Chemical Kinetics
What you must know
Understand rate as change in concentration over time, and the factors affecting it: concentration, temperature, surface area, pressure, catalysts. Know collision theory - effective collisions require sufficient energy and correct orientation. Interpret rate–concentration graphs, order of reaction (zero, first, second), half-life and rate constants. Understand the Arrhenius relationship qualitatively: higher temperature increases the fraction of molecules with energy above the activation energy. Distinguish homogeneous and heterogeneous catalysts and catalysts versus intermediates.
Traps
- Confusing rate with rate constant; k depends on temperature, not concentration.
- Assuming a catalyst changes the equilibrium position (it does not; it only speeds attainment).
- Forgetting that increasing surface area affects heterogeneous but not homogeneous reactions.
- Misreading order: order is experimental, not taken from the balanced equation.
Worked example
For a first-order reaction, doubling the concentration doubles the rate; the half-life is constant. If k = 0.05 s⁻¹, the half-life t½ = ln2/k ≈ 0.693/0.05 ≈ 13.9 s. Rule of thumb: for first order, rate ∝ concentration; for zero order, rate is independent of concentration.