Page 102 - Elements of Chemical Reaction Engineering 3rd Edition

74 pate Laws and Stoichiometry Chap. 3

the kinetic rate law is

This reaction is second-order with respect to nitric oxide, first-order with
respect to oxygen, and overall is a third-order reaction. In general, first- and
second-order reactions are more commonly observed than zero- and third-
order reactions.
co+c1, -
The overall order of a reaction does not have to be an integer, nor does
the order have to be an integer with respect to any individual component. As
an example, consider the gas-phase synthesis of phosgene:

COC1,

in which the kinetic rate law is
- rco = kCCOCi:22

This reaction is first-order with respect to carbon monoxide, three-halves order
with respect to chlorine, and five-halves order overall.
Sometimes reactions have complex rate expressions that cannot be sepa-
rated into solely temperature-dependent and concentration-dependent portions.
In the decomposition of nitrous oxide over platinum,

2N,O & 2N2+0,

the kinetic rate law is

- ~N~OCN,O
-
‘”NZo - 1 + k‘ Coz

Both kNZO and k‘ are strongly temperature-dependent. When a rate expression
such as the one described above occurs, we can speak of reaction orders only
under certain limiting conditions. For example, .at very low concentrations of
oxygen, the second term in the denominator would be negligible (1 9 k’Co2)
and the reaction would be “apparent” first-order with respect to nitrous oxide
and first-order overall. However, if the concentration of oxygen were large
enough so that the number 1 in the denominator were insignificant in compar-
Apparent reaction ison with the second term, k‘Co2(k’Coz+ l), the apparent reaction order
orders
would be - 1 with respect to oxygen and 1 with respect to nitrous oxide. Rate
expressions of this type are very common for liquid and gaseous reactions pro-
moted by solid catalysts (see Chapter 10). They also occur occasionally in
homogeneous reaction systems (see Chapter 7).
The units of the specific reaction rate, kA, vary with the order of the
reaction. Consider a reaction involving only one reactant, such as
A - products