For a given reaction the half life
of a
reactant 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:
- For a first-order reaction, the half life of the reactant may be called the half life
of the reaction.
- For a reaction involving more than one reactant, with the concentrations of the reactants
in their stoichiometric 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
stoichiometric ratios, there are different half lives for different reactants, and one cannot speak
of the half life of the reaction.
Source:
PAC, 1996, 68, 149
(A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC
Recommendations 1996))
on page 167
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1119
PAC, 1996, 68, 957
(Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations
1996))
on page 975
