Nitrogen (N)

Nitrogen exists as a diatomic molecule with a very strong triple bond \(\mathrm{N\equiv N}\). The bond requires a high activation energy to break, so most reactions of \(\mathrm{N_2}\) proceed slowly at ambient conditions unless catalyzed or at elevated temperature/pressure.

The ground-state configuration is \([He]2s^2 2p^3\) with three unpaired p-electrons. Nitrogen displays oxidation states from \(-3\) to \(+5\):

• \(-3\): \(\mathrm{NH_3}\), amines
• \(+1\) to \(+4\): \(\mathrm{N_2O}\), \(\mathrm{NO}\), \(\mathrm{N_2O_3}\), \(\mathrm{NO_2}\)
• \(+5\): \(\mathrm{HNO_3}\), \(\mathrm{NO_3^-}\)

The Haber–Bosch synthesis uses an Fe-based catalyst at high pressure and moderate temperature:

\(\mathrm{N_2(g) + 3\,H_2(g) \rightleftharpoons 2\,NH_3(g)}\;\;\Delta H^{\circ} < 0\)

High pressure favors ammonia formation (fewer moles of gas), while lower temperature favors equilibrium yield but slows kinetics; practical conditions balance both.

Certain microbes (e.g., Rhizobium in legume root nodules) reduce atmospheric \(\mathrm{N_2}\) to ammonia via nitrogenase:

\(\mathrm{N_2 + 8\,H^+ + 8\,e^- + 16\,ATP \rightarrow 2\,NH_3 + H_2 + 16\,ADP + 16\,P_i}\)

This converts inert nitrogen into bioavailable forms, supporting the biosynthesis of amino acids and nucleotides.

The nitrogen cycle includes:

1. Fixation (biological/industrial/lightning) to \(\mathrm{NH_3/NO_3^-}\)
2. Nitrification: \(\mathrm{NH_4^+ \rightarrow NO_2^- \rightarrow NO_3^-}\)
3. Assimilation into biomolecules
4. Ammonification: organic N \(\rightarrow\) \(\mathrm{NH_4^+}\)
5. Denitrification: \(\mathrm{NO_3^- \rightarrow N_2}\) returning N to the atmosphere

The Ostwald process oxidizes ammonia:

1. \(\mathrm{4\,NH_3 + 5\,O_2 \xrightarrow{Pt/Rh} 4\,NO + 6\,H_2O}\)
2. \(\mathrm{2\,NO + O_2 \rightarrow 2\,NO_2}\)
3. \(\mathrm{3\,NO_2 + H_2O \rightarrow 2\,HNO_3 + NO}\) (with absorption and recycling)

Resulting \(\mathrm{HNO_3}\) is a strong acid and oxidizing agent.

Nitrogen’s variable oxidation states lead to several oxides with distinct properties:

• \(\mathrm{N_2O}\) (dinitrogen monoxide): anesthetic, mild oxidizer
• \(\mathrm{NO}\) (nitric oxide): signaling molecule, oxidizes to \(\mathrm{NO_2}\)
• \(\mathrm{NO_2}\) (nitrogen dioxide): brown, toxic, forms \(\mathrm{HNO_3}\)

These species participate in atmospheric chemistry and acid rain formation.

Nitrates \(\mathrm{NO_3^-}\) and nitrites \(\mathrm{NO_2^-}\) are oxyanions of nitrogen. Excess agricultural runoff of nitrates can cause eutrophication, leading to algal blooms and hypoxia in water bodies. Nitrite can also convert hemoglobin to methemoglobin, reducing oxygen transport.

Liquid nitrogen (b.p. \(\approx 77\,\mathrm{K}\)) is used for cryogenics, food freezing, biological sample storage, and classroom demonstrations.

• Wear insulated gloves and face protection; avoid skin contact (frostbite).
• Use in ventilated areas; nitrogen gas can displace oxygen and cause asphyxiation.
• Never seal in a closed container—rapid boil-off can cause explosion.

In the classic test, add freshly prepared \(\mathrm{FeSO_4}\) to the solution, then carefully layer concentrated \(\mathrm{H_2SO_4}\) down the side of the tube. A brown ring appears at the interface due to formation of \([\mathrm{Fe(H_2O)_5NO}]^{2+}\), indicating \(\mathrm{NO_3^-}\).