How is the overall reaction order determined?

How is the overall reaction order determined?

• A. By adding up the stoichiometric coefficients of the balanced equation
• B. By adding up the exponents in the rate law
• C. By comparing forward and reverse rates
• D. By measuring the activation energy of the slow step

Answer: (B)

The overall reaction order comes from how the rate depends on each reactant’s concentration, as shown in the rate law. If the rate law is written as rate = k [A]^m [B]^n (and so on), the exponents m and n are the orders with respect to each reactant, and their sum m + n is the overall order. These exponents are determined experimentally, describing how sensitive the rate is to changes in each concentration. For elementary reactions, the exponents match the stoichiometric coefficients, but for most real reactions the mechanism means the rate law doesn’t simply reflect those coefficients. That’s why the sum of the rate-law exponents, not the stoichiometric coefficients, gives the overall order. The other ideas—using stoichiometric coefficients, comparing forward and reverse rates, or looking at activation energy—don’t determine how the rate scales with concentration. For example, if rate = k [A]^2 [B], the overall order is 3.

The overall reaction order comes from how the rate depends on each reactant’s concentration, as shown in the rate law. If the rate law is written as rate = k [A]^m [B]^n (and so on), the exponents m and n are the orders with respect to each reactant, and their sum m + n is the overall order. These exponents are determined experimentally, describing how sensitive the rate is to changes in each concentration. For elementary reactions, the exponents match the stoichiometric coefficients, but for most real reactions the mechanism means the rate law doesn’t simply reflect those coefficients. That’s why the sum of the rate-law exponents, not the stoichiometric coefficients, gives the overall order. The other ideas—using stoichiometric coefficients, comparing forward and reverse rates, or looking at activation energy—don’t determine how the rate scales with concentration. For example, if rate = k [A]^2 [B], the overall order is 3.