Dinitrogen

Preparation

Dinitrogen is manufactured on an industrial scale by liquefying air and then separating it by fractional distillation: liquid nitrogen, which boils at the lower temperature of 77.2 K, distils off first, leaving liquid oxygen (b.p. 90 K) behind.

In the laboratory, a convenient route is to treat an aqueous solution of ammonium chloride with sodium nitrite:

This reaction also throws off small amounts of $NO$ and $HNO_3$ as impurities, which can be scrubbed out by bubbling the gas through aqueous sulphuric acid containing potassium dichromate. Dinitrogen can equally well be obtained from the thermal decomposition of ammonium dichromate:

For a much purer sample, the thermal decomposition of sodium or barium azide is used, for example:

Properties

Dinitrogen is a colourless, odourless, tasteless and non-toxic gas, and its nitrogen atoms occur naturally as two stable isotopes, $^{14}N$ and $^{15}N$. It dissolves only sparingly in water (23.2 cm$^3$ per litre of water at 273 K and 1 bar) and, in keeping with its weak intermolecular forces, has low freezing and boiling points (see Table 7.1).

At room temperature $N_2$ is quite unreactive, a direct consequence of the very high bond enthalpy of its $N\equiv N$ triple bond — but its reactivity rises sharply as the temperature is increased. At elevated temperature it will combine directly with certain metals to give predominantly ionic nitrides, and with non-metals to give covalent nitrides, for instance:

Around 773 K, and in the presence of a catalyst, dinitrogen combines with hydrogen in the industrially vital Haber process to give ammonia:

Dinitrogen only reacts with dioxygen at very high temperature — around 2000 K — to give nitric oxide:

$N_2 + O_2(g) \xrightarrow{\text{Heat}} 2NO(g)$ …