Use experimental evidence to build a rate equation, predict rate changes and derive the correct units of k.
A rate equation is an experimental model
In rate = k[A]ᵐ[B]ⁿ, square brackets mean concentration in mol dm⁻³, m and n are the orders with respect to A and B, and k is the rate constant at the stated temperature. The overall order is m + n. The order is the power to which a concentration is raised in the experimentally determined rate equation. AQA uses individual orders 0, 1 and 2 in this section.
The overall balanced equation tells you stoichiometry, not the experimental orders. A catalyst can appear in a rate law even though it cancels from the overall equation. A zero-order reactant can still be essential to the reaction; its concentration does not affect the measured rate over the range investigated.
For fixed conditions and reaction pathway, k does not fall merely because reactants are being used up. Their changing concentrations change the rate. A change of temperature or catalyst can change k.
Isolate the effect of each reactant
Compare experiments in which only one concentration changes. State the concentration factor and the rate factor, then identify the power connecting them. With noisy data, use the trend and experimental precision rather than demanding exact integer ratios.
| Order | Doubling concentration | Rate against concentration |
|---|---|---|
| Zero | Rate unchanged. | Horizontal line within the measured range. |
| First | Rate doubles. | Straight line through origin. |
| Second | Rate quadruples. | Upward curve; rate against concentration squared is straight. |
Worked initial-rate table
These are original illustrative data at one fixed temperature. All concentrations are initial concentrations after mixing.
| Run | [A] / mol dm⁻³ | [B] / mol dm⁻³ | Rate / mol dm⁻³ s⁻¹ |
|---|---|---|---|
| 1 | 0.120 | 0.200 | 2.40 × 10⁻⁴ |
| 2 | 0.240 | 0.200 | 4.80 × 10⁻⁴ |
| 3 | 0.240 | 0.400 | 1.92 × 10⁻³ |
When two concentrations change together
If A is known to be first order, doubling A accounts for a factor of two in rate. If B also triples and the total rate rises by eighteen, B accounts for 18/2 = 9; therefore B is second order because 3² = 9. Do not assign the whole rate change to B.
A log(rate)-versus-log(concentration) plot has gradient equal to the order when all other relevant concentrations and conditions are fixed. Use the same logarithm base on both axes. This is a useful way to test a wider dataset.
Derive units rather than guessing
Rearrange k = rate/(concentration terms), insert units, then cancel powers. The rate unit must match the measurement: if time is in minutes, the time factor in k is min⁻¹. Standard concentration-rate calculations here use seconds.
| Overall order | Units of k |
|---|---|
| 0 | mol dm⁻³ s⁻¹ |
| 1 | s⁻¹ |
| 2 | dm³ mol⁻¹ s⁻¹ |
| 3 | dm⁶ mol⁻² s⁻¹ |
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Tripling A multiplies the rate by nine at fixed B. What is the order in A?Show answer
Second order: 3² = 9.
Q2. For rate = k[A]²[B], what happens if A halves and B doubles?Show answer
The factor is (½)² × 2 = ½. The rate halves.
Q3. Rate = 3.60 × 10⁻⁴ mol dm⁻³ s⁻¹ at [A] = 0.0300 mol dm⁻³ for rate = k[A]². Find k.Show answer
k = (3.60 × 10⁻⁴)/(0.0300²) = 0.400 dm³ mol⁻¹ s⁻¹.
Q4. A and B both double and the rate increases eightfold. A is first order. Find B’s order.Show answer
A contributes ×2. B contributes ×4, so B is second order.
Q5. Can an overall coefficient of two prove second order?Show answer
No. The rate law must be determined experimentally unless an elementary-step model is explicitly supplied and applicable.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- AQA 7405 physical chemistry specification — 3.1.9 coverage and required skills.
- Chemrevise: Rate equations — Coverage checklist; explanations, data exercises and quick checks on this page are original Finesse material.
- AQA June 2023 Paper 2 mark scheme — Q01–02, pp11–13; report p3: tangents, measured rate units, relative rates and a proposed rate-determining step.
- AQA June 2023 Paper 2 examiner report — Read alongside the question-specific marking guidance; not a universal wording checklist.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.