Rate Constant Units

Question

What are the units of the rate constant for a reaction that is found experimentally to be second order overall?

RankGuru · Accepted Answer

The units of a rate constant depend on the overall order because the rate always has units of concentration per time, mol L^-1 s^-1, regardless of order. For a reaction of order n, the general unit of k is (mol L^-1)^(1-n) s^-1. For a second order reaction n = 2, so the units become (mol L^-1)^(1-2) s^-1 = (mol L^-1)^-1 s^-1 = L mol^-1 s^-1. The option s^-1 corresponds to a first order reaction. The option mol L^-1 s^-1 corresponds to a zero order reaction, where k equals the rate itself. The option L^2 mol^-2 s^-1 corresponds to a third order reaction. Deriving rate-constant units from the order is a standard NCERT skill and a quick check on a proposed rate law. Plausibility check: substituting L mol^-1 s^-1 multiplied by (mol L^-1)^2 from the rate law gives back mol L^-1 s^-1 for the rate, confirming the units are consistent for second order.

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