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Home » Class 12 » Chemistry » Chemical Kinetics » Half Life Period of a Reaction

Half Life Period of a Reaction

Last Updated on July 3, 2023 By Mrs Shilpi Nagpal

Half life period of a reaction is defined as the time during which the concentration of a reactant is reduced to half of its initial concentration.

or

The time in which half of a reaction is completed. It is generally denoted as t½

The half life period of a first order reaction may be calculated as given below:

The first order rate equation for the reaction

A ———–> Products

Half life period of a reaction

Now, half life period corresponds to time during which the initial concentration, [A]0 is reduced to half i.e.

[A] = [A]0 /2   at t= t½

Then half life period, t½ becomes

Half life period of a reaction

Thus, half life period of a first order reaction is independent of the initial concentration of the reactant. Half life period for the first order reaction is inversely proportional to the rate constant.

For example,

 

(i) time required to complete 1/3 of the reaction will be given as:

[A]o = a ,  [A] = a – a/3=2/3a

ime required to complete 1/3 of the reaction

(ii) time required to complete 3/4 of the reaction will be

[A]o =  a , [A] = a – 3/4 a = 1/4 a

time required to complete 3/4 of the reaction

Half Life period of zero order and second order reactions

The integrated rate equation is ,

kt = [A]o -[A]

For half life period, t½ , [A] = [A]0/2

t½ = [A]o /2k

Similarly for second order reaction,

Half Life period of zero order and second order reactions

zero first second order reaction

zero first second order reaction

Filed Under: Chemical Kinetics, Chemistry, Class 12

About Mrs Shilpi Nagpal

Author of this website, Mrs. Shilpi Nagpal is MSc (Hons, Chemistry) and BSc (Hons, Chemistry) from Delhi University, B.Ed. (I. P. University) and has many years of experience in teaching. She has started this educational website with the mindset of spreading free education to everyone.

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