Q.Write any two characteristics of first order reaction.

A first-order reaction has a rate constant with units of reciprocal time, and its half-life (and the time for any fixed fractional completion) is independent of the starting concentration.

For a first order reaction, $A \rightarrow \text{products}$, the integrated rate law is: $k=\dfrac{2.303}{t}\log\dfrac{a}{a-x}$

Characteristic 1 — Unit of k: Since $k = \dfrac{1}{t}\ln\dfrac{a}{a-x}$ and $\ln(a/(a-x))$ is a dimensionless ratio of concentrations, $k$ carries only the reciprocal of time as its unit, e.g. $s^{-1}$, $min^{-1}$ or $hr^{-1}$ — unlike higher order reactions where the unit of $k$ also involves concentration.

Characteristic 2 — Concentration-independent half-life: Half-life is the time for the concentration to fall to half its initial value, i.e. $x=a/2$:

$t_{1/2}=\dfrac{2.303}{k}\log\dfrac{a}{a-a/2}=\dfrac{2.303}{k}\log 2=\dfrac{0.693}{k}$ …