In the rate law expression Rate = k[A]^m[B]^n, what do m and n represent?

• A. Reaction orders, determined experimentally.
• B. Stoichiometric coefficients from the balanced equation.
• C. Rate constants for each reactant.
• D. Activation energies.

Answer: (A)

The numbers m and n are the reaction orders with respect to A and B, telling you how sensitive the rate is to each reactant’s concentration. They’re determined experimentally by measuring how the rate changes when you vary [A] or [B] while keeping the other variables constant.

They’re not the stoichiometric coefficients from the balanced equation, which tell you how many moles react but not how fast. They’re not rate constants—the rate constant is k, while m and n describe the dependence on concentrations. They’re not activation energies, which relate to the energy barrier and temperature via the Arrhenius equation.

A useful way to think about it is that the overall rate law Rate = k[A]^m[B]^n says the rate scales with [A] to the power m and with [B] to the power n. If you double [A] with [B] held fixed, the rate changes by a factor of 2^m; similarly, changing [B] changes the rate by a factor of 2^n. The sum m+n is the overall order of the reaction. Exponents can be zero or fractional, and in more complex mechanisms they reflect the details of the rate-determining steps rather than simple stoichiometry.