Question

What is the rate limiting step in a chemical reaction?
A) Fastest step
B) Slowest step
C) Any step involved in a chemical reaction
D) None of the above

Answer 1

We have to remember that the rate limiting step can be studied by the concept of funnel as a fluid flowing through a funnel in which the rate of flowing fluid in a funnel depends on the flowing rate through the neck of the funnel and not on the speed of pouring fluid in funnel.

Complete answer:
We also know that the speed at which a chemical reaction takes place defines a rate for a chemical reaction. The rate limiting step is the slowest step in a chemical reaction. Slow step of a reaction therefore determines the rate of a reaction. You can look at this concept as a fluid flowing through a funnel in which the rate of flowing fluid in a funnel depends on the flowing rate through the neck of the funnel and not on the speed of pouring fluid in the funnel. There are many steps in a reaction but only the slowest step is considered to be the rate determining step.
How we can look at chemical reaction for rate determining step:
$NO + NO\overset {Faststep} \longleftrightarrow {N_2}{O_2}$
${N_2}{O_2} + {O_2}\xrightarrow{{slowstep}}2N{O_2}$
$Rate = K\left[ {{N_2}{O_2}} \right]\left[ {{O_2}} \right]$
Option A) this is an incorrect option as the fastest step does not determine the rate determining step.
Option B) This is a correct option as the slowest step is the rate determining step as seen in the above example given.
Option C) this is an incorrect option as any step cannot be rate determining step.
Option D) this is an incorrect option as we got option B as the correct answer.

Note:
We also remember that there cannot be more than one step which represents the rate. In several steps reaction it is only the slowest which represents the rate determining step. The slowest step determines the rate of the chemical reaction. As explained in the above example, rate is determined by the slowest step.