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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