Molecularity and Order of Reaction Molecularity of an elementary reaction is the number of molecules involved in the reaction,and this has been found to have the values of one,two,or occasionally three. Note that the molecularity refers only to an elementary reaction. 30
30 • Molecularity and Order of Reaction • Molecularity of an elementary reaction is the number of molecules involved in the reaction, and this has been found to have the values of one, two, or occasionally three. • Note that the molecularity refers only to an elementary reaction
-4=CCB..CD a+b+...+d=n Where a,b,......d are not necessarily related to the stoichiometric coefficients.We call the powers to which the concentrations are raised the order of the reaction. ·Above reaction is ath order with respect to A bth order with respect to B ·nth order overall 31
31 • Where a,b,......d are not necessarily related to the stoichiometric coefficients. We call the powers to which the concentrations are raised the order of the reaction. • Above reaction is • ath order with respect to A • bth order with respect to B • nth order overall r k C C C a b ....... d n d D b B a − A = A ...... + + + =
·Rate constant k In a rate expression-=kCCB..C the dimensions of rate constant k for nth- order reaction are (time)(concendration)-" For a first order reaction,it becomes (time) 32
32 • Rate constant k • In a rate expression • the dimensions of rate constant k for nthorder reaction are • For a first order reaction, it becomes ...... d D b B a − rA = k CA C C ( ) ( ) 1 1 n time concendration − − ( ) 1 time −
Representation of an elementary reaction Any measure equivalent to concentration can be used to express a rate,for example: -r=kpip.Po Whatever measure we use leaves the order unchanged;but it will affect the rate constant k. 33
33 • Representation of an elementary reaction • Any measure equivalent to concentration can be used to express a rate, for example: • Whatever measure we use leaves the order unchanged; but it will affect the rate constant k . ...... d D b B a − rA = k PA P P
Elementary reactions are often represented by an equation showing both the molecularity and the rate constant. 。A reaction2Ak→2 R represents a bimolecular irreversible reaction with second-order rate constant k,implying that the rate of reaction is -r4 =FR=kCa But:A>R implying -r=r=k C 34
34 • Elementary reactions are often represented by an equation showing both the molecularity and the rate constant. • A reaction represents a bimolecular irreversible reaction with second-order rate constant k1 , implying that the rate of reaction is 2A 2R 1 ⎯⎯k → A R A k But : A ⎯1 →R implying : − r = r = k1 C 2 A R 1 CA − r = r = k