Topic 3 of 5
Test a mechanism and design a measurement
Connect elementary steps with observable rates and suitable signals.
A-Level 9476 (2026-2027)
A mechanism must match both the overall equation and the rate law
Intermediates cancel from the overall equation; a proposed rate law must use measurable reactant concentrations.
A mechanism is a sequence of elementary steps. The rate-determining step is the slow step that controls the overall rate under the specified conditions. For an elementary slow step involving A and B, the simplest collision model gives rate = k[A][B]. A later fast step can consume another reactant without that reactant appearing in the observed rate law.
Worked example
Test a two-step mechanism
Proposed steps are A + B → I (slow), followed by I + B → P (fast). The observed rate is k[A][B]. Is the mechanism consistent?
- Add the steps and cancel intermediate I: A + 2B → P.
- The elementary slow step predicts first order in A and first order in B.
- This matches the observed rate law even though B has coefficient 2 in the overall equation.
It is consistent with both pieces of evidence. Agreement supports the mechanism but does not prove it is the only possible mechanism.
Worked example
Use a fast pre-equilibrium to remove an intermediate
A proposed mechanism is 2A ⇌ I (fast equilibrium), then I + B → P (slow). Predict the rate law.
- The slow elementary step gives rate = k2[I][B].
- For the preceding equilibrium, K = [I]/[A]2, so [I] = K[A]2.
- Substitute: rate = k2K[A]2[B]; at fixed temperature, combine k2K into the observed constant k.
Rate = k[A]2[B], second order in A and first order in B. This prediction relies on the stated fast-equilibrium approximation.
A proposed elementary step cannot have an arbitrary concentration exponent unrelated to the species participating in that step. Conversely, an observed zero order does not mean the substance is absent from the reaction: its concentration may not control the slow process over the measured range. Always use the mechanism supplied rather than inventing a slow step from the overall coefficients.
Choose a signal that follows a changing species
The method must be fast enough, selective enough and controlled enough for the question.
| Observable change | Possible method | Key control or limitation |
|---|---|---|
| Gas produced | Record gas-syringe volume against time, or loss of mass if gas escapes. | Check for leaks, gas solubility and temperature/pressure changes; account for mixing delay. |
| Coloured species consumed or formed | Record absorbance with a colorimeter at a suitable wavelength. | Calibrate absorbance against concentration; use a blank and avoid interference from other coloured species. |
| Acid, base or redox species changes | Withdraw timed aliquots, quench if needed, then titrate. | Quenching must stop the reaction without changing the amount being analysed. |
| A small fixed amount of product triggers a colour change | Use a clock method and measure time to the same endpoint. | 1/time compares initial rates only when the fixed threshold is small and consistent. |
To compare concentration effects, change one initial concentration while holding total volume, temperature and other reactant concentrations constant, often by replacing part of a solution volume with water. Use an appropriate initial gradient, because reactant concentrations change during a run. Keep solid particle size or surface area constant if a solid participates.
Worked example
Plan a rate measurement for calcium carbonate and acid
How could the effect of acid concentration be investigated using the carbon dioxide produced?
- Use a gas-tight flask connected to a gas syringe; use equal masses and a controlled size range of carbonate pieces.
- Use acid in excess and hold its total volume and temperature constant between runs.
- Start timing reproducibly at mixing and record gas volume at short intervals; use the initial tangent rather than the final gas volume.
- Repeat to assess scatter and check the apparatus for leaks.
Initial gas-volume gradients compare initial rates. The final volume mainly reflects the limiting amount, so it cannot by itself show which reaction was faster.
This is a valid theoretical planning method. The 9476 practical syllabus excludes gas collection by gas syringe or water displacement as a hands-on examination operation; interpreting supplied gas-volume data or discussing an appropriate method remains useful chemical reasoning.