## Wikipedia - Demi-vie (fr) Wikipedia - Emivita (fisica) (it) half life, $$t_{1/2}$$

https://doi.org/10.1351/goldbook.H02716
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.