Notes On Half Life of a Reaction - CBSE Class 12 Chemistry
Half life period of a reaction is defined as the time taken for the concentration of a reactant to reduce to one half of its initial concentration. For a zero order reaction, t1/2 can be calculated as follows, A → Products The integrated equation [A] = -kt + [A]o At t =t1/2, [A] = [A]0/2 [A]0/2 = -kt1/2 + [A]o [A]0/2 - [A]o = -kt1/2 -[A]0/2 = -kt1/2 t1/2 = [A]0/2 t1/2 ∝ [A]0 Half life for a zero order reaction is directly proportional to the initial concentration of the reactants . For a first order reaction, t1/2 can be calculated as, A → Products The integrated equation k = 2.303/t . log([A0]/[A]) t = t½; [A]:[A0] k = 2.303/t½ . log([A0]/[A0/2]) t½ = (2.303 x log 2)/k t½ = (2.303 x 0.301)/k t½ = 0.693/k Half life period for a first order reaction is independent of the initial concentration of the reactant.

#### Summary

Half life period of a reaction is defined as the time taken for the concentration of a reactant to reduce to one half of its initial concentration. For a zero order reaction, t1/2 can be calculated as follows, A → Products The integrated equation [A] = -kt + [A]o At t =t1/2, [A] = [A]0/2 [A]0/2 = -kt1/2 + [A]o [A]0/2 - [A]o = -kt1/2 -[A]0/2 = -kt1/2 t1/2 = [A]0/2 t1/2 ∝ [A]0 Half life for a zero order reaction is directly proportional to the initial concentration of the reactants . For a first order reaction, t1/2 can be calculated as, A → Products The integrated equation k = 2.303/t . log([A0]/[A]) t = t½; [A]:[A0] k = 2.303/t½ . log([A0]/[A0/2]) t½ = (2.303 x log 2)/k t½ = (2.303 x 0.301)/k t½ = 0.693/k Half life period for a first order reaction is independent of the initial concentration of the reactant.

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