Nitrogen Family (Group 15) — NH3 (Haber) & HNO3 (Ostwald) Why Group 15 matters for NEET and for life Fertilizers that grow our food and nitric acid that enables explosives, dyes and polymers — both start from Group 15 chemistry. Understand dinitrogen’s stubborn triple bond, phosphorus’s allotropes, ammonia’s Lewis basicity, and two industrial giants: the Haber and Ostwald processes. This gives direct NEET questions and connects inorganic chemistry to real-world plants. Haber-Bosch ammonia underpins global food security: nitrogen fertilizers from NH3 help feed billions. remember Meet the nitrogen family (Group 15): foundation Members: nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi). Valence electronic configuration is ns 2 np 3 . As we move down the group: atomic radius increases, ionization enthalpy decreases, electronegativity decreases, and character changes from non-metal (N, P) to metalloid (As, Sb) to metal (Bi). Common oxidation states are +3 and +5; due to the inert-pair effect, the +3 state becomes more stable down the group ( Bi 3+ is more common and stable than Bi 5+ ). Nitrogen shows a maximum covalency of 4 (no vacant d orbitals). Heavier members (P, As, Sb, Bi) can exceed octet; classic examples include PCl5 and the hexacoordinate anion [PF6] −. Group 15 order: Naughty People Are Stubborn Beggars (N, P, As, Sb, Bi). Atomic radius Increases Shielding rises → larger size Ionization enthalpy Decreases Easier to remove electrons down group Electronegativity Decreases N is most EN; Bi least Metallic character Increases Non-metal → metalloid → metal Oxidation states +3, +5 (both); +3 more stable down group Inert-pair effect strengthens ( Bi 3+ favored) Maximum covalency N: 4; P, As, Sb, Bi: up to 6 N lacks d orbitals; P can form PCl5, [PF6] − Aspect Property Trend down the group Short note / NEET hook Group 15 trends — exam-oriented view Dinitrogen (N2): why so inert? Diatomic nitrogen (dinitrogen, N2; SMILES: N N) has a very strong N N triple bond with very high bond enthalpy (about 941–945 kJ mol −1 ). At room temperature it is chemically inert towards most reagents — a blessing for life (stable atmosphere) but a challenge for making reactive nitrogen compounds. Industry must apply high pressure, elevated temperature, and catalysts to make ammonia from N2 and H2. Nitrogen gas (N2) is reactive at room temperature. N2 is very inert at room temperature due to the strong N N triple bond (bond enthalpy ≈ 941–945 kJ mol −1 ). Reactivity requires special conditions (e.g., Haber process). Industrial N2 and O2 are obtained by liquefying air and fractional distillation — N2 boils at 77 K and distills off before O2 (90 K). Source gases for Haber and Ostwald processes come from air separation. gpt-image-2 Apparatus diagram for liquefaction of air and fractional distillation: compressor, heat exchangers, expansion valve, distillation column with trays. Label boiling points (N2 77 K, O2 90 K). Clean 2D vector schematic, arrows in red, neutral palette, no internal text beyond labels. Schematic: Air compression, cooling to liquefy, fractional distillation — nitrogen top product (bp 77 K), oxygen bottom (bp 90 K). 2026-05-26T17:04:57.270Z Allotropy of phosphorus: white, red, black Phosphorus shows striking allotropy. - White phosphorus (tetraphosphorus, P4): discrete tetrahedral P4 molecules, highly strained bonds, very reactive, glows in dark (chemiluminescence often called phosphorescence), stored under water, very toxic. - Red phosphorus: polymeric network obtained by heating white P in absence of air; more stable, less reactive, non-toxic — used in safety match striking surfaces. - Black phosphorus: layered, the most thermodynamically stable allotrope at room conditions; a semiconductor (puckered layers). Allotrope Structure Reactivity / Handling Typical uses Allotropes of phosphorus — structure and reactivity White P (P4) Discrete tetrahedral P4 molecules Very reactive; glows in dark; stored under water; highly toxic Chemistry demos (careful), starting material for PCl3/PCl5 Red P Polymeric chains/networks Stable, less reactive, non-toxic Safety matches, pyrotechnics Black P Layered (sheet-like), most stable Least reactive; semiconductor Research electronics, reference allotrope gpt-image-2 Contrasting structures: tetrahedral P4 (white), chain/polymeric red P, and layered black P. Three-panel structural comparison: Panel 1 shows tetrahedral P4 (white P) with P atoms at vertices; Panel 2 shows red P as polymeric chain; Panel 3 shows stacked puckered layers of black P. Labels for each allotrope; clean vector chemistry style; arrows in red for bonds; white background. 