Chemical Kinetics
Chemical Kinetics examines rate laws, order, integrated rate equations, half-life and temperature dependence. The exam favours first-order calculations and Arrhenius comparisons.
Rate laws and order
Rate is change in concentration per unit time. Order is experimental; molecularity is the number of species in an elementary step. Zero order: [A] = [A]0 - kt, t1/2 = [A]0/(2k). First order: ln[A] = ln[A]0 - kt, t1/2 = 0.693/k. Second order: 1/[A] = 1/[A]0 + kt, t1/2 = 1/(k[A]0). Rate constant units are mol L^-1 s^-1 (zero), s^-1 (first) and L mol^-1 s^-1 (second).
Temperature and catalysis
Arrhenius: k = A e^(-Ea/RT). A plot of ln k against 1/T has slope -Ea/R. For two temperatures, ln(k2/k1) = (Ea/R)(1/T1 - 1/T2). A catalyst lowers Ea and speeds both directions equally; it does not change delta H or the equilibrium constant.
Worked example
A first-order reaction has t1/2 = 10 min, so k = 0.693/10 = 0.0693 min^-1. After 30 min, three half-lives have passed, so [A] = [A]0(1/2)^3 = 0.125[A]0. Using ln([A]0/[A]) = kt gives the same 12.5% remaining. The reasoning is that first-order half-life is independent of initial concentration.
Traps
Using order as molecularity, giving first-order k units of mol L^-1 s^-1, and assuming a catalyst shifts equilibrium are common mistakes.