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:

(B) and (C) only

The correct answer is Option (2) → (B) and (C) only

Molecularity refers to the number of reacting species (atoms, ions, or molecules) that collide simultaneously in a single elementary step of a reaction.

Important characteristics of molecularity:

• It is defined only for elementary reactions.
• It is always a whole number (1, 2, or 3).
• It cannot be zero or fractional.
• Reactions with molecularity greater than three are extremely rare.

Analysis of the statements

(A) Molecularity is equal to the number of reacting species involved in the overall reaction

Molecularity refers to the number of reacting species in an elementary step, not necessarily in the overall reaction. Therefore this statement is incorrect.

(B) Decomposition of ammonium nitrite is an example of unimolecular reaction

Decomposition reaction:

NH₄NO₂ → N₂ + 2H₂O

Only one molecule decomposes, so the molecularity is one (unimolecular). Hence this statement is correct.

(C) Reactions with the molecularity three are very rare

Termolecular reactions require simultaneous collision of three molecules, which is statistically unlikely. Therefore such reactions are very rare. This statement is correct.

(D) Molecularity can only be determined experimentally

This statement describes order of reaction, not molecularity. Molecularity can be determined from the mechanism of the elementary reaction, not purely from experimental data. Therefore this statement is incorrect.