Page 184 - Instant notes

Physical chemistry 170

1. The half-life is always inversely proportional to k, but its dependence on [A] 0 depends
on the order of the reaction. The half-life relationships may be used to determine the
order of a reaction and its rate coefficient in two ways:
(i) The reaction is followed over several half-lives and the dependence of the sequence of
half-lives on the concentration at the start of each half-life period is examined.
(ii) Several experiments can be performed, each with a different initial concentration, and
the dependence on concentration of the first half-life for each experiment measured.
A useful diagnostic for first order reactions is that t 1/2 is independent of initial
concentration, i.e. the concentration of A will fall to [A]/2 in an interval ln2/k whatever
the value of [A].