Chemical Kinetics
Chemical Kinetics tests rate expressions, order versus molecularity, integrated rate laws, half-life, Arrhenius behaviour and collision theory. First-order numericals dominate.
Core rules
- Rate = k[A]^m[B]^n; order = m + n, determined experimentally, not from stoichiometry.
- Zero order: [A] = [A]0 - kt; first order: k = (2.303/t) log([A]0/[A]); t1/2 = 0.693/k.
- Second order: 1/[A] = 1/[A]0 + kt.
- Arrhenius: k = A e^(-Ea/RT); log(k2/k1) = Ea/2.303R (1/T1 - 1/T2).
- A catalyst lowers Ea and increases k without changing ΔH or the equilibrium position.
Worked example
A first-order reaction has t1/2 = 20 min. Find k and the time for 75% completion. k = 0.693/20 = 0.03465 min^-1. 75% completion leaves 25%, i.e. two half-lives, so t = 40 min. Reasoning: after one half-life 50% remains, after two 25% remains; for first order the half-life is constant.
Traps
Treating order as the coefficients in the balanced equation, using minutes with k in s^-1, and confusing rate with rate constant. For zero order, concentration falls linearly but the half-life is not constant.