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

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.

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.