How does the rate determining step affect the observed rate law?

Question

Caleb Adams · Accepted Answer

For the concentration of a reactant to appear in the rate law equation, that substance must appear in the reaction mechanism during or prior to the rate determining step.

The ability of a reaction mechanism to account for the rate law is one of the key requirements for deeming it acceptable; in other words, the mechanism must demonstrate a correlation between the coefficients of the reactants that show up in the mechanism and the exponents in the rate law. For illustration purposes, let's say that the reaction's mechanism was #A+B rarr AB# #AB + C rarr D+E# The reaction rate is independent of [C], so if the rate law is rate=k[A][B], we anticipate that the first step is the rate-determining step. Conversely, we anticipate that the second step will be the rate-determining step if the rate law is rate=k[A]{B]{C]. To be clear, we know the rate law because of experimentation; it is what it is, and chemists have to work to piece together a mechanism that makes sense of the rate law, including deciding which step determines the rate. In other words, the rate law dictates the kind of mechanism that we think is feasible, not the other way around!

Cameron Andrews · Answer

undefined The rate-determining step directly influences the form of the observed rate law. It is the slowest step in a reaction mechanism and determines the overall rate of the reaction. The coefficients of the reactants in the rate-determining step become the exponents in the rate law equation. Therefore, the rate law is directly proportional to the concentration of the reactants involved in the rate-determining step.