Reaction Kinetics and Reaction Mechanisms
This domain tests rate laws, collision theory, the Arrhenius equation and reaction mechanisms. STPM expects orders to be deduced from experimental data, not from stoichiometry.
Rate and order
Rate = k[A]m[B]n, where m and n are experimental orders. Zero-order rate is independent of concentration; first-order rate is proportional to concentration; second-order depends on concentration squared. The units of k depend on overall order. For a first-order reaction, t1/2 = ln2/k.
Collision theory and Arrhenius
Reacting particles must collide with energy at least equal to the activation energy Ea and with correct orientation. Rate increases with concentration, temperature, surface area and catalysts. Arrhenius: k = Ae−Ea/RT, so ln k = ln A − Ea/RT. A graph of ln k against 1/T has gradient −Ea/R.
Mechanisms
A mechanism is a sequence of elementary steps. The rate-determining step controls the overall rate. Intermediates appear in mechanisms but not in the overall equation. Catalysts provide an alternative pathway with lower Ea.
Worked example
A first-order reaction has t1/2 = 20 min. k = ln2/20 = 0.693/20 = 0.03465 min−1. For 75% completion, [A] = 0.25[A]0. Using t = ln([A]0/[A])/k = ln4/0.03465 = 1.386/0.03465 = 40 min.
Traps that cost marks
Do not assume order equals the coefficients in the balanced equation. Check the units of k. Half-life is constant only for first-order reactions. A catalyst lowers Ea for both forward and reverse reactions and does not change ΔH. Temperature changes k, not Ea.