True or false: The rate law for a reaction must have exponents equal to the coefficients in the balanced equation.

• A. False
• B. True
• C. Cannot be determined
• D. Sometimes

Answer: (A)

The rate law is about how the rate depends on reactant concentrations, and the exponents (the reaction orders) come from experiments. They do not have to match the coefficients in the balanced equation. For many reactions the mechanism involves steps where the slow (rate-determining) step includes a different combination of species, or even fractional or zero orders, so the exponents in the rate law aren’t simply the numbers in the balanced equation. Only for an elementary reaction—where the overall equation directly represents a single molecular event—do the exponents often equal the stoichiometric coefficients. In general, though, the rate law is determined experimentally, not by the coefficients. So the statement is false.