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Reaction Mechanisms - PowerPoint PPT Presentation


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Reaction Mechanisms. Rxn Mechanism. The series of steps that chemical reactions occur by main points of kinetics is to learn these steps. NO 2(g) +CO (g) -->NO (g) +CO 2(g). Rate = k[NO 2 ] 2 Tells us about reactants and products and stoichiometry, but nothing about mechanism.

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rxn mechanism
Rxn Mechanism
  • The series of steps that chemical reactions occur by
  • main points of kinetics is to learn these steps
no 2 g co g no g co 2 g
NO2(g)+CO(g)-->NO(g)+CO2(g)
  • Rate = k[NO2]2
  • Tells us about reactants and products and stoichiometry, but nothing about mechanism
no 2 g co g no g co 2 g1
NO2(g)+CO(g)-->NO(g)+CO2(g)
  • Mechanism thought to be:
  • NO2(g)+NO2(g)-->NO3(g)+ NO(g)
  • NO3(g)+CO(g)-->NO2(g)+CO2(g)
  • Two new rate constants k1, k2
no 3 g
NO3(g) ???
  • INTERMEDIATE- species that is neither a reactant or a product, but is formed and used during the rxn sequence
elementary steps
Elementary Steps
  • Reactions whose rate law can be written from its molecularity
molecularity
Molecularity
  • Number of species that must collide to produce the reaction indicated by that step
unimolecular
Unimolecular
  • Rxn involving one molecule
  • A-->prod
  • Rate = k[A]
bimolecular
Bimolecular
  • Involving two molecules
  • A + A --> prod
  • A + B --> prod
  • Rate = k[A]2
  • Rate = k[A][B]
termolecular
Termolecular
  • Involving 3 molecules
  • rare- hard for 3 to collide simultaneously
  • A + B + C --> prod
  • Rate = k[A][B][C]
rxn mech precisely
Rxn mech- precisely
  • 1. The sum of elementary steps must give overall balanced eqn for rxn.
rxn mech precisely1
Rxn mech- precisely
  • 2. The mechanism must agree with the experimentally determined rate law
question
Question
  • Does our mechanism balance?
  • YES
rate determining step
Rate-determining step
  • The SLOW step in a rxn
  • Reactants can only become products as fast as their slow step will allow
back to example
Back to example
  • Assume that the first rxn is the SLOW step
  • Formation of NO3 occurs more slowly than its reaction with CO
therefore
Therefore...
  • The rate of CO2 production is controlled by the rate of formation of NO3
since
Since...
  • It is an elementary step, we can write the rate law from the molecularity
  • Rate = k[NO2]2
and look
And look!
  • The overall rate can be no faster than the slowest step
  • Overall rate = k[NO2]2 !
finding mechanism
Finding mechanism
  • Always find rate law first
  • Then use chemical intuition and try to make mechanism
but remember
But Remember
  • Mechanism can NEVER be proved absolutely
2no 2 g f 2 g 2no 2 f g
2NO2(g) + F2(g)  2NO2F(g)

Rate Law = k[NO2][F2]

Mechanism

NO2 + F2 NO2F + F slow

F + NO2  NO2F fast