Describe the methods for the preparation of dinitrogen $(N_{2})$.

(N/A) $(I)$ Commercial preparation: Commercially,dinitrogen is produced by the liquefaction and fractional distillation of air. Liquid dinitrogen (b.p. $77.2 \ K$) distills out first,leaving behind liquid oxygen (b.p. $90 \ K$).
$(II)$ Laboratory preparation: In the laboratory,dinitrogen is prepared by treating an aqueous solution of ammonium chloride with sodium nitrite.
$NH_{4}Cl_{(aq)} + NaNO_{2(aq)} \rightarrow NaCl_{(aq)} + N_{2(g)} + 2H_{2}O_{(l)}$
Small amounts of $NO$ and $HNO_{3}$ are also formed in this reaction; these impurities can be removed by passing the gas through aqueous sulphuric acid containing potassium dichromate.
$(III)$ By thermal decomposition: Thermal decomposition of ammonium dichromate and metal azides such as $Ba(N_{3})_{2}$ or $NaN_{3}$ yields nitrogen.
$(NH_{4})_{2}Cr_{2}O_{7} \xrightarrow{\Delta} N_{2} + 4H_{2}O + Cr_{2}O_{3}$
$Ba(N_{3})_{2} \xrightarrow{\Delta} Ba + 3N_{2(g)}$
$2NaN_{3} \xrightarrow{\Delta} 2Na + 3N_{2(g)}$
Very pure dinitrogen is obtained from metal azides.