What is rate constant?

• A. A constant of proportionality between the rate and the concentrations of reactants that appear in the rate law.
• B. A constant number of reactants.
• C. The temperature at which the reaction occurs.
• D. The overall number of moles in the reaction.

Answer: (B)

The rate constant is the proportionality factor in the rate law that relates how fast a reaction proceeds to the concentrations of the reactants. When the rate law is written as rate = k [A]^m [B]^n, the symbol k is the rate constant. It is a property of the reaction at a given temperature and sets the scale of the rate for those concentrations; it does not depend on how much reactant is present, though it does depend on temperature (it changes if you change the temperature, via the Arrhenius relationship). It is not a count of reactants, nor the temperature itself, nor the total number of moles.