What is the rate law for a reaction that is first order in A and zero order in B?

• A. Rate = k[A]
• B. Rate = k[A]^0
• C. Rate = k[B]
• D. Rate = k[A][B]

Answer: (A)

In rate laws, each reactant contributes according to its reaction order: a first-order reactant makes the rate proportional to its concentration, while a zero-order reactant means the rate is independent of its concentration. Since the reaction is first order in A, the rate is proportional to [A]. Since it is zero order in B, the rate does not depend on [B], so B^0 = 1 does not affect the rate. Putting this together gives Rate = k[A]^1[B]^0, which simplifies to Rate = k[A]. The overall reaction order is 1. The other forms would imply different dependencies: no dependence on A, dependence on B, or dependence on both A and B.