Rate Determining Step
The slowest step controls the overall rate
Core Concept
Overall reaction rate = rate of slowest elementary step
Like a bottleneck: can't go faster than the narrowest point
Example Mechanism
Overall: 2NO₂ + F₂ → 2NO₂F
Step 1 (slow): NO₂ + F₂ → NO₂F + F
Rate₁ = k₁[NO₂][F₂]
Step 2 (fast): NO₂ + F → NO₂F
Rate₂ = k₂[NO₂][F]
Overall rate law: Rate = k₁[NO₂][F₂]
Determined by slow step only
Identifying Rate-Determining Step
- Has highest activation energy (Ea)
- Slowest elementary step in mechanism
- Determines rate law for overall reaction
- Species in slow step appear in rate law
Key Points
- If slow step is first: rate law uses reactants directly
- If slow step is later: may need pre-equilibrium approximation for intermediates
- Fast steps after slow step don't affect rate law
- Sum of elementary steps = overall balanced equation