Surface Chemistry
Surface Chemistry covers adsorption isotherms, catalysis, and colloid behaviour, with the colloid part dominating objective questions.
Adsorption
- physisorption: van der Waals forces, ΔH about 20–40 kJ mol⁻¹, multilayer, reversible, decreases with temperature
- chemisorption: chemical bonding, ΔH about 80–240 kJ mol⁻¹, monolayer, often irreversible, needs activation energy
- Freundlich isotherm: x/m = k·p^(1/n) with 0 < 1/n < 1; a plot of log(x/m) against log p gives slope 1/n and intercept log k; the isotherm fails at high pressure
- Langmuir model assumes a saturated monolayer of identical sites
Catalysis
- homogeneous (same phase) versus heterogeneous (surface reaction)
- promoters increase activity (Mo in Fe for Haber); poisons destroy it (CO on Pt)
- enzymes are highly specific and work best at an optimum pH and temperature
Colloids
- particle size 1–1000 nm; Brownian motion, Tyndall scattering, electrophoresis
- coagulation: Hardy–Schulze rule states that the ion carrying charge opposite to the sol is effective, and effectiveness rises steeply with its charge - Al³⁺ ≫ Ca²⁺ > Na⁺ for a negative sol
- gold number measures protective power of a lyophilic colloid; a lower value means better protection
- dialysis removes crystalloids through a membrane, not the colloid
Worked example
As2S3 forms a negatively charged sol. Adding AlCl3 coagulates it at a far lower concentration than NaCl because Al³⁺ has three positive charges, while Na⁺ has one, so Al³⁺ neutralises the negative sol far more effectively.
Traps that cost marks
- Confusing adsorption with absorption (adsorption is surface-only).
- Reversing the temperature trend for physisorption.
- Saying charge on the sol itself decides coagulation; it is the opposite ion's charge.