Illustrated Glossary of Organic Chemistry

Second order (2^nd order): A rate expression involving either the concentration of two species (each to the first power): rate = k [species1] [species2], or the concentration of a single species squared: rate = k [species]². Also refers to a reaction or reaction mechanism having this rate expression.

NaI Acetone

Rate = k [CH3Cl] [I-]

This S_N2 reaction has second-order kinetics, because its rate expression involves the concentration of methyl chloride to the first power times the concentration of iodide ion to the first power.

Water

Rate = k [(CH3)3CCl]

This S_N1 reaction has first-order kinetics, because its rate expression involves only the concentration of tert-butyl chloride to the first power.