If two reactant concentrations always change by the same factor, initial-rate data may reveal their combined order while leaving the individual orders unresolved. Add a run that changes one concentration and holds the other constant.
Before looking at the result, calculate what each proposed rate law predicts for that run. A useful run gives distinct predictions under the same conditions.
- Compare rates using concentration factors.
- Check which orders the original runs actually determine.
- Choose an extra run that makes the candidate predictions diverge.
Worked example | The first three runs agree with both models
In run 1, [A] and [B] are both 0.10 mol dm⁻³ and the initial rate is 2.0 × 10⁻⁴ mol dm⁻³ s⁻¹. In run 2, both concentrations are 0.20 mol dm⁻³ and the rate is 8.0 × 10⁻⁴. In run 3, both are 0.30 mol dm⁻³ and the rate is 1.8 × 10⁻³, in the same rate units.
Doubling both concentrations multiplies the rate by four. Tripling both multiplies it by nine. Both rate = k[A]² and rate = k[A][B] match these ratios. Under the assumed power-law model, these runs establish an overall order of two, but leave the individual orders unresolved.
Design a fourth run that separates the predictions
Set [A] to 0.20 mol dm⁻³ and hold [B] at the run 1 value of 0.10 mol dm⁻³. Keep the temperature and other conditions the same. Compare both predictions with the rate in run 1.
If rate = k[A]², doubling [A] quadruples the rate to 8.0 × 10⁻⁴ mol dm⁻³ s⁻¹. If rate = k[A][B], doubling [A] with [B] fixed only doubles the rate to 4.0 × 10⁻⁴ mol dm⁻³ s⁻¹. An observed rate close to the latter supports the second model over the first.
Try changing B instead
Keep [A] at 0.10 mol dm⁻³ and raise [B] to 0.20 mol dm⁻³. What does each model predict?
Answer. The first model predicts an unchanged rate of 2.0 × 10⁻⁴ mol dm⁻³ s⁻¹. The second predicts twice that rate, 4.0 × 10⁻⁴ mol dm⁻³ s⁻¹. A measurement that differs substantially from both predictions would prompt a check of the data and assumptions.
Frequently asked questions
Would a fourth run with both concentrations doubled again help?
It could help check repeatability if it repeats an earlier run. Since both models give the same prediction for that run, it cannot separate the proposed orders.
Can the balanced chemical equation settle the orders?
An overall balanced equation gives stoichiometric ratios. Determine the rate law from kinetic evidence unless the question gives you a justified elementary step and asks about that step.
Official information
SEAB, 2027 GCE A-Level H2 Chemistry syllabus (9476), Reaction Kinetics, initial rates. Syllabus checked 11 November 2026. The worked examples are original.