Termolecular steps are rare in elementary reactions because

• A. Collisions among three reactant molecules are low probability.
• B. They involve high activation energy.
• C. They never occur.
• D. They are always rate-limiting.

Answer: (A)

The main idea is that three-body collisions are incredibly unlikely. For a reaction to happen in one elementary step, three molecules must meet at the same place at the same time with the correct relative orientations and enough energy to react. This simultaneous meeting is far less probable than a simple two-body collision, so an elementary termolecular step has a very small probability and a correspondingly tiny rate constant. In practice, this makes true termolecular elementary steps exceedingly rare, and what we often see instead are fast bimolecular steps that form an intermediate, which then reacts with another molecule. Activation energy isn’t the defining factor here, and termolecular steps do occur only very infrequently, rather than always being required or always controlling the rate.