Page 174 - Instant notes

Physical chemistry 160

exponent, or sum of exponents, equals two the reaction is described as second order with
respect to that species, or second order overall, respectively, and so on. Both the rate
laws:
2
rate=k 1[A]
and
rate=k 2[A][B]

are second order overall, but whereas the first rate law is second order in species A only,
the second rate law is first order in each of species A and B.
If a reactant species appears in the balanced chemical equation for the reaction but
does not appear in the rate law then the reaction is zero order with respect to that
species. Zero order terms are not usually written in rate law equations since the
concentration of any species to the power zero is just unity. For example, the rate law for
the aqueous phase iodination of propanone:
+
+
I 2+H +CH 3COCH 3→CH 2ICOCH 3+HI+H
is:
+
rate=k[H ][CH 3COCH 3]
The reaction is therefore zeroth order in iodine concentration, first order in each of the
hydrogen ion and propanone concentrations, and second order overall.
The exponents do not have to be integers, and for rate laws not of the general form
β
α
[A] [B] …the order is not a definable quantity. The rate law for the formation of HBr
from H 2 and Br 2 is:

so the reaction is first order with respect to H 2 concentration, but has an indefinite order
with respect to both Br 2 and HBr concentrations and an indefinite order overall (see
Topic F6).

Molecularity

The molecularity of a reaction is the number of species which come together in the
reaction. The complex rate law for the H 2+Br 2 reaction indicates that the reaction does
not proceed through a single step collision between undissociated hydrogen and bromine
molecules, but consists of several separate elementary reactions. One of these is the
reaction between hydrogen atoms and bromine molecules:
H+Br 2→HBr+Br