-in-water or water-in-oil; an emulsifier stabilises them, and demulsification breaks them.
Worked example
An As₂S₃ sol is negatively charged because it adsorbs S²⁻. To coagulate it, add cations. AlCl₃ supplies Al³⁺, CaCl₂ supplies Ca²⁺ and NaCl supplies Na⁺, so the coagulating power order is AlCl₃ > CaCl₂ > NaCl. A lower concentration of AlCl₃ produces the same effect because a triply charged ion neutralises the charge far more effectively.
Traps that cost marks
Water adsorbs on silica by physisorption, but H₂ adsorbs on Ni by chemisorption; do not generalise. Dialysis removes crystalloids, not the colloidal particles. Tyndall scattering proves heterogeneity, not surface charge; electrophoresis shows charge. Gold number is an inverse measure of protective power, so a small gold number means a good protective colloid.