In a rate law, what does k represent?

• A. The activation energy
• B. The equilibrium constant
• C. The rate constant
• D. The stoichiometric coefficient

Answer: (C)

The rate constant k is the temperature-dependent proportionality factor that links the overall rate to the reactant concentrations in the rate law. It acts as a single number that makes the rate match the concentrations raised to their reaction orders, and it changes with temperature according to the Arrhenius equation. It is not the activation energy (the energy barrier Ea), not the equilibrium constant (which describes balance at equilibrium), and not the stoichiometric coefficients (which come from the balanced equation and, except in special cases of elementary steps, do not determine the rate). For example, in a rate law like rate = k [A]^2 [B], k sets how fast the reaction proceeds at a given temperature, independent of the concentrations themselves.