Derive an expression for the rate constant of a first order reaction. Give its units.

Let us consider a first order reaction,

A → product

Suppose at the beginning of the reaction, that is, when t=0, the concentration of A is amolL and when t=t, the concentration of A is (a-x)molL.

For a first order reaction, the rate of reaction,

where k is the rate constant.


Here, c is the constant of integration.

At, t=0, x=0 ; then from equation (i) we have,

putting the value of c in equation (i), we get,

ln a – ln(a-x) = kt

Equation (ii) is the required expression.


Unit of k:
From equation (ii) we found that the unit of k depends on the unit of t . The unit of k may be s-1, min-1, h-1, day-1 and so on.

Author: Tanmay Chakrabarty

Tanmay Chakrabarty is a former CSE student, currently working as a Senior Software Engineer with 5+ years of experience in the field of Web Application development in PHP+MySQL platform with strong skills in Javascript, JQuery, JQuery UI and CSS. He tries to write notes every week but fails due to heavy loads of duty.

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