In a bimolecular reaction with B in large excess, which expression defines k_obs?

• A. k_obs = k[B]
• B. k_obs = k
• C. k_obs = k[A]
• D. k_obs = k[A]^2

Answer: (A)

When one reactant is in large excess, its concentration stays effectively constant during the reaction, so the bimolecular rate law rate = k[A][B] reduces to a pseudo-first-order form with respect to A: rate = (k[B]) [A]. Here the observed rate constant is k_obs = k[B], since [B] is constant. This is why the expression for k_obs is k times the constant concentration of the excess reactant. The other forms would misrepresent the dependence: k alone ignores the constant [B], k[A] would imply first-order in A with a variable coefficient, and k[A]^2 would imply second-order in A, which isn’t aligned with the scenario of B being in large excess.