1. Modified by Jed Macosko
Reaction Dynamics
Potential Energy
Modified by Jed Macosko
1/2/2019

2. Collision Theory 1
In the simple ________ __________ model of molecular collisions, the impact parameter (distance of closest approach) is the _______ of the radii of the collision pair. rA rB
d = rA+rB
Collision __________
area s
collision volume swept by A per sec.
Collision __________
collision frequency per A molecule
number of collisions per unit time per unit volume
= (m-3) (m-3) (m2) (m s-1)  m-3 s-1
Modified by Jed Macosko
1/2/2019

3. Modified by Jed Macosko
Collision Theory 2
_______
where
is the __________ mass.
__________ Energy: Only a fraction of collisions have sufficient kinetic energy to overcome the activation barrier.
molecules m-3 s-1
__________ Rate Constant
mol m-3 s-1
L = Avogadro’s no.
mol m-3 s-1
mol m-3 s-1
mol dm-3 s-1
mol-1 dm3 s-1
M-1 s-1
Modified by Jed Macosko
1/2/2019

4. Modified by Jed Macosko
Collision Theory 3
Basic collision theory has several deficiencies, which can be partially overcome by making more sophisticated models.
__________ Dependence The reactions of polyatomic molecules typically depend on their mutual orientation.
Solution: Replace s with s*, the __________ cross-section:
steric factor
__________ Interactions Molecules are not incompressible hard spheres!
rij u
U(r)
Hard sphere
Square well
__________
The collision energy depends on the impact parameter, which is angular dependent.
There is no way to predict the activation energy.
Modified by Jed Macosko
1/2/2019

5. Potential Energy Surfaces 1
A reaction ________ is a plot of the energy of a reaction system as a function of all the independent variables (bond ________ and bond _______). Even a collinear triatomic reaction such as
A + BC  AB + C
needs a 3-D plot:
rA-B
A + BC
AB + C E
rB-C
rA-B
or a contour plot:
_________vibration of A-B-C
minimum energy path
rB-C
Modified by Jed Macosko
1/2/2019

6. Potential Energy Surfaces 2
Modified by Jed Macosko
1/2/2019

7. __________ State Theory (TST)
Assume that in the course of a reaction there is some __________ surface (point on a one-dimensional reaction path) past which reaction to products is inevitable.
Assume that this __________ state is in effective equilibrium with the reactants.
reactants
products U
reaction coordinate kp K‡
A full discussion of kp and K‡ requires quantum chemistry and statistical mechanics, and leads to the
________ Equation
transmission coefficient or tunneling factor
Modified by Jed Macosko
1/2/2019

8. Thermodynamic Formulation of TST
For gases,
where
Substitute into the Eyring Equation:
“hide” k in the DS factor
Since
(for gases only)
Modified by Jed Macosko
1/2/2019