Choose the statements that are valid for the molecularity of a reaction
(A) Molecularity is equal to the number of reacting species involved in the overall reaction
(B) Decomposition of ammonium nitrite is an example of unimolecular reaction
(C) Reactions with the molecularity three are very rare
(D) Molecularity can only be determined experimentally
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (B) and (C) only
The molecularity of a reaction is the number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously to bring about the reaction. It is a theoretical idea that belongs to a single step; it is always a whole number (1, 2 or 3) and has no meaning for a complex reaction as a whole. The order, by contrast, is found by experiment.
(A) Molecularity is equal to the number of reacting species involved in the overall reaction — Incorrect. Molecularity refers to an elementary step, not to the whole balanced equation; a complex reaction with many molecules in its equation goes through several steps of molecularity one or two.
(B) Decomposition of ammonium nitrite is an example of unimolecular reaction — Correct. One species breaks up:
\( \displaystyle \ce{NH4NO2 -> N2 + 2H2O} \)
(C) Reactions with the molecularity three are very rare — Correct. The chance that three molecules collide at the same instant is very small, so termolecular reactions are rare and slow.
(D) Molecularity can only be determined experimentally — Incorrect. Molecularity is worked out from the proposed mechanism; it is the order of a reaction that is determined experimentally.
NCERT (2026-27 edition):
Molecularity of a Reaction
Molecularity of a Reaction – “… (i) Order of a reaction is an experimental quantity. It can be zero and even a fraction but molecularity cannot be zero or a non integer.”