Question Details

Which of the following relation about first order and zero order reaction is correct?


Options

A

(t1/2) first order = 2 (t100%) first order

B

(t100%)first order=Infinite time oft1/2

C

(t1/2)zero order=2(t100%)zero order

D

 (t100%)zero order=Infinite time oft1/2

Show Answer

Correct Answer :

Option B

(t100%)first order=Infinite time oft1/2

Solution :

To determine the correct relationship between first-order and zero-order reactions, let us analyze the kinetics and timescales associated with each type of reaction.

First, let us look at a first-order reaction. The rate of a first-order reaction depends on the concentration of only one reactant. The integrated rate equation for a first-order reaction is given by:
t=2.303klog[A]0[A]
where [A]0 is the initial concentration of the reactant, [A] is the concentration of the reactant at time t, and k is the rate constant.

The half-life (t1/2) is the time required for the reactant concentration to decrease to half of its initial value, meaning [A]=[A]02. Substituting this into the first-order rate equation gives:
t1/2=ln(2)k0.693k
This shows that the half-life of a first-order reaction is a constant, independent of the initial concentration.

The time required for 100% completion of the reaction (t100%) corresponds to the concentration of reactant reaching zero ([A]=0). If we substitute [A]=0 into the integrated rate equation:
t100%=1kln[A]00=
Because the term ln() approaches infinity, a first-order reaction theoretically takes an infinite amount of time to reach 100% completion. Therefore, we can state that:
(t100%)first order=Infinite time oft1/2

For comparison, in a zero-order reaction, the rate is independent of the reactant concentration. The integrated rate equation is:
[A]=[A]0-kt
The time for 100% completion ([A]=0) is a finite value:
t100%=[A]0k
While the half-life is:
t1/2=[A]02k
Thus, for a zero-order reaction, the relationship is t100%=2(t1/2), which is not represented correctly in the incorrect options.

Hence, the correct statement is:
(t100%)first order=Infinite time oft1/2

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