Half Life of a reaction is time required to reduce the initial concentration to half. The half-life of a reaction depends on the order of reaction. The variation of half-life with order is given as:

**Integrated rate law expression**

For Zeroth order reaction

For first order reaction

Integrating on both sides, we get,

So, half-life of first order reaction is independent of initial concentration.

**Numerical**

The half-life of a first order reaction is 50 mins. Calculate the time required to complete 75% of the reaction.

Given,

Again,

initial concentration (a) = 100 (let) then

at time t,

Concentration left (a-x) = 100-75 = 25

We have,

**Calculate the half period of first order reaction when rate constant is 5 year-1.**

We have,

For 1st order reaction

t1/2 = 0.693/5

= 0.1386 year

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