2026-05-26T17:04:57.352Z White and red phosphorus have the same properties because both are 'P4'. White P is discrete P4 molecules, whereas red P is a polymeric solid. Different structures → different reactivity, toxicity, and uses. Ammonia (NH3): a Lewis base and complexing agent Ammonia (NH3; SMILES: N) donates its lone pair, so it is a Lewis base. It forms hydrogen bonds, is highly soluble in water, and forms ammine complexes with transition metals: - Deep-blue tetraamminecopper(II), [Cu(NH3)4] 2+ - Diamminesilver(I), [Ag(NH3)2] + (Tollens’ reagent) Ammonia is a key feedstock for fertilizers (urea, ammonium nitrate, ammonium sulfate) and many nitrogen chemicals. Manufacture of ammonia — the Haber–Bosch process Exothermic overall ( H -92.4 kJ mol -1 ); fewer moles on product side. Haber equilibrium (gas phase) Synthesis gas (N2 + H2) is compressed and passed over an iron-based catalyst at high temperature; ammonia is condensed and unreacted gases recycled to improve overall yield. Dinitrogen reactant Dinitrogen Dihydrogen reactant Dihydrogen Ammonia Azane product Drying and CO/CO2 removal; sulfur scrubbing Feed preparation: N2 (from air separation) and H2 (e.g., from steam reforming) are purified to remove poisons (CO, CO2, H2S). ≈200 atm; ≈700 K (industrial compromise between rate and equilibrium) Compression and heating: gases are compressed and heated before entering the catalyst bed. Catalyst: Fe (magnetite/Fe3O4-derived) with promoters Al2O3, K2O; Mo may be used as a promoter Catalytic synthesis: over iron catalyst with promoters, N≡N bond is activated and NH3 forms. Le Chatelier: high P favours ammonia (fewer moles); lower T favours yield but slows rate Cooling and condensation: reactor effluent is cooled; NH3 liquefies and is separated. Ammonia condenses; unreacted N2/H2 recycled Haber–Bosch synthesis of ammonia Haber plant flow: N2 + H2 → compression → Fe-catalyst reactor (≈700 K, ≈200 atm) → cooling → NH3 separation → recycle. At typical conditions, single-pass conversion is only about 15–20%. Continuous recycling of unreacted N2 and H2 gives high overall yield. Haber process gives 100% ammonia in one pass. Higher temperature increases rate but decreases equilibrium yield for exothermic reactions. Industry uses a compromise: high pressure, moderate temperature, and a catalyst to balance rate and yield. High temperature always increases both rate and yield for exothermic reversible reactions like Haber. Rate vs equilibrium trap: For N 2 + 3H 2 2NH 3 , higher T speeds up reaction but shifts equilibrium left; higher P shifts right. Catalyst only speeds rate, it does not change equilibrium composition. neet-alert Catalysts for Haber and Contact processes are the same. Haber uses iron-based catalysts (with promoters like K2O, Al2O3; Mo as promoter). Contact process for sulfuric acid uses V2O5 — a different catalyst. Nitric acid (HNO3): the Ostwald process A hot Pt–Rh gauze catalyzes the first, highly exothermic step (industrial practice ≈ 1100 K). Stage 1: catalytic oxidation of ammonia Brown gas NO2 forms on cooling and mixing with air. Stage 2: oxidation of nitric oxide NO is regenerated during absorption and recycled. Stage 3: absorption to nitric acid Ammonia reactant Azane Oxygen Dioxygen oxidant Nitric oxide intermediate Nitrosyl radical (NO) Nitrogen dioxide intermediate Nitrogen dioxide Nitric acid Nitric acid product Ammonia is oxidized to NO over Pt–Rh at high temperature, NO is oxidized to NO2, and NO2 is absorbed in water (with air) to give nitric acid; NO is recycled. ≈ 1100 K; short contact time to maximize NO selectivity NH3 oxidation over Pt–Rh gauze to NO and H2O. NO oxidizes to NO2 in air on cooling. Gas-phase oxidation; lower temperature favours NO2 formation Efficient gas–liquid contact; tail-gas NOx recovery NO2 is absorbed in water (with O2) in packed towers to produce HNO3; NO is released and recycled. Ostwald process (3-stage industrial route to HNO3) Ostwald process: (1) NH3 → NO on Pt–Rh at high T, (2) NO → NO2 in air, (3) absorption to HNO3 with NO recycle. Strong oxidizing agent: oxidizes non-metals and many metals; brown fumes of NO2 may appear Nitrating agent for aromatics (with