What is the rate constant?

• A. An experimentally determined proportionality constant that gives some measure of the intrinsic reactivity of the reaction
• B. The rate of the reaction
• C. The rate of the reaction per unit concentration
• D. A constant that depends on temperature

Answer: (A)

The rate constant is the proportionality constant in a rate law that relates the reaction rate to the concentrations of the reactants. In a rate law like rate = k [A]^m [B]^n, k carries information about how fast the reaction would proceed independent of how much A or B is present, and it is determined experimentally, reflecting the intrinsic reactivity of the system under the studied conditions. Although k depends on temperature (follows the Arrhenius relationship), that temperature dependence is a consequence of the molecular kinetics, not what the constant fundamentally represents. The other statements miss this essential role: the rate is the product of k and concentration terms, not k itself; and k is not simply “the rate per unit concentration.”