Intuition The big picture (WHY this subtopic exists)Nitrogen is the gateway element of Group 15. It locks up 78% of the atmosphere as nearly inert N 2 N_2 N 2 reactive nitrogen (NH₃, HNO₃, nitrates). This subtopic is the story of how we tame inert N₂ (Haber), then oxidise NH₃ stepwise up to HNO₃ (Ostwald), and how nitrogen's many oxidation states (−3 to +5) give a zoo of oxides. Everything below flows from one fact : N forms a very strong triple bond and N–N single bonds are weak.
N 2 N_2 N 2 : N ≡ N : :N\equiv N: : N ≡ N : triple bond = one σ \sigma σ π \pi π σ 2 s σ 2 s ∗ π 2 p x = π 2 p y σ 2 p z \sigma_{2s}\,\sigma^*_{2s}\,\pi_{2p_x}=\pi_{2p_y}\,\sigma_{2p_z} σ 2 s σ 2 s ∗ π 2 p x = π 2 p y σ 2 p z = 10 − 4 2 = = = 3 = = =\dfrac{10-4}{2}= ==3== = 2 10 − 4 === 3 ==
Common mistake Steel-man: "N₂ is inert because it's a noble-gas-like full octet."WHY it feels right: each N has an octet, looks "satisfied." Fix: O₂ also has full octets but is reactive! The real cause is the kinetic barrier from the 941 kJ/mol triple bond , not the octet. Inertness here is kinetic , not thermodynamic.
N 2 ( g ) + 3 H 2 ( g ) ⇌ 2 N H 3 ( g ) Δ H = − 92 kJ/mol N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)\qquad \Delta H = -92\ \text{kJ/mol} N 2 ( g ) + 3 H 2 ( g ) ⇌ 2 N H 3 ( g ) Δ H = − 92 kJ/mol fewer moles of gas on the right (4 → 2).
Intuition HOW to choose conditions (Le Chatelier + kinetics fight!)
Exothermic ⇒ low T favours more NH₃ (equilibrium). But low T ⇒ reaction too slow . Compromise ≈ 700 K plus a catalyst.Fewer gas moles on product side ⇒ high pressure pushes equilibrium right. Use ≈ 200 atm .Catalyst: finely divided Fe with K 2 O K_2O K 2 O A l 2 O 3 Al_2O_3 A l 2 O 3 both directions equally — it only reaches equilibrium faster , it does not shift it.
Worked example Forecast-then-Verify: raise TForecast: raising T on an exothermic reaction. Verify: equilibrium shifts left (less NH₃), but rate increases. Yield ↓ but you reach equilibrium quicker. Industry picks T to balance both. ✔
We already make NH₃ (N at −3). To reach HNO₃ (N at +5 ) we oxidise nitrogen in stages: − 3 → + 2 → + 4 → + 5 -3 \to +2 \to +4 \to +5 − 3 → + 2 → + 4 → + 5
Common mistake Steel-man: "Step 3 gives only HNO₃."WHY it feels right: water + acid oxide → acid, seems clean. Fix: it's a disproportionation — one NO₂ goes up to +5, another down to +2 (NO). You must recycle the NO, else you waste a third of your nitrogen.
Definition Properties of HNO₃ (oxidising acid)
Concentrated HNO₃ is a strong oxidiser : dilute reacts with metals giving mostly NO , concentrated giving NO₂ .
