Which of the following best illustrates a bimolecular elementary step?

• A. A + B -> Products
• B. A -> Products
• C. A + B + C -> Products
• D. 2A -> Products

Answer: (A)

A bimolecular elementary step involves exactly two molecules colliding and reacting in one discrete event, so the rate law depends on both reacting species. When the step is A plus B forming products, the rate is proportional to the product [A][B], which cleanly shows two different molecules coming together to react in a single step. A unimolecular step, like A turning into products, depends on a single molecule and gives a rate proportional to [A]. A step with three reactants, A plus B plus C, would be a termolecular process, which is rare and usually not an elementary single step, with rate proportional to [A][B][C]. Although two A molecules colliding, 2A, is also a two-molecule collision and thus bimolecular, it involves identical reactants and is represented by a rate law like rate = k[A]^2, which is less straightforward to illustrate the concept of two different species colliding. Therefore, the example with A plus B best illustrates a bimolecular elementary step.