Dinitrogen and Nitric Oxide
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Direct answer
Dinitrogen and nitric oxide are nitrogen's two minimal molecules with opposite personalities. N2, seventy-eight per cent of air, is nearly inert at room temperature because the N≡N triple bond carries about 941 kJ/mol over just 110 pm — among the strongest bonds known — breakable by lightning, by the Haber converter's catalyst, or by nitrogenase enzymes. NO is an odd-electron (eleven valence electrons), paramagnetic, colourless gas with bond order 2.5, made industrially by ammonia oxidation over a hot platinum-rhodium gauze and in the laboratory from dilute nitric acid reacting with copper.
What you must remember
- N2 numbers: bond enthalpy 941 kJ/mol, bond length 110 pm, bond order 3; liquid at 77 K — the coolant of cryobiology and the inert gas flushed into chip packets to delay oxidation.
- N2 preparations: laboratory NH4Cl + NaNO2 → N2 + NaCl + 2H2O on warming, or (NH4)2Cr2O7 → N2 + Cr2O3 + 4H2O; the purest from barium azide Ba(N3)2 → Ba + 3N2; industrial by fractional distillation of liquid air.
- Fixation reactions: N2 + 3H2 ⇌ 2NH3 (Haber); 6Li + N2 → 2Li3N at room temperature, lithium alone in its group; N2 + O2 → 2NO only at lightning-arc temperatures.
- NO identity: eleven valence electrons, paramagnetic, bond order 2.5, bond length 115 pm between N2 and a double bond; colourless gas that browns instantly in air (2NO + O2 → 2NO2).
- NO preparations: Ostwald stage 4NH3 + 5O2 → 4NO + 6H2O over Pt-Rh gauze near 1200 K; laboratory 3Cu + 8HNO3 (dilute) → 3Cu(NO3)2 + 2NO + 4H2O.
- NO reactions: dimerises weakly to N2O2 only in condensed phases; 2NO + Cl2 → 2NOCl; binds haem iron — the biological signalling angle behind its fame.
- Brown ring test: fresh FeSO4 added to a nitrate solution, then concentrated H2SO4 poured down the wall — a brown ring of [Fe(H2O)5NO]2+ forms at the junction, formally iron(I) with nitrosonium.
- Contrast molecule: NO+ is isoelectronic with N2 and CO — bond order 3, diamagnetic — the comparison JEE prefers for bonding questions.
Two molecules, two personalities
The inertness of N2 is pure energetics: to react, the triple bond must break or stretch, and 941 kJ/mol buys near-total room-temperature indifference — which is why nitrogen flushing keeps packaged food fresh while fixation demands the Haber converter or a lightning bolt. NO inverts every property. With its odd electron count, the antibonding π* orbital holds the unpaired spin, making the gas paramagnetic and instantly oxidised by air to brown NO2 — a built-in visual detector. Follow the brown ring closely, since it earns marks: in the presence of nitrate and ferrous sulphate, the concentrated acid liberates nitric oxide in situ, and NO replaces one water in [Fe(H2O)6]2+ to give the pentaaquanitrosyl complex; its position at the density boundary between salt solution and heavy acid is what draws the ring. Remove the odd electron instead — ionise to NO+ — and the bond shortens towards N2's, the moment drops to zero, and the order climbs to three: one electron, three different molecules.
Nitrogen questions that repeat
JEE Main samples recall: why N2 is inert (the bond-enthalpy one-liner), the brown ring formula, NO's colour change in air, barium azide as the source of the purest nitrogen. Advanced probes the bonding — NO's bond order 2.5 and paramagnetism against NO+'s order 3 and diamagnetism, the ionisation removing an antibonding electron and strengthening the bond — plus the stoichiometry chain from ammonia through NO and NO2 towards nitric acid. Two slips recur: calling NO brown (NO2 is the brown one), and forgetting that NO dimerises only in liquid or solid phases — the gas stays monomeric. Group 15 chemistry, NO included, remains on both current syllabi, and the Ostwald-brown-ring pairing is among the most repeated inorganic segments in the JEE archive.
Frequently asked questions
Why is dinitrogen inert at room temperature?
The N≡N triple bond's 941 kJ/mol enthalpy over 110 pm is among the largest in chemistry, so thermal activation fails without a catalyst, an arc, or an enzyme.
How is the brown ring test for nitrate performed?
Add freshly prepared ferrous sulphate to the nitrate solution, then pour concentrated sulphuric acid carefully down the wall; a brown ring of [Fe(H2O)5NO]2+ appears at the junction of the two layers.
Why is nitric oxide paramagnetic?
Its eleven valence electrons leave one unpaired in a π* antibonding orbital — bond order 2.5, and the spin shows a measurable magnetic moment.
Which industrial step produces NO?
Catalytic oxidation of ammonia: 4NH3 + 5O2 → 4NO + 6H2O over a platinum-rhodium gauze — the Ostwald process's first stage towards nitric acid.
Which compound gives the purest dinitrogen on heating?
Barium azide: Ba(N3)2 → Ba + 3N2, decomposing without oxide or other gaseous contamination to leave pure nitrogen.