Choose the correct statement.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Reactions with molecularity three are very rare.
The molecularity of an elementary reaction is the number of reacting species that must collide at the same time for the reaction to occur. A reaction needing three species to collide together, such as \( \ce{2NO + O2 -> 2NO2} \), is termolecular. The chance of three molecules colliding at the same instant is very small, so reactions with molecularity three are very rare and slow; molecularity above three is not observed.
Why the other options are incorrect
Option (2) Order of reaction cannot have a fraction value. — Incorrect. Order is the sum of the powers in the experimental rate law, and it can be a fraction or zero, e.g. rate = k[A]1/2[B] is of order 3/2. It is molecularity that is always a whole number.
Option (3) Molecularity of a reaction is meaningful for complex reactions only. — Incorrect. It is the reverse: molecularity applies only to elementary reactions. A complex reaction goes through several steps, and only each step has its own molecularity.
Option (4) Order of a reaction is always determined theoretically. — Incorrect. Order cannot be read from the balanced equation; it is determined experimentally, by measuring how the rate changes with concentration.
NCERT (2026-27 edition):
Molecularity of a Reaction – …
