A balanced chemical equation tells us which substances are consumed and produced, but it does not necessarily describe the sequence of events that makes the reaction happen. This lesson introduces reaction mechanisms and the elementary steps that make them up.
A reaction mechanism, also called a reaction path, is a proposed sequence of elementary reactions that describes how an overall chemical change occurs. The overall balanced equation summarizes the net change; the mechanism provides a step-by-step account.
All flashcards
What is a reaction mechanism?
A proposed sequence of elementary reactions describing how an overall chemical change occurs.
What distinguishes an intermediate from a catalyst?
An intermediate is formed and later consumed; a catalyst is consumed and later regenerated.
What is molecularity?
The number of reactant entities participating in one elementary reaction.
Why are termolecular elementary steps uncommon?
Three particles are less likely to collide simultaneously under suitable conditions than two.
How can a fast equilibrium help derive an overall rate law?
It relates an intermediate concentration to reactant concentrations so the intermediate can be eliminated.
Quiz questions
What can a balanced overall chemical equation establish by itself?Answer: The net reactants and products, but not the reaction mechanism or rate law.
What must a proposed reaction mechanism account for to be consistent with experimental evidence?Answer: The observed rate law and the balanced overall reaction.
What does molecularity count in an elementary reaction?Answer: The reactant entities participating in that single step.
Why are termolecular elementary reactions less common than bimolecular ones?Answer: Three reactant particles must meet simultaneously under suitable conditions.
For a mechanism with a fast reversible step before a slow step, what can the fast-step equilibrium provide?Answer: A relationship for replacing an intermediate concentration with reactant concentrations.
Based on Chemistry 2e by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.