01The slow step rules
The rate-determining step (slowest) sets the rate law. If it's the first step, write its rate law directly.
If the slow step contains an intermediate, substitute using the fast equilibrium before it.
02Notebook box
Rate law must contain only reactants (and catalysts), never intermediates.
Worked example: Step 1 slow: NO₂ + NO₂ → NO₃ + NO → rate = k[NO₂]²; CO doesn't appear.