3.7. Half-life

The variation in concentration with time provides a detailed description of how fast a reaction is occurring. In many circumstances, though, it is desirable to have a simple, approximate measure of the reaction rate, and the half-life (t1/2)provides such a measure. The half-life is the time it takes for one-half of the original amount of material to react (assuming the compound in question is a limiting reactant). If the initial concentration of a reactant A is 1.0 mole L1, the half-life is the time at which cA= 0.5 mole L1. Intuitively, the faster the reaction, the shorter the half-life. The rate of the reaction is proportional to the rate coefficient; thus the larger the rate constant, the shorter the half-life. Dependence of the half-time on the rate coefficient could be complicated, however, for first order reactions , and the half-time does not depend on the initial concentration of A (reagent).