Page 109 - Elements of Chemical Reaction Engineering 3rd Edition

Sec. :3.1 Basic Definitions 81

- IAO = kAc.40c& (E3-3.6)

Equation (E3-3.6) does not agree with Equation (E3-3.2) and therefore the rate law
given by Equation (E3-3.5) is not valid.
The one-half power in the rate law suggests that we might take the square
root of Equation (E3-3.4):

CDe
[KC2] is in units of - (E3-3.7)

Rearranging gives

CDe
112
c,, c,, - - 0 (E3-3.8)
=
Kc2
Using this new equilibrium constant, Kc, , we can formulate another suggestion for
thle reaction rate expression:

(E3-3.9)

Note that thiis expression satisfies both the thermodynamic relationship (see ithe def-
inition of Kc2) and reduces to the irreversible rate law when C, = 0. The Form of
thie irreversible rate law provides a big clue as to the form of the reversible reaction
rate expression.

3.1.5 Rlonelementary Rate Laws and Reactions

It is interesting to note that although the reaction orders correspond to the
stoichiometric coefficients for the reaction between hydrogen and iodine, the
rate expression for the reaction between hydrogen and another halogen, bro-
mine, is quite complex. This nonelementary reaction
H2+Br2 --+ 2HIBr

proceeds by a free-radical mechanism, and its reaction rate law is

(3-18)

Another reaction involving free radicals is the vapor-phase decomposition of
acetaldehyde:
CH3CHO CH,+CO

At a temperature of about 500"C, the order of the reaction is three-halves with
respect to acetaldehyde.