Page 136 - Introduction to chemical reaction engineering and kinetics

118 Chapter 6: Fundamentals of Reaction Rates

Bimolecular Steps:
l Bimolecular association/recombination-two species combine:

H,C’ + CH: -+ C,H,

l Bimolecular exchange reactions-atoms or group of atoms transferred:

OH’ + C,H, + H,O + C,H;

l Energy transfer-this is not actually a reaction; there is no change in bonding; but it
is nevertheless an important process involving another molecule M:

R*+M-+R+M*

The asterisk denotes an excited state-a molecule with excess energy (more than
enough energy to enable it to undergo a specific reaction step).

Termolecdar Steps:
l Termolecular steps are rare, but may appear to arise from two rapid bimolecular steps
in sequence.

6.2.1.2 Photochemical Elementary Reactions
Light energy (absorbed or emitted in a quantum or photon of energy, hu, where h is
Planck’s constant (6.626 x 1O-34 J s), and v is the frequency of the light, s-l) can change
the energy content of a molecule enough to produce chemical change.
l Absorption of light (photon):

Hg+hv-+Hg*

l Photodissociation:

l Photoionization (electron ejected from molecule):

CH, + hv -+ CH,+ + e-

l Light (photon) emission (reverse of absorption):

Ne* + Ne+hv

6.2.1.3 Elementary Reactions Involving Charged Particles (low, Electrons)
These reactions occur in plasmas, or other high-energy situations.
l Charge exchange:

X-+A+A-+X
M++A+M+A+