Paper 2 Q7(b) — Reaction order from half-lives

Kinetics — constant half-life means first order; reading order from a concentration–time graph

What the examiners said

“A small number thought constant half-life indicated zero or second order and some thought a curve indicated second order… Common incorrect responses included ‘straight line so second/first order’ or gave zero order with no reason.”

Try the question

On a concentration–time graph, the reactant’s half-life is measured as constant (successive halvings take equal times). What is the order with respect to that reactant?

Examiner report: “Some thought constant half-life meant zero/second order, or read order from 'straight/curve' alone.”

Explore — read the order off the half-lives

Switch between zero-, first- and second-order decay and mark successive half-lives. Only first order keeps them equal — the diagnostic the question rewards.

Order: first

The half-life fingerprint: constant → first order, shortening → zero order, lengthening → second order. A curved graph alone cannot tell first from second.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H1 Chemistry 8873 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

Made by lookang, using Claude Fable 5. More resources: iwant2study.org