How Initial-Rate Comparisons Reveal Reaction Order

A rate law describes how changing concentration changes reaction speed. Determine its exponents from controlled measurements rather than copying coefficients from an overall reaction equation.

Change one concentration at a time

For rate = k[A]ᵐ[B]ⁿ, compare trials at the same temperature and in the same solvent. If only [A] doubles, the rate ratio is 2ᵐ. A doubled rate indicates m = 1; an unchanged rate indicates m = 0. Repeat independently for B, then add the exponents for overall order.

A calculated illustration

Suppose a model follows rate = k[A][B]. Tripling [A] while halving [B] predicts a rate factor of 3 × 0.5 = 1.5. This is an algebraic illustration, not a reported laboratory observation.

Concentration dependence is not the whole mechanism

Changing solvent or temperature may change k, so those trials cannot isolate a concentration exponent. A rate law constrains a proposed mechanism, but stereochemistry and other evidence are needed to distinguish mechanisms that share the same concentration dependence.