Consider the following three-step representation of a reaction mechanism
Explanation
When the rate-determining step is not the first step, deriving the rate law for the overall reaction becomes more complex because the concentration of intermediates, such as AB in this case, is not necessarily directly proportional to the concentration of the reactants as they appear in the balanced equation of the overall reaction. Instead, the rate law for the overall reaction depends on the mechanism as a whole and how the concentration of intermediates is related to the concentration of reactants.
In summary, the rate-determining step dictates the rate at which the overall reaction can proceed, but the presence of a fast initiatory step can alter the relationship between reactant concentrations and the overall rate law.
Final answer
The rate law for the overall reaction differs from that of the rate-determining step because the initial fast equilibrium affects the concentration of intermediates which, in turn, affects the overall rate law.Which explains why the rate law for the overall equation is not the same as the rate equation for the rate-determining step?
When the rate-determining step is preceded by a step involving a rapidly reversible reaction, the rate law for the overall reaction may be more difficult to derive.