An elementary rate law is followed when the reaction orders are identical to which quantities?

• A. The temperature exponents
• B. The pre-exponential factors
• C. The stoichiometric coefficients
• D. The activation energies

Answer: (C)

An elementary rate law mirrors the molecularity of the step: the rate depends on how many reactant molecules must collide. In that case, the exponents in the rate expression match the numbers of each reactant involved in the step, i.e., the stoichiometric coefficients. So if two molecules of A and one of B must react in that step, the rate would be proportional to [A]^2[B], and the overall order equals the sum of those coefficients. The other items—temperature exponents, pre-exponential factors, and activation energies—belong to how the rate constant k changes with temperature (Arrhenius form) and do not determine the reaction orders. In a non-elementary overall reaction, the observed orders may differ from the stoichiometric coefficients because the mechanism dictates the rate law.