However, 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 way:

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 rate law is

(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 this chain 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.