Chemical Kinetics and Surface Chemistry
Chemical Kinetics and Surface Chemistry tests rate laws, order, half-life, Arrhenius behavior, collision theory, catalysis, adsorption, and colloidal systems. Numerical questions often combine rate constants with time or concentration.
Core rules
- Rate = k[A]^m[B]^n; order is experimental, not taken from stoichiometry.
- Zero order: [A] = [A]0 − kt. First order: ln([A]0/[A]) = kt; t1/2 = 0.693/k. Second order: 1/[A] = 1/[A]0 + kt.
- Arrhenius: k = A e^(−Ea/RT); a higher Ea makes k more temperature-sensitive.
- A catalyst lowers Ea and speeds both forward and reverse reactions without changing equilibrium.
- Physisorption involves weak forces; chemisorption involves bond formation. Freundlich isotherm: x/m = kP^(1/n).
- Colloids show Tyndall scattering, Brownian motion, and coagulation by electrolytes.
Worked example
A first-order reaction has t1/2 = 20 min. Find k and the time for 75% reaction. k = 0.693/20 = 0.0347 min−1. After 75% reaction, 25% remains, which is two half-lives: 20 + 20 = 40 min. Alternatively, ln(100/25) = kt gives t = ln4/0.0347 = 40 min.
Traps
Do not infer order from coefficients. Do not use first-order half-life for zero- or second-order reactions. Check units of k: first order is time−1. A catalyst does not increase the equilibrium yield. Do not confuse adsorption with absorption in surface chemistry.