Bimolecular reaction

• A. Involves three reactant molecules.
• B. Occurs in a single elementary step.
• C. Involves two reactant species.
• D. Involves the decomposition of a single particle.

Answer: (C)

Bimolecular means two reactant species must come together to react. In practice, the rate of such a reaction depends on the concentrations of two reactants (for example, A and B with rate = k[A][B]), or on the square of a single reactant’s concentration if two identical molecules collide (rate = k[A]^2). This is why the description involving two reactant species is the best fit.

The other ideas aren’t defining features: involving three reactants would be termolecular, which is a different category; a unimolecular process involves decomposition or rearrangement of a single molecule, and while a bimolecular reaction can occur in a single elementary step, that is not what defines it.