For a first order reaction. k=t2.303log10a−xa when t=t1/2 k=t1/22.303log10a−2aa or t1/2=k2.303log102=kln2 A Caution The half-life offirst order reaction is constant because the initial concentration of the reactant has no influence on the half-life of the reaction, i.e. the half-life is constant regardless of the concentration of the reactant.