For a particular reaction,the initial concentration of the reactant is 2M.The product of half life and rate constant for this reaction is 1M then the order of reaction is

t12×K=1 M;               K=1Mt50%; Units of K=mole/littime; ∴ it is a zero order reaction ; Alternate method : t12×K=1 M ;        a=2M ; Thus, t12×K=a2;               t12=a2K; ∴it is a zero order reaction ;