Mastering Group 15 Nitrogen Family for JEE and NEET

Group 15, the nitrogen family (N, P, As, Sb, Bi), runs from inert nitrogen gas to metallic bismuth. Its headliners are ammonia, the oxides of nitrogen, nitric acid, and the allotropes of phosphorus.

The group matters because it contains the fertilizer industry (ammonia, nitric acid, phosphoric acid), the chemistry of explosives, and some of the most-asked oxidation-state and preparation questions in exams.

Key Concepts

• ns2 np3 configuration: 5 valence electrons; nitrogen is limited to covalence 4 (no d orbitals), while phosphorus onwards expand to 5 and 6 (PCl5, SF6 analogues like PF5).
• Oxidation states: nitrogen spans -3 (NH3) to +5 (HNO3); the +3 and +5 states dominate phosphorus chemistry (H3PO3, H3PO4).
• Ammonia: NH3, pyramidal, basic, hydrogen-bonded; made industrially by the Haber process.
• Oxides of nitrogen: N2O (+1), NO (+2), N2O3 (+3), NO2 (+4), N2O5 (+5); NO and NO2 are the key atmospheric pollutants.
• Phosphorus allotropes: white phosphorus (P4 tetrahedra, ignites near 30 C, stored under water), red phosphorus (polymeric, safer), black phosphorus (most stable, layered).

Formulas

• Haber process: N2 + 3 H2 ⇌ 2 NH3, delta H = -92 kJ/mol. Conditions: 450 C, 200 atm, iron catalyst with molybdenum promoter. Low temperature favours yield (exothermic) but high temperature is needed for speed: a compromise.
• Ostwald process (nitric acid): 4 NH3 + 5 O2 -> 4 NO + 6 H2O (Pt gauze, 900 C); 2 NO + O2 -> 2 NO2; 3 NO2 + H2O -> 2 HNO3 + NO.
• Phosphoric acid: H3PO4 is tribasic (3 replaceable H), made from phosphate rock: Ca3(PO4)2 + 3 H2SO4 -> 2 H3PO4 + 3 CaSO4.
• Oxidation-state check: in HNO3, H is +1 and O is -2 x 3 = -6, so N = +5; in NO2, N = +4; in NH3, N = -3.

Worked Example

Question: What mass of ammonia is theoretically obtained from 28 g of nitrogen in the Haber process?

Step 1: Equation: N2 + 3 H2 ⇌ 2 NH3. Molar mass of N2 = 28 g/mol, so 28 g = 1 mol of N2.

Step 2: 1 mol N2 gives 2 mol NH3 by the 1:2 mole ratio.

Step 3: Molar mass of NH3 = 17 g/mol, so 2 mol = 34 g of ammonia.

Step 4: Real plants get less (equilibrium limits conversion to about 15 percent per pass; unreacted gases are recycled), but 34 g is the theoretical maximum from 28 g of N2.

Common Misconceptions

• "Nitrogen is unreactive because it is a noble gas." N2 is inert because of the 945 kJ/mol triple bond, not a full shell; once split, nitrogen reacts vigorously.
• "All nitrogen oxides are acidic." N2O and NO are neutral oxides; only N2O3, NO2, and N2O5 form acids with water.
• "White and red phosphorus are just colours." They are distinct allotropes: white P4 is molecular and pyrophoric, red is polymeric and stable enough for matchboxes.

Real-World Applications

• Fertilizers: ammonia, urea, nitric acid (for nitrates), and phosphoric acid (for phosphates) feed roughly half the world's population.
• Explosives and propellants: TNT, nitroglycerin, and ammonium nitrate chemistry all start from nitric acid.
• Red phosphorus strikes safety matches; phosphorus compounds make detergents, flame retardants, and steel additives.
• Bismuth subsalicylate treats digestive upsets; arsenic and antimony compounds appear in semiconductors (GaAs) and flame retardants.

Group 15 spans nitrogen's triple-bonded inertness to bismuth's metallic character, unified by the ns2 np3 configuration. Master the Haber and Ostwald conditions, the nitrogen oxide oxidation states, and the phosphorus allotropes, and the group's long question list collapses into a few patterns.