Half-Life
Definition
The time for half the reactant to be used up. For first-order reactions it is constant: the same whether you start with a ton or a gram.
Symbol: · Unit: s
Technical definition
Half-life is the time required for a reactant's concentration to fall to half its initial value. For first-order reactions it equals ln(2)/k and is independent of initial concentration, while for other orders it depends on the starting concentration.
Example
A first-order reaction with k = 0.0693 s^-1 has a half-life of 10 s. After 30 s, three half-lives, only 1/8 of the reactant remains.
Common confusion
Constant half-life is a first-order exclusive. For second-order reactions the half-life gets longer as reactant runs out, so half-life alone does not identify the order.