Surface Chemistry
Surface Chemistry
This domain covers adsorption, catalysis, colloids and emulsions, with emphasis on distinguishing physisorption from chemisorption and explaining colloid stability.
Core rules
- Physisorption: van der Waals forces, low ΔH (20–40 kJ mol⁻¹), reversible, multilayer. Chemisorption: chemical bonds, high ΔH, irreversible, monolayer.
- Freundlich isotherm: x/m = kp^(1/n); log(x/m)=log k+(1/n)log p, with 1/n between 0 and 1.
- Heterogeneous catalysis: reactants adsorb, reaction occurs on the surface, products desorb. Promoters increase activity; poisons reduce it.
- Colloids have a dispersed phase and a dispersion medium. Lyophilic sols are stable; lyophobic sols need electrolytes.
- Coagulation follows Hardy–Schulze: higher charge coagulates better; Al³⁺ > Ca²⁺ > Na⁺ for a negative sol.
- Tyndall effect, Brownian motion, electrophoresis and dialysis identify or purify colloids.
Worked example For adsorption obeying log(x/m)=1.0+0.50 log p, at p=100: log(x/m)=1.0+0.50×2=2.0, so x/m=100 in the units used. The slope 1/n=0.5 indicates favourable adsorption.
Traps
- Saying chemisorption is multilayer.
- Reversing Hardy–Schulze: coagulating power depends on the charge of the ion opposite to the sol.
- Soap micelles form above CMC; below it, soap behaves as a normal electrolyte.