Page 63 - Basic physical chemistry for the atmospheric sciences

Chemical kinetics 49

d[NOz(g)]
- k[N02(g)][03(g)]
dt
what can be concluded about the relative speeds of steps (i) and (ii)?
Solution. (a) The overall chemical reaction is obtained by adding the
elementary processes and canceling the species that appear on both
sides of the equation. Hence, adding Reactions (i) and (ii), we get

or,

(b) Since N03(g) is formed in Reaction (i) and consumed in Reaction
(ii), it is an intermediate.
(c) Since Reaction (i) is a bimolecular proce s , its rate law is second
s
order and is given by

Similarly, for Reaction (ii)

(d) Comparing the rate law for the overall reaction

with the rate laws given in (c) for the two elementary processes, we
see that it is identical to the rate law for Reaction (i) . Hence, it is this
process that is rate controlling. Therefore, Reaction (i) proceeds at a
slower speed than Reaction (ii).
Exercise 3 .3. The reaction mechanism for
2
H2(g) + Br2(g)-7 H B r (g)
which is important in volcanic plumes is
(ia) l

(fast equilibrium)
(ib)