Question

describe the kinetics of a first order reaction why is first order reaction never completed

Answer 1

we know,
$k = \frac{1}{t} ln \frac{a}{a - x} \\ kt = ln( \frac{a}{a - x } ) \\ {e}^{ - kt} = \frac{a}{a - x} \\ a - x = a {e}^{ - kt}$
when reaction us ceased then x=a
so
${e}^{ - kt} = 0 \\ t = \infty$
in this reason the first order reaction never completed.