For a two-reactant system, what is the general form of the rate law and how is it determined?

• A. Rate law is always first order in each reactant.
• B. Rate law expresses rate as rate = k[A]^m[B]^n; determine by varying concentrations and fitting data to rate = k[A]^m[B]^n.
• C. Rate constant k is independent of conditions.
• D. Reaction order equals the sum of exponents and equals stoichiometric coefficients.

Answer: (B)

When two reactants are involved, the rate at which the reaction proceeds is described by a rate law that shows how the rate depends on each concentration. The general form is rate = k [A]^m [B]^n, where m and n are the orders with respect to A and B. These exponents tell you how sensitive the rate is to changes in each concentration.

Crucially, m and n are determined experimentally, not from the balanced equation or stoichiometry. You figure them out by varying one concentration at a time (or using a set of experiments), measuring the rate, and fitting the data to the form rate = k [A]^m [B]^n. The sum m + n gives the overall reaction order, but this overall order does not have to equal the stoichiometric coefficients in the balanced equation.

The rate constant k depends on conditions like temperature (often described by the Arrhenius equation) and other factors such as catalysts or solvent. So changing temperature or a catalyst can change k even if the exponents m and n stay the same.

In short, the correct view is that the rate for a two-reactant system is rate = k [A]^m [B]^n, with m and n determined experimentally by analyzing how rates change with concentration.