The molecularity of the following elementary reaction is
\(NH_4NO_2 → N_2 + 2H_2O\)
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → one
The molecularity of an elementary reaction is the number of reacting species (atoms, ions or molecules) that must collide at the same time to bring about the reaction. In
\( \ce{NH4NO2 -> N2 + 2H2O} \)
only one species, ammonium nitrite, takes part, so the reaction is unimolecular; its molecularity is one.
Why the other options are incorrect
Option (1) zero — Incorrect. Molecularity counts the species that collide, so it is always a whole number of at least one; it cannot be zero.
Option (3) two — Incorrect. A bimolecular reaction needs two species to collide, e.g. the dissociation of hydrogen iodide, 2HI → \( \ce{H2} \) + \( \ce{I2} \).
Option (4) three — Incorrect. A trimolecular reaction needs three species to collide, e.g. 2NO + \( \ce{O2} \) → \( \ce{2NO2} \). The 2 in front of \( \ce{H2O} \) is a product coefficient and does not count.
NCERT (2026-27 edition):
Molecularity of a Reaction – …
