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
Ch ical chemistr Reaction Kinetic Reaction mechanisms For the consecutive unimolecular reactions A-M>B-2>C Suppose now that k2 >>k1 Then whenever a b molecule is formed it decays rapidly into C kt <<已 k1 k k e e -k24 k2-k (1741) reduces to [C]=[4b-e kt The formation of c depends on only the smaller of the two rate constants A-k >b is called the rate-determining step of the reaction 12
12 Physical Chemistry 2 1 k k Reaction Mechanisms A B C k k ⎯⎯1→ ⎯⎯2→ k t k t e e − 2 − 1 2 1 2 k −k k Suppose now that Then whenever a B molecule is formed it decays rapidly into C. − + − = − −k t −k t o e k k k e k k k C A 1 2 2 1 1 2 1 2 [ ] [ ] 1 (17.41) reduces to ( ) k t o C A e 1 1 [ ] [ ] − = − The formation of C depends on only the smaller of the two rate constants A B k ⎯⎯1→ is called the rate-determining step of the reaction. For the consecutive unimolecular reactions Reaction Kinetics