Under pseudo-first-order conditions, what does the rate law look like?

• A. Rate = k [A]
• B. Rate = k' [A]
• C. Rate = k' [B]
• D. Rate = k [A][B]

Answer: (B)

Pseudo-first-order conditions occur when one reactant is in large excess, so its concentration stays effectively constant during the reaction. For a reaction A + B → products, the true rate law is rate = k[A][B]. Since [B] is essentially constant, we absorb it into a new, effective rate constant k' = k[B]0. The rate law then simplifies to rate = k' [A]. This means the reaction behaves as first-order with respect to A, with B's influence captured in k'. The other forms would either ignore the constant nature of B or show dependence on both concentrations, which isn’t the case under pseudo-first-order conditions.