What is the rate law for a zero-order reaction?

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

Answer: (A)

Zero-order reactions proceed at a constant rate that does not depend on the reactant concentration. The rate law is simply rate = k, where k is the rate constant. In terms of the concentration change, -d[A]/dt = k. Integrating gives [A] = [A]0 − kt, so the reactant concentration falls in a straight line with time. The rate constant k has units of concentration per time (for example M s^-1), matching the overall units of the rate. This contrasts with first- and second-order laws, which involve [A] and/or [A]^2, and with nonstandard forms like rate = k/t that don’t fit the typical rate-law framework.