Aqua regia = 3 HCl : 1 HNO₃ , dissolves Au, Pt.Brown ring test for N O 3 − NO_3^- N O 3 − [ F e ( H 2 O ) 5 N O ] 2 + [Fe(H_2O)_5NO]^{2+} [ F e ( H 2 O ) 5 N O ] 2 +
Definition Memorise by oxidation state
Oxide
N oxid. state
Name
Nature
N 2 O N_2O N 2 O +1
nitrous oxide (laughing gas)
neutral
N O NO N O +2
nitric oxide
neutral, paramagnetic
N 2 O 3 N_2O_3 N 2 O 3 +3
dinitrogen trioxide
acidic (anhydride of H N O 2 HNO_2 H N O 2
N O 2 NO_2 N O 2 +4
nitrogen dioxide
acidic, paramagnetic, brown
N 2 O 4 N_2O_4 N 2 O 4 +4
dinitrogen tetroxide
acidic (NO₂ dimer)
N 2 O 5 N_2O_5 N 2 O 5 +5
dinitrogen pentoxide
acidic (anhydride of H N O 3 HNO_3 H N O 3
Intuition WHY NO & NO₂ are paramagnetic; WHY NO₂ dimerises
N O NO N O odd electrons (11 valence e⁻) → one unpaired → paramagnetic.N O 2 NO_2 N O 2 odd electron on N. Two NO₂ pair their unpaired electrons forming an N–N bond → N 2 O 4 N_2O_4 N 2 O 4 2 N O 2 ( brown, paramagnetic ) ⇌ N 2 O 4 ( colourless, diamagnetic ) 2NO_2 \;(\text{brown, paramagnetic}) \rightleftharpoons N_2O_4\;(\text{colourless, diamagnetic}) 2 N O 2 ( brown, paramagnetic ) ⇌ N 2 O 4 ( colourless, diamagnetic ) N 2 O 4 N_2O_4 N 2 O 4
Worked example Worked: assign oxidation state of N inN 2 O 5 N_2O_5 N 2 O 5 x x x 2 x + 5 ( − 2 ) = 0 2x + 5(-2)=0 2 x + 5 ( − 2 ) = 0 Why this step? Molecule is neutral, O is −2.
2 x = 10 ⇒ x = = = + 5 = = 2x = 10 \Rightarrow x = ==+5== 2 x = 10 ⇒ x === + 5 ==
Recall Why exactly is N₂ inert? (3 reasons)941 kJ/mol triple bond (high activation energy), nonpolar/no attack site, large HOMO–LUMO gap. Inertness is kinetic .
Recall Haber conditions and reason for each~200 atm (fewer product gas moles), ~700 K (compromise — exothermic wants low T but rate wants high T), Fe catalyst + K₂O/Al₂O₃ promoters (speeds equilibrium, no shift).
Recall Three Ostwald steps with oxidation states4 N H 3 + 5 O 2 → 4 N O + 6 H 2 O 4NH_3+5O_2\to4NO+6H_2O 4 N H 3 + 5 O 2 → 4 N O + 6 H 2 O 2 N O + O 2 → 2 N O 2 2NO+O_2\to2NO_2 2 N O + O 2 → 2 N O 2 3 N O 2 + H 2 O → 2 H N O 3 + N O 3NO_2+H_2O\to2HNO_3+NO 3 N O 2 + H 2 O → 2 H N O 3 + N O
Recall Feynman: explain to a 12-year-oldAir is full of nitrogen, but it's like a couple holding hands so tightly (triple bond) they won't dance with anyone — that's "inert." To make plant food (ammonia), we force them apart using high squeeze (pressure), warm heat, and a helper (iron catalyst). Then we slowly let nitrogen grab oxygen, one step at a time, until it becomes strong acid (nitric acid). Nitrogen can wear many "outfits" of oxygen = the different oxides.
Mnemonic Oxides ladder & OstwaldOxides: "1 2 3 4 4 5 " → N 2 O , N O , N 2 O 3 , N O 2 , N 2 O 4 , N 2 O 5 N_2O, NO, N_2O_3, NO_2, N_2O_4, N_2O_5 N 2 O , N O , N 2 O 3 , N O 2 , N 2 O 4 , N 2 O 5 Ostwald chain: "A mmonia → NO → NO₂ → HNO₃ " = "Ammonia Naturally Nears Nitric" .
Haber: "HIGH P, low-ish T, Fe " = "Pressure Pushes, Heat Hurts, Iron Helps."
N2 molecule MO diagram — bond order & paramagnetism logicLe Chatelier Principle — drives Haber condition choicesDisproportionation Reactions — Step 3 of Ostwald, NO₂ in waterOxidation States and Redox — assigning N states across oxidesGroup 15 Hydrides PH3 vs NH3 — trends down the groupAqua regia and Noble Metals — HNO₃ + HCl chemistry
Why is N₂ kinetically inert? Very high triple-bond dissociation enthalpy (941 kJ/mol) → high activation energy; also nonpolar with no easy attack site and large HOMO–LUMO gap.
