Page 189 - Modeling of Chemical Kinetics and Reactor Design

Reaction Rate Expression 159

Another useful technique in kinetic studies is the measurement of
the total pressure in an isothermal constant volume system. This
method is employed to follow the course of homogeneous gas phase
reactions that involve a change in the total number of gaseous molecules
present in the reaction system. An example is the hydrogenation of
an alkene over a catalyst (e.g., platinum, palladium, or nickel catalyst)
to yield an alkane:

Pt, Pd, or Ni catalyst
CH 2 n + H  → C H 2 n 2 (3-203)
+
n
2
n
Nickel is the least active of these catalysts and requires an elevated
temperature and pressure, whereas platinum and palladium function
adequately at ordinary temperatures and pressures. An example is
butylene to butane:

C H + H → C H (3-204)
4 8 2 4 10
In gaseous reactions, the composition term in the rate equation is
often expressed as the partial pressure of the reacting species. These
pressures are then transformed to concentration.
Consider the reaction A → products. The rate equation is:

− ( r ) = k p n = kC n (3-205)
A p A A
where

− ( r ) = mol  mol 
A 3  
•
m • s ls
k = mol  mol 
p
n
n
3
•
m • Pa • s  l atm • s 
p = Pa n ( atm )
n
n
A
k =  mol  1 − n s −1  mol  1 − n s −1
 m 3   ls • 
For ideal gases, the partial pressure is expressed as: