Chemical Kinetics and Surface Chemistry
This section covers Chemical Kinetics and Surface Chemistry: rate laws, order and molecularity, first-order integrated equations, half-life, Arrhenius behaviour, adsorption, colloids and coagulation. Numerical questions usually involve first-order kinetics.
Core rules
- Rate = k[A]^m[B]^n; the exponents give order. Order is experimental and may be zero or fractional; molecularity is the number of species in an elementary step.
- First order: k = (2.303/t) log(a/(a − x)); t₁/₂ = 0.693/k.
- Arrhenius: k = A e^(−Ea/RT); a higher Ea makes k more temperature-sensitive.
- Physisorption is weak and reversible; chemisorption involves chemical bonds. Freundlich isotherm: x/m = kP^(1/n). Hardy–Schulze rule: a coagulating ion is more effective when its charge is higher.
Worked example
A first-order reaction is 75% complete in 30 min. Since 75% completion means two half-lives, t₁/₂ = 15 min. Then k = 0.693/15 = 0.0462 min⁻¹. Alternatively, k = (2.303/30) log(100/25) = 0.0462 min⁻¹. The same k applies at this temperature; it would change if temperature changed.
Traps
Do not confuse order with molecularity; check units of k (first order: time⁻¹); do not say a catalyst shifts equilibrium; distinguish adsorption from absorption; do not compare coagulating power by ion size instead of charge.