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4.3 Dependence of Rate on Concentration 75
We take advantage of the fact that pnO is constant for the first two experiments, and PA0
is constant for the first and third. Thus, from the first two and equation (l),
-1.6 kAp(12)Y34)~ 1 (y
- = k,,(36)"(34)P = 3
- 14.4
0
from which
cY=2
Similarly, from the first and third experiments,
p=1
(The overall order, 12, is therefore 3.) Substitution of these results into equation (1) for any
one of the three experiments gives
k = 3.27 X 10e4 kPaF2 mm’
AP
From equation 4.2-8,
kA = (RT)2kAp = (8.314)2(1073)23.27 X 10p4/60 = 434 L2 molp2 s-l
4.3.4 Other Orders of Reaction
From the point of view of obtaining the “best” values of kinetics parameters in the rate
law, equation 4.1-3, the value of the order can be whatever is obtained as a “best fit”
of experimental data, and hence need not be integral. There is theoretical justification
(Chapter 6) for the choice of integral values, but experiment sometimes indicates that
half-integral values are appropriate. For example, under certain conditions, the decom-
position of acetaldehyde is (3/2)-order. Similarly, the reaction between CO and Cl, to
form phosgene (COCl,) is (3/2)-order with respect to Cl, and first-order with respect
to CO. A zero-order reaction in which the rate is independent of concentration is not
observed for reaction in a single-phase fluid, but may occur in enzyme reactions, and in
the case of a gas reacting with a solid, possibly when the solid is a catalyst. The basis for
these is considered in Chapters 8 and 10.
4.3.5 Comparison of Orders of Reaction
In this section, we compare the effect of order of reaction n on cAIcAO = 1 - .& for
various conditions of reaction, using the model reaction
A + products (4
with rate law
(-I*) = kAcL (3.4-1)
We do this for isothermal constant-density conditions first in a BR or PFR, and then in
a CSTR. The reaction conditions are normalized by means of a dimensionless reaction
number MA,, defined by
