Surface Chemistry
Surface Chemistry tests adsorption types, adsorption isotherms, catalysis, colloids and emulsions. The exam often asks you to distinguish physisorption from chemisorption and to apply the Freundlich equation.
Core rules
- Adsorption is surface accumulation; absorption is penetration into the bulk.
- Physisorption: weak van der Waals forces, low heat, reversible, multilayer.
- Chemisorption: chemical bonds, high heat, irreversible, monolayer.
- Freundlich: x/m = k p^(1/n); log(x/m) = log k + (1/n) log p.
- Colloids: particle size 1–1000 nm; Tyndall effect, Brownian motion, electrophoresis.
- Coagulation follows Hardy–Schulze: the ion of opposite charge with higher valency is most effective.
Worked example
For a Freundlich adsorption, a plot of log(x/m) against log p has slope 0.50 and intercept 0.30. Find x/m at p = 10. Reasoning: log(x/m) = 0.30 + 0.50 log 10 = 0.30 + 0.50 = 0.80. Therefore x/m = 10^0.80 = 6.3.
Traps
- Physisorption is reversible and has low enthalpy; chemisorption is not.
- A positive sol is coagulated by an anion, not a cation.
- The Tyndall effect is scattering of light by colloidal particles, not a test for true solutions.
- Emulsions are liquid-in-liquid; o/w and w/o are not the same.