Ch ical chemistr Reaction Kinetic Reaction mechanisms The rate -Determining-Step approximation The reaction mechanism is assumed to consist of one or more reversible reactions that stay close to equilibrium during most of the reaction, followed by a relatively slow rate-determining step which in turn is followed by one or more rapid reactions The number of molecules that react in an elementary step The molecularity of the elementary reaction A→pos Unimolecular A+B→> products2A→> products bimolecular A+B+C→> products2A+B→> products3A→> products trimolecular(termolecular)11 Physical Chemistry Reaction Mechanisms A→ products The Rate-Determining-Step Approximation The reaction mechanism is assumed to consist of one or more reversible reactions that stay close to equilibrium during most of the reaction, followed by a relatively slow rate-determining step, which in turn is followed by one or more rapid reactions. A+ B → products 2A → products A+ B +C → products 2A+ B → products 3A → products unimolecular The number of molecules that react in an elementary step The molecularity of the elementary reaction bimolecular trimolecular (termolecular) Reaction Kinetics