molecular reaction dynamics. collision theory of kinetics with few exceptions, the reaction rate...
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Molecular Reaction Dynamics
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Collision Theory of KineticsWith few exceptions, the reaction rate
increases with increasing temperature temperature
If we assume that a chemical reaction takes place due to collisions between reactant molecules
i.e. rate number of collisions / unit time
A2 + B2 product rate = k2[A2][B2]
“collision of A2 and B2 causes a reaction”
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Rate constant vs. T
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The Reaction Profile
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The Activation EnergyThe minimum amount
of energy need for initiation of a chemical reaction is the activation energy (Ea).
Colliding reactant molecules possess kinetic energy > the activation energy or Ea.
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The Activated ComplexActivated Complex
the species temporarily formed by the reactant molecules as a result of collisions
A small fraction of molecules usually have the required kinetic energy to get to the transition state the concentration of the activated complex is
extremely small.
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The Arrhenius EquationArrhenius showed how the rate constant
depended on temperature.
/RT A - E k alnln
The frequency factor
The activation energy
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The Arrhenius Equation
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Estimating Rate ConstantsWe can use collision theory to provide a
basis estimating the rate constant.What if all collisions gave products?
AB
2d Zdt
AdN
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The Energy RequirementSince we know only a fraction of collisions
will give products
RTE
ABAB2d
a
eZfZdt
AdN
RTaE
ef
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Converting to Molar Amounts RT
E
ABAB2d
a
eZfZdt
AdN
V
AnNVAN
AN 2Avo22d
RTE
Avo
AB2a
eNZ
dtAd
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2d2d2
AB
21
ABAB BNANd
RT8Z
22RT
E
Avo2
AB
21
AB
2 BAeNdRT8
dtAd a
RTE
Avo2
AB
21
AB2
a
eNdRT8
k
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The Geometrical RequirementThere are many collisions of sufficient energy
that do not yield productsDefine the steric factor P to account for local
properties of the moleculeOrientations during collisionsSubstituent effects
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Effective Collisions
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Ineffective Collisions
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The Steric Factor and k2
RTE
Avo2
AB
21
AB2
a
eNdRT8
Pk
Note values of P can be quite small Usually in the range < 0.001
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Limitations of Collision TheorySimple Collision Theory
Best suited for studying reactions between simple species (atoms, diatomic molecules).
P factor indicates how reactants collide becomes very important when the species get bigger.