IB Chemistry Rate Laws & Reaction Mechanisms (HL): Your Questions Answered

First, work out what you're actually given — concentration-time data, initial rates, or a proposed mechanism — and pick the matching tool. Use the initial rates method to find orders, integrated graphs for half-life work, and always check that a mechanism's slow step matches the experimental rate law you derived. Show units at every stage.

A rate law is the experimentally measured equation rate = k[A]^m[B]^n linking concentration to rate — you cannot read it off a balanced equation. A reaction mechanism is the proposed sequence of elementary steps showing how bonds actually break and form. The rate law tests and constrains which mechanisms are plausible.

Use the initial rates method: compare experiments where only one reactant's concentration changes and see how the rate responds. Doubling [A] while doubling the rate means first order in A; doubling [A] while quadrupling the rate means second order. Multiply the individual orders' effects together to get the overall rate law.

The rate-determining step is the slowest elementary step in a multi-step mechanism, and it's this step — not the overall equation — that dictates the experimental rate law. Only species involved up to and including that slow step appear in the rate expression, which is why intermediates from later steps never show up in it.

Mostly, yes. SL students study collision theory and qualitative rate factors, but quantitative rate laws, orders of reaction, the rate-determining step, half-life calculations, and the Arrhenius equation are HL-only content. According to the IB Chemistry guide (first exams 2025), this sits under sub-topic Reactivity 2.2, with the extra maths reserved for HL.

Expect "determine" (calculate a numerical value, e.g. order or k), "deduce" (reach a conclusion using given data, e.g. propose the rate law), "sketch" (a labelled graph shape, e.g. concentration against time), and "suggest" (a plausible mechanism consistent with the rate law). Each demands a different depth of working shown.

HL Paper 2 is marked out of 90, and kinetics typically appears as one structured, data-based sub-question worth roughly 6–10 marks — often combining a rate-data table, a calculation, and a mechanism or graph-sketching part. It rarely stands alone; it's usually bundled with equilibrium or energetics in the same question.

The single biggest error I mark every year: students assume reaction order matches the stoichiometric coefficients in the balanced equation. Order can only come from experimental data. The second common trip-up is sign errors and log manipulation in Arrhenius equation questions, where a mishandled negative sign flips the whole activation energy answer.

Practise the initial rates method until finding orders is automatic, memorise that k's units change with overall reaction order, and always link your calculated rate law back to a plausible mechanism's slow step. Past-paper data questions are the fastest way to build this — the topic rewards pattern recognition more than raw memory.

Yes — HL kinetics adds a genuine layer of maths and abstraction that SL doesn't touch: calculating orders from data, deriving rate constants with correct units, and using the Arrhenius equation to find activation energy. SL stops at qualitative collision theory. If your child enjoys graphs and algebra, HL kinetics tends to click quickly.

Both cover orders of reaction, rate laws, and the Arrhenius equation to a similar mathematical depth, but IB HL assesses it through open-ended data-based questions rather than mostly closed-form calculations. A-Level tends to isolate kinetics into its own paper section; IB weaves it into broader structured questions, which many students find tests understanding more than recall.

Look for resources with plenty of data-response practice, not just theory summaries — this topic is learned by doing calculations repeatedly, not by re-reading notes. Topical worksheets isolating kinetics questions, a mock paper with full mark schemes, and concise revision notes covering the Reactivity 2.2 sub-topic are the most efficient combination for HL students.