Chemical Kinetics
Chemical Kinetics tests rate expressions, order and molecularity, integrated rate laws, half-life and the Arrhenius equation. First-order numericals and graph interpretation are frequent.
Core rules
- Rate = -d[A]/dt for a reactant; rate law is experimentally determined.
- Order is the sum of exponents; molecularity is the number of species in an elementary step.
- Zero order: [A] = [A]0 - kt; half-life t1/2 = [A]0/2k.
- First order: ln([A]0/[A]) = kt; t1/2 = 0.693/k, independent of initial concentration.
- Arrhenius: k = A e^(-Ea/RT); log k vs 1/T gives slope -Ea/2.303R.
Worked example
A first-order reaction has k = 2.0 x 10-3 s-1. Calculate its half-life and the time for 75% decomposition. Reasoning: t1/2 = 0.693/k = 0.693/(2.0 x 10-3) = 346.5 s. After 75% decomposition, 25% remains, which is two half-lives: 2 x 346.5 = 693 s.
Traps
- Order can be zero or fractional; molecularity must be a small integer.
- The units of k depend on order: s-1 for first order, mol-1 L s-1 for second order.
- For a first-order reaction, t1/2 does not change with initial concentration.
- A catalyst lowers Ea and increases k; it does not change delta H or the equilibrium constant.