18.4 Kinetics-Rxn Mechanism and Rate-2020 (1).pdf -...

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Chapter18:ChemicalKinetics
18.1RatesofChemicalReactions
18.2RateLaws
18.3ReactionMechanisms
18.4ReactionMechanismsandRate
18.5EffectofTemperatureonReactionRates
18.6 MolecularTheoriesofElementaryReactions
18.7ReactionsinSolution
18.8CatalysisCumulativeExercise:SulfiteandSulfateKineticsinAtmosphericChemistry
Contents
Chapter18.4:
ReactionMechanismsandRate
Summary
18.4&18.4ReactionMechanismsandRate
Contents
ReactionMechanismandRate
• Tomakeaconnectionofthemacroscopically
determinedreactionrate( experiment )totheration
inthemolecularlevelreactionrate
• Ifthereactionisonlya elementarystep (
molecular
level
),then
aA + bB cC + dD 
m Rate = k[A] [B] m a, n b n ,  
m Rate = k[A] [B] m a, n b n ,  
ReactionMechanismandRate
• Forexample,theelementarysteps
• Foramacroscopicreaction,breakintomany
elementarysteps
A products, Rate = k[A] 
2 A + A products, Rate = k[A] 
A + B products, Rate = k[A][B] 
1 2
3 4

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ReactionMechanismandRate
• Inmanyreactionmechanisms,onestepis
significantlyslowerthanalltheothers:
– Thatslowstepisthe rate‐determiningstep
– Themostimportantstepofall
– Alwaysstartwiththatstepforanalysis
ReactionMechanismandRate
• Ifthe rate‐determiningstepisthefirstone ,the
analysisisparticularly simple ,e.g.
• Apossiblemechanism
• Theoverallrate( fromslowstep#1 )
     
2 2 2
2NO g + F g 2NO F g 
 
obs 2 2
The experiment observed rate = k NO F [ ]
         
       
1
k
2 2
k
2 2
NO g + F g NOF g + F g slow
NO g + F g NO F g fas t


2
 
1 2 2
rate = k NO F [ ]
ReactionMechanismandRate
• Therate‐determiningstepisnotthefirststep,i.e. the
firststepisnottheslowstep
– Couldbeareactionof ordergreaterthan2 ,
– Couldbean inverseconcentrationdependence ononeof
thespecies
– Couldbe anon‐integral reactionorder
• Forexample
• theexperimentalratelawis
     
2 2
2NO g + O g 2NO g 
obs 2 2
[N rate = k O ] [ ] O
2
ReactionMechanismandRate: ExampleI
• Forexample
• theexperimentalratelawis
• Whatistheratelawfromaproposedmechanism?