Summarizing Reaction Mechanisms

Key Concepts and Summary

The sequence of individual steps, or elementary reactions, by which reactants are converted into products during the course of a reaction is called the reaction mechanism. The overall rate of a reaction is determined by the rate of the slowest step, called the rate-determining step. Unimolecular elementary reactions have first-order rate laws, while bimolecular elementary reactions have second-order rate laws. By comparing the rate laws derived from a reaction mechanism to that determined experimentally, the mechanism may be deemed either incorrect or plausible.

Glossary

bimolecular reaction

elementary reaction involving the collision and combination of two reactant species

elementary reaction

reaction that takes place precisely as depicted in its chemical equation

intermediate

molecule or ion produced in one step of a reaction mechanism and consumed in another

molecularity

number of reactant species (atoms, molecules or ions) involved in an elementary reaction

rate-determining step

(also, rate-limiting step) slowest elementary reaction in a reaction mechanism; determines the rate of the overall reaction

reaction mechanism

stepwise sequence of elementary reactions by which a chemical change takes place

termolecular reaction

elementary reaction involving the simultaneous collision and combination of three reactant species

unimolecular reaction

elementary reaction involving the rearrangement of a single reactant species to produce one or more molecules of product