In a reaction with rate law v = k[A], what is the order of the reaction?

• A. Zero order
• B. Second order
• C. Mixed order
• D. First order

Answer: (D)

In kinetics, the rate law shows how the rate depends on reactant concentrations, and the exponent on a concentration tells you the order with respect to that reactant. Here, the rate is proportional to [A] to the first power because v = k[A]. That means the reaction is first order (in A), and if A is the only reactant, the overall reaction order is also first order. This differs from zero order, where the rate is independent of [A], and from second order, where the rate goes with [A]^2. For a first-order process, the integrated form A = [A]0 e^{-kt} leads to a straight line when plotting ln[A] versus time, and the half-life t1/2 = 0.693/k is constant.