What you'll be able to do: Evaluate a proposed mechanism against the overall equation and the experimental rate law using the rate-determining step.
Best after: Rate Laws and Reaction Order from Initial Rates, Collision Theory and Activation Energy
Most reactions do not happen in one collision. A mechanism is the sequence of single-collision events that adds up to the overall equation, and the slowest of those events sets the pace for everything.
These are recommended, not required. You can start this lesson at any time.
An elementary step is a single molecular event with no hidden stages. Because it is exactly what the equation says, its rate law can be written directly from its coefficients. This is the only situation in kinetics where coefficients become exponents.
Unimolecular means one particle rearranges or breaks apart, giving rate = k[A]. Bimolecular means two particles collide, giving rate = k[A][B] or k[A]². Termolecular steps require three particles to meet simultaneously and are rare because such a triple collision is very improbable.
An intermediate is produced in one step and consumed in a later one, so it cancels when the steps are added and never appears in the overall equation. A catalyst is the reverse: consumed early and regenerated later, so it appears at the start and is returned at the end.
If one step is much slower than the others, it acts as a bottleneck. The overall rate cannot exceed the rate of that step, so the experimental rate law matches the rate law of the slow step. When the slow step is first, the prediction is immediate because only reactants appear in it.
Two tests must both pass. First, the steps must sum to the overall balanced equation with intermediates cancelling. Second, the rate law predicted by the rate-determining step must match the experimental rate law. Passing both makes a mechanism plausible, never proven, because another mechanism might make the same predictions.
A single molecular event whose rate law follows directly from its own coefficients.
The number of particles taking part in one elementary step.
A species produced in one step and consumed in a later step; it never appears in the overall equation.
When one step is much slower, the overall rate law is that step's rate law.
Consistency with the data supports a mechanism but cannot prove it uniquely.
aA + bB → products gives rate = k[A]ᵃ[B]ᵇ
A proposed mechanism is: Step 1 (slow) NO₂ + NO₂ → NO₃ + NO; Step 2 (fast) NO₃ + CO → NO₂ + CO₂. Find the overall equation, the intermediate, and the predicted rate law.
Try it first: Add the two steps and cross out anything appearing on both sides.
0 of 4 steps revealed.
A reaction A + 2 B → C is found experimentally to be rate = k[A][B]. Can the overall equation be a single elementary step?
0 of 3 steps revealed.
Classify each step by molecularity: (i) O₃ → O₂ + O, (ii) O + O₃ → 2 O₂.
0 of 2 steps revealed.
Why it's wrong: Only elementary steps allow coefficients to be used as exponents.
Check instead: Use the rate-determining step, or experimental data.
Why it's wrong: Intermediates cancel because they are produced and then consumed.
Check instead: Add the steps and cancel species appearing on both sides.
Why it's wrong: Any step can be the slow one; the problem must state or imply which.
Check instead: Locate the step labelled slow before writing the rate law.
Why it's wrong: Different mechanisms can predict identical rate laws.
Check instead: Describe a consistent mechanism as supported rather than proven.
No practice questions are available for this topic yet. You can still practice the whole unit.
An elementary step describes what happens in a single molecular event, so for that step alone the rate law can be written from its coefficients. Molecularity counts the reactant particles in the step: unimolecular, bimolecular, and rarely termolecular. Adding all steps must reproduce the overall balanced equation with intermediates cancelling. When the first step is rate-determining, the rate law of that step is the rate law of the reaction, and a mechanism is only acceptable if it matches both the overall equation and the experimentally measured rate law.
Chemistry 2e, OpenStax, Rice University, licensed CC BY 4.0. License