In the rate law, what does the exponent for a reactant represent?

• A. The exponent equals the stoichiometric coefficient
• B. The exponent equals the reaction order with respect to that reactant
• C. The exponent is always 2
• D. The exponent is irrelevant to the rate law

Answer: (B)

The exponent in the rate law shows how the rate depends on the concentration of that reactant; it is the reaction order with respect to that reactant, not its stoichiometric coefficient. The rate law is determined experimentally and can be first order, second order, fractional, or even zero order with respect to a given reactant. For a rate law written as rate = k[A]^m[…], the exponent m is the order with respect to A, meaning doubling [A] changes the rate by a factor of 2^m. Since these exponents come from experiments and reflect the mechanism, they are not fixed by the balanced equation and are not always equal to 2.