Page 62 - Basic physical chemistry for the atmospheric sciences

48 Basic physical chemistry

- d[A]
dt = k[A][B]
For the special case of a bimolecular reaction

A
A + � products
that s ,
i
2A� products
- d[A]
= k[A]2
dt
We conclude that the order f o r each reactant in a single-step ( ele­
mentary) process is equal to the coefficient o f that reactant in the
chemical equation f o r that process, and, f o r an elementary process,
the overall order is the same as the molecularity (i.e . , a u n imolecular
c
i
process s first order, a bimolecular process is second order, et . ) The
converse does not hold ; that i s , not all first-order chemical reactions
e
are unimolecular elementary process s , etc.
How is the order and rate of an overall chemical reaction related to
the orders and rates of the elementary processes that comprise the
reaction? The answer is simple for most reaction . Since the overall
s
reaction can be no faster than its slowest step (called the rate-de­
termining step), the rate law for the overall reaction is closely related
to the rate law for this step.
A common method for determining a reaction mechanism is first to
determine the rate law experimentall y , and then to postulate one or
more elementary processes that are consistent with the overall rate
law. The following two problems illustrate the general approach.
Exercise 3 .2. The following mechanism has been proposed for the
formation of N20 (g) from NOz(g) and Oig) in the gas phase within
5
clouds
(i)

(ii)
(a) Write down the overall chemical reaction. (b) What is the interme­
diate? (c) What is the rate law for each step? (d) If the experimentally
determined rate law for the overall chemical reaction is