Page 440 - Elements of Chemical Reaction Engineering Ebook

Chap. 7 Questions and Problems 41 1
H20+0* + 20H. (P7-3.2)
COJrOH. CO2+H. (P7-3.3)

H*+02 + OH*+O* (P7-3.4)
The last two reactions are rapid compared to the first two. When HCl is intro-
duced to1 the flame, the following additional reactions occur:
H2 + C1.
'H. + HC1 4
IH. -t- C1. HC1
Assume that all reactions are elementary and that the PSSH holds for the O.,
OH., and C1. radicals.
(a) Derive a rate law for the consumption of CO when no retardant is present.
(b) Derive an equation for the concentration of H. as a function of time
assuming constant concentration of 02, CO, and H20 for both uninhib-
ited combustion and combustion with HCl present. Sketch He versus
time for both cases.
(c) Sketch a reaction pathway diagram for this reaction.
More elaborate forms of this problem can be found in Chapter 6, where the
PSSH is not invoked.
P74A The pyrolysis of acetaldehyde is believed to take place according to the fol-
lowing sequence:

CH3CHO '' > CH3. + CHO.
CH3. + CH3CHO k2 > CH3. + CO + CH4
CHO- + CH3CHO " > CH3. + 2CO + Hz

~cH~. k4 ) c2 H6
(a) Derive the rate expression for the rate of disappearance of acetaldehyde,
- J-AC.
(b) Under what conditions does it reduce to Equation (7-2)?
(c) Sketch a reaction pathway diagram for this reaction.
W-5B The gas-phase homogeneous oxidation of nitrogen monoxide (NO) to dioxide
(NO,),
2N0 +02 2N02
is known to have a form of third-order kinetics which suggests that the reac-
tion is elementary as written, at least for low partial pressures of the nitrogen
oxides. However, the rate constant k actually decreases with increasing abso-
lute temperature, indicating an apparently negative activation energy. Because
the activation energy of any elementary reaction must be positive, some
explanation is in order.
Provide an explanation, starting from the fact that an active intemiedi-
ate species, NOS, is a participant in some other known reactions that involve
oxides of nitrogen.
W-6, For the decomposition of ozone in an inert gas M, the rate expression is

Suggest a mechanism.