Which expression correctly represents the rate law of an elementary step that involves three molecular entities A, B, and C?

• A. rate = k [A][B]
• B. rate = k [A]^2 [B]
• C. rate = k [A]
• D. rate = k [A][B][C]

Answer: (D)

In an elementary reaction, the rate law mirrors the molecularity of the step—the number of molecules that must collide at once. If three different molecules A, B, and C must come together in a single collision to react, the rate is proportional to the product of their concentrations. That gives rate = k [A][B][C]. Each reactant contributes to the collision frequency, so you multiply their concentrations, one factor for each molecule involved.

The other forms would describe steps involving fewer colliding bodies: a two-body collision would yield rate = k [A][B], and a unimolecular rearrangement would yield rate = k [A]. For a three-molecule elementary step, the three-factor product correctly captures the dependence on all three reactants. The rate constant k in this case has units that make the overall rate have units of concentration per time.