Chemistry · Unit 5 Kinetics · 14 min read · Updated 2026-05-11
Introduction to Reaction Mechanisms — AP Chemistry
AP Chemistry · Unit 5 Kinetics · 14 min read
1. Core Fundamentals of Reaction Mechanisms★☆☆☆☆⏱ 3 min
A reaction mechanism describes the step-by-step sequence of bond-breaking and bond-forming events that convert starting reactants to final products at the molecular level. Unlike the overall net reaction, which only shows starting and final species, mechanisms reveal the intermediate species that form and are consumed during the reaction.
2. Elementary Reactions and Molecularity★★☆☆☆⏱ 4 min
Molecularity is the number of reactant particles that participate in an elementary step. There are three common classifications:
Unimolecular: One reactant particle reacts, rate = $k[A]$, first order overall
Bimolecular: Two reactant particles collide and react, rate = $k[A][B]$ (or $k[A]^2$ for $2A$), second order overall
Termolecular: Three reactant particles collide simultaneously (very rare), rate = $k[A][B][C]$, third order overall
3. Reaction Intermediates and Catalysts★★☆☆☆⏱ 3 min
Two types of species do not appear in the final net reaction: reaction intermediates and catalysts. You must be able to distinguish between these two for AP exam questions.
To distinguish between the two, track the order of appearance: intermediate = product first, reactant second; catalyst = reactant first, product second.
4. Rate-Determining Step and Rate Law Derivation★★★★☆⏱ 6 min
The overall rate of a reaction mechanism is limited by the slowest step in the mechanism, called the **rate-determining step (RDS)**. The overall rate law is exactly equal to the rate law of the RDS.
If the RDS is the first step with no intermediates, you can write the rate law directly from the RDS stoichiometry. If the RDS comes after one or more fast reversible steps, the RDS will contain an intermediate from the fast step. You must substitute the intermediate concentration using the **pre-equilibrium approximation**: the fast step reaches equilibrium quickly, so the rate of the forward step equals the rate of the reverse step.
Why: Students confuse the rule for elementary reactions with overall reactions, and the rate law sometimes matches by coincidence, leading to incorrect assumptions
Why: Students only remember both are canceled from the net reaction, so they mix up the classification
Why: Students forget intermediates are not starting reactants, so their concentration depends on starting reactant concentrations
Why: Most introductory examples have RDS as the first step, so students assume this pattern always holds
Why: Students assume all steps contribute equally to the overall reaction rate