Theories of reaction rates Transition state theory and Collision theory ry

Theories of reactions

1. 1
Collision Theory
•Molecules must collide before they can react
•In a gas, molecules are traveling with different
velocities and have different kinetic energies. The
average kinetic energy is dependent on
temperature (Kinetic Theory of Gases)
•Colliding molecules must together have enough
kinetic energy to break existing bonds.
•The minimum kinetic energy required for a
reaction to occur is the activation energy,
symbol E
a

2. 2
Energy diagram

3. 3
Collision Theory
•Molecules must also collide with a
favorable orientation for reaction to occur
•The factors affecting the rate of reaction
are:
–Fraction of collisions which occur with
required orientation (p)
–Collision frequency (Z)
–Fraction of collisions with required energy (f)
•Most collisions do not result in reaction

4. 4
Effective vs. ineffective collisions

5. 5
Collision Theory and the Rate
Constant
•For elementary processes, where the rate
law expression is: Rate = k[A][B]
k = pxZxf
•It can be shown that the fraction of
collisions with sufficient energy is:
f = e
-Ea/RT
–R = gas constant
–T = temperature in Kelvin

6. 6
Collision Theory and the Rate
Constant
•Substitute into the collision theory equation
for the rate constant:
k = pxZxe
-Ea/RT
•The larger the activation energy, the smaller
the fraction of molecules having enough
energy to react, so the slower the rate

7. 7
Limitations of Collision Theory
•Collision theory does not provide any
prediction of p, the “steric factor”
•Calculated values for the rate constant are
usually too high compared with measured
values
•Measured activation energies are lower than
the energies of the bonds that have to be
broken in reactions

8. 8
Transition State Theory
•At the moment of collision, molecules with
enough energy and the right orientation
combine to form an activated complex
•The activated complex can either revert
back to reactants or decompose to products
•The exact nature of the activated complex is
difficult to determine
•Less energy is required to form an activated
complex than to beak bonds

9. 9
Energy diagram