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Chemical Kinetics

Rate laws and order, the integrated first-order law with its half-life, and the Arrhenius equation. Most questions are one formula with logarithms.

Questions in the bank
126
q/paper in 2025–26
1.37
Calculation
71%
Notes pages
7

Strand: Calculate

When you’ll see it

A rate, a rate law or an order, a time to some fraction decomposed, a half-life, a gas pressure over time, or a rate constant at two temperatures.

How this chapter is tested

Two equations carry most of the chapter: the first-order law, k = (2.303/t) log([A]₀/[A]) with t½ = 0.693/k, and the Arrhenius equation, k = A e^(−Ea/RT). The rest is bookkeeping: dividing a rate by its coefficient, finding a reactant's pressure from a total pressure, or reading an order from a table, a half-life or a graph.

Most questions want a number, usually to the nearest integer, and there is no calculator. Keep log 2 = 0.301, log 3 = 0.477 and ln 10 = 2.303 ready. Answers are built so that the logs come out clean: one-eighth left is three half-lives, and one-thousandth left takes three times as long as one-tenth left.

Arrhenius questions are common, and marks slip there on the factor 2.303 between ln and log, on kelvin, and on joules against kilojoules for Ea. Mechanism and energy-profile questions are reading, not arithmetic: the slow step writes the rate law, and a catalyst lowers both barriers without changing ΔH.

The sub-skills

The distinct skills inside the chapter, in the order to learn them.

  • Rate and stoichiometry

    Rate = −(1/a)d[A]/dt = (1/c)d[C]/dt; divide each species' rate by its coefficient before comparing.

  • Rate law and order

    Rate = k[A]ᵐ[B]ⁿ with exponents from experiment; pick runs where one concentration changes; the unit of k gives the order.

  • First order and half-life

    k = (2.303/t) log([A]₀/[A]); t½ = 0.693/k does not depend on [A]₀; time ratios are ratios of logs.

  • Gas decomposition and decay

    For A(g) → B(g) + C(g), p_A = 2pᵢ − Pₜ; radioactive decay and bacterial growth are first order.

  • Zero order and finding the order

    [A] = [A]₀ − kt, t½ = [A]₀/2k; t½ ∝ [A]₀^(1 − n) or the straight plot names the order.

  • Arrhenius equation

    log(k₂/k₁) = (Ea/2.303R)(1/T₁ − 1/T₂); slope of ln k against 1/T is −Ea/R; same A gives ln(k₂/k₁) = (Ea₁ − Ea₂)/RT.

  • Mechanisms and catalysts

    Rate law from the slow step with intermediates removed; ΔH = Ea,f − Ea,b; a catalyst changes neither ΔH, ΔG nor K.

Traps to expect

Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.

  • Percent decomposed used as [A]

    Seventy parts decomposed out of a hundred leaves thirty. Use log(100/30), not log(100/70).

  • Order read from the equation

    2N₂O₅ → 4NO₂ + O₂ is first order. Coefficients give exponents only for an elementary step.

  • Total pressure in the log

    The first-order law needs the reactant's own pressure. For A → B + C that is 2pᵢ − Pₜ, not Pₜ.

  • ln read as log

    The slope of log k against 1/T is −Ea/2.303R, not −Ea/R. Using the wrong one puts Ea out by 2.303; Ea in kJ with R in J puts it out by 1000.

  • Half-lives counted as if first order

    For zero order the second half-life is half the first, so one-quarter is reached at 1.5 t½, not 2 t½.

Learn it before you drill it

This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.

Chemical Kinetics notes

Drill every Chemical Kinetics question

126 questions from the bank, across 7 subtopics.

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Drill one subtopic at a time

The 7 subtopics, in teaching order.

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