half life, \(t_{1/2}\)

For a given reaction the half life \(t_{1/2}\) of a @R05163@ is the time required for its concentration to reach a value that is the arithmetic mean of its initial and final (equilibrium) values. For a reactant that is entirely consumed it is the time taken for the reactant concentration to fall to one half its initial value: The half life of a reaction has meaning only in special cases:
  1. For a first-order reaction, the half life of the reactant may be called the half life of the reaction.
  2. For a reaction involving more than one reactant, with the concentrations of the reactants in their @S06021@ ratios, the half life of each reactant is the same, and may be called the half life of the reaction.
If the concentrations of reactants are not in their @S06021@ ratios, there are different half lives for different reactants, and one cannot speak of the half life of the reaction.
See also:
Green Book, 2nd ed., p. 55 [Terms] [Book]
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1119 [Terms] [Paper]
PAC, 1996, 68, 149. (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 167 [Terms] [Paper]
PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 975 [Terms] [Paper]