10) Compare and contrast unimolecular reactions, bimolecular recombination reactions
and ligand exchange reactions. Be sure to consider kinetics, collision times, key
processes that determine activation barriers and rates.
Basic reaction
Energy/momentum
barriers to reaction
Main cause of
activation barrier
Collision times
Preexponentials
determined by
Kinetics
Typical
preexponential size
Typical activation
barrier
Unimolecular
A+XB+X
Recombination
A + B + X  AB +
X
Accumulating
Removing energy
energy in reactants
and momentum
from products
Barrier to bond
Quantum effects
stretching/ distortion (often change
electronic config)
Hot complex may
last 10-9 to 10-11 sec
i) Number of modes
to accumulate
energy, ii)
efficiency of heat
transfer into the
bond that needs to
break
Hot complex may
last 10-9 to 10-11 sec
i) Number of modes
to hold energy of
reaction, ii)
efficiency of heat
transfer to the
surroundings, iii)
angular momentum
barrier to reaction
Looks second order
at low pressure first
order at high
pressure
1015 Ǻ3/moleculesec
varies
Looks third order at
low pressure second
order at high
pressure
1015 Ǻ6/molecule2sec
0-3 kcal/mole
Ligand exchange
A + BC  AB + C
Transfer energy and
momentum from A
to C
Pauli repulsions
associated with
bringing reactants
close enough to
react
10-12 to 10-13 sec
i) Collision rate, ii)
ease that energy and
momentum
transferred from
bond that breaks to
bond that forms,
iii) angular
momentum barrier
to reaction
True second order
reaction
1013 Ǻ3/moleculesec
Varies