Do rate laws always reflect the stoichiometric coefficients?

• A. Yes
• B. No; only for elementary steps; overall rate laws can differ
• C. Sometimes
• D. Only for reactions in solution

Answer: (B)

The rate law tells us how the reaction rate depends on the concentrations of species involved, and it mirrors the mechanism, not just the overall balanced equation. In an elementary step, the reaction happens in a single molecular event, so the rate law directly uses the stoichiometric coefficients as exponents: for A + B → products, the rate is proportional to [A][B]. But most reactions proceed through multiple steps, and the slowest step controls the overall rate. When you derive the overall rate law from a mechanism, you often must express intermediates in terms of the starting reactants, which can give exponents that differ from the overall stoichiometric coefficients. So rate laws are not always tied to the stoichiometry; they reflect the actual step-by-step process, and only for elementary steps do the exponents match the coefficients. This is independent of being in solution or not, so that option isn’t generally correct.