Elementary Steps and the Rate-Determining Step (RDS)
Most reactions proceed via a sequence of elementary steps called a reaction mechanism. The slowest step with the highest activation energy is the rate-determining step (RDS) and dictates overall kinetics. The molecularity of an elementary step is the number of reacting particles involved in that single 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
Verifying a Two-Step Reaction Mechanism
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). ✅
Reactive Intermediates vs Catalysts
Intermediates are generated in an early elementary step and consumed in a later step; they never appear in the overall balanced equation. In contrast, catalysts are present initially, react in an early step, and are regenerated in a later step.
Three Rules for Validating a Reaction Mechanism
- The sum of all elementary steps must equal the overall stoichiometric equation.
- The molecularity of the RDS (plus any prior fast equilibria) must match the experimentally observed reaction orders.
- All reaction intermediates must cancel completely out of the overall equation.
Thermodynamics vs Elementary Kinetic Steps
A stoichiometric equation gives only the initial state of reactants and final state of products (thermodynamic net change). It conveys nothing about the microsecond sequence of bond-breaking and bond-forming collisions along the reaction coordinate.
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