Bond order of N₂? 3 (one σ + two π); MO (10−4)/2 = 3.
Compare 3×(N–N single) vs N≡N enthalpy. 3×159 = 477 kJ/mol ≪ 941 kJ/mol, so the triple bond is hugely favoured.
Haber equation with ΔH. N₂ + 3H₂ ⇌ 2NH₃, ΔH = −92 kJ/mol.
Why high pressure helps Haber? 4 gas moles → 2; high P shifts equilibrium toward fewer moles (NH₃).
Why ~700 K and not lower for Haber? Reaction exothermic so low T favours yield, but rate is too slow; ~700 K is a kinetic/equilibrium compromise.
Haber catalyst and promoters? Finely divided Fe with K₂O and Al₂O₃ promoters.
Does a catalyst increase NH₃ yield? No — it speeds attainment of equilibrium equally both ways; yield (Kp) unchanged.
Ostwald Step 1 (catalyst & equation)? 4NH₃ + 5O₂ →(Pt/Rh, 500 K) 4NO + 6H₂O; N: −3→+2.
Ostwald Step 2? 2NO + O₂ → 2NO₂; N: +2→+4.
Ostwald Step 3 and its type? 3NO₂ + H₂O → 2HNO₃ + NO; disproportionation (+4 → +5 and +2); NO recycled.
Aqua regia composition? 3 parts conc. HCl : 1 part conc. HNO₃; dissolves Au, Pt.
Which N oxides are paramagnetic? NO and NO₂ (odd number of electrons).
Why does NO₂ dimerise to N₂O₄? NO₂ has an unpaired electron; pairing forms an N–N bond giving diamagnetic N₂O₄ (favoured at low T/high P).
Oxidation state of N in N₂O, NO, N₂O₃, NO₂, N₂O₅? +1, +2, +3, +4, +5.
Anhydride of HNO₃ and HNO₂? N₂O₅ (HNO₃), N₂O₃ (HNO₂).
Brown ring test detects? Nitrate ion NO₃⁻; brown [Fe(H₂O)₅NO]²⁺ forms.
Fe catalyst, 200 atm, 700 K
Le Chatelier: low T + high P favour
oxidation states -3 to +5
Intuition Hinglish mein samjhoDekho, hawa ka 78% nitrogen hai par wo "inert" hai — matlab aaram se react nahi karta. Iska asli reason octet nahi, balki uska triple bond (N≡N) hai jiska bond enthalpy bahut zyada (941 kJ/mol) hota hai. Itni strong bond todne ke liye huge activation energy chahiye, isliye N₂ kinetically lazy hai. Yaad rakho: inertness ka kaaran kinetic hai, thermodynamic nahi.
Ab industry ko reactive nitrogen chahiye, to Haber process se NH₃ banate hain: N 2 + 3 H 2 ⇌ 2 N H 3 N_2 + 3H_2 \rightleftharpoons 2NH_3 N 2 + 3 H 2 ⇌ 2 N H 3 high pressure (~200 atm) se yield badhti hai; reaction exothermic hai isliye theoretically low T chahiye, par tab rate slow ho jaati hai, isliye ~700 K ka compromise + Fe catalyst lagate hain. Catalyst sirf equilibrium jaldi laata hai, yield nahi badhata.
Phir Ostwald process se HNO₃ banta hai — basically NH₃ ko step-by-step oxidise karke N ko −3 se +5 tak le jaate hain: pehle Pt par NO (+2), phir NO₂ (+4), phir paani me ghol ke HNO₃ (+5) + NO (recycle). Last step ek disproportionation hai, isliye NO wapas recycle hota hai.
Oxides ka zoo bas oxidation state se yaad karo: N₂O(+1), NO(+2), N₂O₃(+3), NO₂(+4), N₂O₅(+5). NO aur NO₂ me odd electron hone se wo paramagnetic hain; isliye do NO₂ milke N₂O₄ ban jaate hain (low T, high P pe). Bas itna clear ho gaya to poora topic 80/20 me cover ho gaya.
