the rate of the forward reaction
when little or no HBr is present is observed to depend on H2
and Br2 concentrations in the following
The rate of production of HBr with time is proportional to the product
of the H2 concentration and the square
root of the Br2 concentration.
Under these conditions, the reaction has
an overall order
of 3/2. After appreciable amounts of HBr have accumulated, the overall
(Notice that this expression reduces to
the simpler form when the ratio [HBr]/[Br2]
is close to zero.) When the rate law becomes this complex, the concept
of reaction order begins to lose meaning.
Such a complex rate behavior occurs because the actual reaction
mechanism is a series of steps, one after the other. We will examine
reaction later on. If the mechanism were simply a collision of
H2 and Br2 molecules,
the rate law would be
The fact that this is observed by experiment to be the wrong rate
law tells us that the simple reaction mechanism also is wrong.