A-Level Chemistry: Kinetics and Rate Equations

Kinetics spans both years of A-Level Chemistry: the qualitative collision-theory story of Year 1 and the quantitative rate-equation machinery of Year 2. This quiz deliberately tests them together, the way synoptic papers do.

The qualitative questions cover the Maxwell-Boltzmann distribution and what a catalyst really does to it — lowering the activation energy so a greater proportion of existing collisions succeed, without touching the curve itself — and the exponential heart of collision theory: why a small temperature rise produces a disproportionately large rate increase.

The quantitative questions are the Year 2 core. A rate = k[A][B] question tests proportional reasoning (double one concentration, halve the other — the rate is unchanged). A full initial-rates data question requires deducing orders from three experiments and assembling the rate equation. The units of k are tested for a third-order reaction — the mol⁻² dm⁶ s⁻¹ result that requires actually doing the dimensional analysis rather than recalling a pattern. The Arrhenius equation appears twice: its qualitative content (temperature raises k; A is the frequency factor) and the physical meaning of the pre-exponential factor including orientation.

Mechanism reasoning closes the set: reading the rate equation off the slow step of a proposed mechanism, and knowing which factors genuinely change k (temperature, catalyst) versus which only change the rate (concentration). A methods question separates the tools — initial rates and concentration-time graphs determine order; Arrhenius determines activation energy.

Explanations show the full deduction each time, which makes the set a compact revision of the entire kinetics strand.

  • Interpret Maxwell-Boltzmann distributions and catalyst effects on them
  • Deduce reaction orders and rate equations from initial-rate data
  • Derive the units of the rate constant for any overall order
  • Use the Arrhenius equation qualitatively, including the meaning of A
  • Connect rate equations to the rate-determining step of a mechanism

Aligned to the kinetics core of the DfE A level science subject content: collision theory and Maxwell-Boltzmann distributions, rate equations and orders of reaction, the Arrhenius equation, and rate-determining steps.

Sample question

In the reaction A + B -> C, the rate equation is determined to be rate = k[A][B]. If the concentration of A is doubled and the concentration of B is halved, how does the initial rate of reaction change?

See the answer

The rate remains unchanged

The rate equation is rate = k[A][B]. If [A] becomes 2[A] and [B] becomes 0.5[B], the new rate is k(2[A])(0.5[B]) = k[A][B]. Thus, the rate remains the same.

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