📘 IB Understanding
Many reactions occur in a series of elementary steps. The slowest step (the rate-determining step) dictates the rate of the overall reaction. The molecularity of an elementary step is the number of reactant particles involved in that step.
Key Definitions
| Term | Definition |
|---|---|
| Reaction mechanism | The step-by-step sequence of elementary steps by which an overall reaction occurs |
| Elementary step | A single molecular event (collision) in the mechanism |
| Rate-determining step (RDS) | The slowest elementary step, which controls the overall rate |
| Intermediate | A species produced in one step and consumed in a later step |
| Molecularity | Number of reactant particles in one elementary step (unimolecular = 1, bimolecular = 2) |
The Golden Rule
The rate law for the overall reaction is determined only by the rate-determining step. For an elementary step, the orders are equal to the stoichiometric coefficients.
Two-Step Mechanism Energy Profile
Worked Example
Overall: NO₂(g) + CO(g) → NO(g) + CO₂(g)
Experimental rate law: Rate = k[NO₂]²
Proposed mechanism:
| Step | Equation | Speed | Molecularity |
|---|---|---|---|
| 1 (RDS) | NO₂ + NO₂ → NO + NO₃ | Slow | Bimolecular |
| 2 | NO₃ + CO → NO₂ + CO₂ | Fast | Bimolecular |
Check: The slow step involves 2 molecules of NO₂, giving Rate = k[NO₂]². This matches the experimental rate law. ✅
Check overall: Steps add to give NO₂ + CO → NO + CO₂ (NO₃ cancels as an intermediate). ✅
⚠️ Key Point
Intermediates (like NO₃ above) are produced in an early step and consumed later. They never appear in the overall equation or the final rate law.
📋 Exam Tip
When asked to "propose a mechanism consistent with the rate law": (1) Write the slow step so its molecularity matches the rate law orders, (2) Add fast step(s) that combine with Step 1 to give the overall equation, (3) Verify intermediates cancel.
Think About It
Why can't we determine the mechanism from the balanced equation alone?
Because multiple mechanisms could produce the same overall equation. The rate law, which can only be found experimentally, tells us which particles are involved in the slowest step, and this is the only way to distinguish between possible mechanisms.