In a bimolecular reaction A + B -> products with B in large excess so rate ≈ k_obs[A], if k = 2.0 M^-1 s^-1 and [B]0 = 0.50 M, what is k_obs?

• A. 2.0 s^-1
• B. 0.5 s^-1
• C. 4.0 s^-1
• D. 1.0 s^-1

Answer: (D)

When one reactant is in large excess, its concentration stays effectively constant, so the reaction behaves as if it were first-order in A. The rate law becomes rate = k_obs [A], with k_obs = k [B]0 under these conditions. Here, k is 2.0 M^-1 s^-1 and the constant B concentration is 0.50 M, so k_obs = 2.0 × 0.50 = 1.0 s^-1. The units work out to s^-1 because M^-1 × M = s^-1. So the observed first-order rate constant is 1.0 s^-1.