Page 157 - Introduction to chemical reaction engineering and kinetics

6.5 Transition State Theory (TST) 139

The formation of C,H, must first involve the formation of the “energized” molecule
C,H;:

which is followed by collisional deactivation:

C,HT, + M + C,H, + M

However, GHT, may convert to other possible sets of products:
(1) Redissociation to Ho and GHS:

C,H; + Ho + C2H;

(2) Dissociation into two methyl radicals:

(3) Formation of stable products:

C,H;, --$ H, + C,H,

The overall process for this last possibility

H’ + C,H; + [C,H,*] + H, + &H,

can be thought of as a bimolecular reaction with a stable molecule on the reaction co-
ordinate (C,H& as illustrated in Figure 6.5. The competition of these other processes
with the formation of ethane can substantially influence the overall rate of ethane de-
hydrogenation. These and similar reactions have a substantial influence in reactions at
low pressures and high temperatures.

6.5 TRANSITION STATE THEORY (TST)

6.51 General Features of the TST

While the collision theory of reactions is intuitive, and the calculation of encounter rates
is relatively straightforward, the calculation of the cross-sections, especially the steric
requirements, from such a dynamic model is difficult. A very different and less detailed
approach was begun in the 1930s that sidesteps some of the difficulties. Variously known
as absolute rate theory, activated complex theory, and transition state theory (TST), this
class of model ignores the rates at which molecules encounter each other, and instead
lets thermodynamic/statistical considerations predict how many combinations of reac-
tants are in the transition-state configuration under reaction conditions.
Consider three atomic species A, B, and C, and reaction represented by

AB+C+A+BC (6.51)

The TST considers this reaction to take place in the manner

Ki
AB+C=ABC$“i-A+BC (6.5-2)