H2SO4): important for explosives and dyes Fertilizer manufacturing (ammonium nitrate), pickling of metals HNO3 — properties and uses neet-alert Identify brown gas: NO2 is reddish-brown — commonly seen during nitration and in Ostwald tail gases. Analytical: the brown-ring test for nitrate (NO3−) Procedure: Add freshly prepared FeSO4 solution to the test solution, then carefully layer concentrated H2SO4 down the side of the test tube. A thin brown ring at the junction indicates nitrate. Explanation: nitrate reduces to NO, which forms a nitrosyl complex with Fe 2+ , [Fe(H2O)5(NO)] 2+ , responsible for the brown ring. gpt-image-2 2026-05-26T17:04:57.381Z Brown-ring test setup: layered FeSO4 solution and conc. H2SO4; thin brown ring forms at the interface if NO3− is present. Test-tube diagram with aqueous FeSO4 on top, carefully added conc. H2SO4 below; label brown ring at interface and complex [Fe(H2O)5(NO)]2+. Clean vector lab schematic, white background, minimal color. Phosphorus oxoacids: formula, basicity, oxidation state Key exam set: recognize structures, count ionizable OH groups (basicity), and deduce oxidation state of phosphorus. Remember: only hydrogens attached to oxygen (–OH) are acidic; P–H hydrogens are not. Acid Formula Basicity (no. of ionizable H+) Oxidation state of P Selected phosphorus oxoacids Phosphorous acid H3PO3 Dibasic (2) +3 Phosphoric acid H3PO4 Tribasic (3) +5 Pyrophosphoric acid H4P2O7 Tetrabasic (4) +5 (each P) Metaphosphoric acid HPO3 (monomeric unit) Monobasic (1) +5 All phosphorus oxoacids are tribasic because they contain multiple hydrogens. Basicity depends on the number of –OH hydrogens. H3PO3 is dibasic (structure HP(=O)(OH)2 has 2 acidic OH groups). H3PO4 is tribasic. Phosphorus halides — quick applications (NEET favorites) Uses of PCl3 and PCl5 PCl3: precursor to phosphites and organophosphorus compounds; chlorinating agent; converts –OH groups to –Cl in certain substrates. PCl5: stronger chlorinating agent; converts alcohols and acids to alkyl and acyl chlorides (R–OH → R–Cl; R–COOH → R–COCl). Cross-link preview: Contact process (Group 16, H2SO4) — coming next Though covered in the next unit, many students mix up catalysts and conditions. Keep in mind: Contact process uses V2O5 at about 720 K and low pressure to convert SO2 to SO3, then absorbs SO3 in conc. H2SO4 to form oleum. Key equilibrium in Contact process Oleum formation Preview (Contact process): multi-bed V2O5 converter, heat exchangers, and absorption tower forming oleum. Full details in NTCH10/04. Students often confuse the conditions that favor a high reaction rate with those that favor a high equilibrium yield, especially for exothermic reversible reactions where high temperature increases rate but decreases yield. For exothermic reversible reactions (e.g., Haber), increasing temperature speeds the reaction but reduces equilibrium yield; high pressure and suitable catalysts are used to balance rate and yield. Haber: iron-based catalyst with promoters (K2O, Al2O3; Mo as promoter). Ostwald: Pt–Rh gauze. Contact: V2O5. Keep them distinct. Incorrectly identifying the specific catalysts used in major industrial processes (e.g., confusing the catalyst for the Haber process with that of the Contact process). Manufacture of ammonia is a continuous loop process. Industrial-scale NH3 via Haber–Bosch with gas purification, compression, Fe-catalyzed synthesis, and recycle loop. Manufacture of nitric acid integrates chemistry and gas handling. Ostwald process: catalytic oxidation of NH3 to NO, oxidation to NO2, and absorption to HNO3 with NOx recycle. pnictogens Group 15 (nitrogen family) Elements with valence configuration ns 2 np 3 : N, P, As, Sb, Bi. Haber–Bosch Haber process Synthesis of NH3 from N2 and H2 over iron catalyst at high P and moderate T. Oxidation of NH3 to HNO3 via NO and NO2 intermediates using Pt–Rh catalyst and absorption towers. Ostwald process N2, an inert diatomic gas with a strong triple bond. Dinitrogen NH3, a Lewis base that forms ammine complexes. Ammonia Nitric acid HNO3, strong oxidizing and nitrating agent. P4 tetrahedral molecules; very reactive and toxic. White phosphorus Red phosphorus Polymeric allotrope; stable and less reactive. Black phosphorus Layered, most stable allotrope; a semiconductor. Qualitative test for nitrate based on [Fe(H2O)5(NO)] 2+ formation. Brown-ring test Glossary — Group 15 and industrial key terms