The reaction \( \ce{2A + B -> A2B} \) takes place in the following two steps:
Step (1): \( \ce{A + B -> AB} \) (slow)
Step (2): \( \ce{AB + A -> A2B} \) (fast)
What is the overall order of the reaction?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 2
When a reaction takes place in steps, the rate of the whole reaction is controlled by the slowest step, the rate determining step. For such a complex reaction, the molecularity of the slowest step is the same as the order of the whole reaction.
The slow step (1) is a collision between one A and one B, so it is bimolecular.
The rate of the whole reaction follows this step:
\( \displaystyle \mathrm{Rate} = k[\mathrm{A}][\mathrm{B}] \)
The order is 1 + 1 = 2.
NCERT (2026-27 edition):
3.2.4 Molecularity of a Reaction – “Similarly, the overall rate of the reaction is controlled by the slowest step in a reaction called the rate determining step.”
“(iii) For complex reaction, order is given by the slowest step and molecularity of the slowest step is same as the order of the overall reaction.”