Question

Define the following terms:
(i) Pseudo first-order reaction
(ii) Half-life period of reaction $\left(t_{1/2}\right)$.

Answer 1

(i) Pseudo first-order reaction
The reactions that have higher order true rate law but are found to behave as first order are called psuedo first order reactions.
For example, consider hydrolysis of methyl acetate.
$C{H}_{3}COOC{H}_3+H_{2}O\to C{H}_{3}COOH+C{H}_{3}OH$
Water participates in the reaction. Hence, the rate law expression is
$rate=k\left[C{H}_{3}COOC{H}_3\right]\left[H_{2}O\right]$
The reaction should follow second order kinetics. However, it is found that the reaction follows first order kinetics as water is present in such a large excess that its concentration remains almost constant. Hence, the rate law expression becomes.
$rate=k^{\prime }\left[C{H}_{3}COOC{H}_3\right]$
Thus the second order reaction behaves as first order reaction. Hence, it is psuedo first order reaction.
(ii) Half-life period of reaction ($t_{1/2}$).
The half life period of a reaction is the time needed for the reactant concentration to fall to one half of its initial value.
For example, if the initial concentration of a reactant is 1 M and in 30 minutes, the concentration reduces to one half, i.e., it becomes 0.5 M, then the half life period is 30 minutes.