Nitrogen and Phosphorus

Nitrogen

Nitrogen is in Group V. Air contains 78% nitrogen.

In the N₂ molecule the triple bond (N≡N) is strong, so N₂ has low chemical reactivity (it is inert).

Oxidation states of nitrogen: from −3 to +5.

Ammonia

Ammonia NH₃ dissolves well in water: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ (weakly alkaline medium).

• In industry: N₂ + 3H₂ ⇌ 2NH₃ (Haber process).
• In the laboratory: 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃ + 2H₂O.
• Qualitative test for NH₄⁺: when heated with an alkali, the sharp smell of NH₃ is released.

Nitric acid (Ostwald process)

1. NH₃ (ammonia)
2. With O₂: 4NH₃ + 5O₂ → 4NO + 6H₂O
3. NO with O₂: 2NO + O₂ → 2NO₂
4. With water: 3NO₂ + H₂O → 2HNO₃ + NO

• NO is a colorless gas, NO₂ is a brown gas.
• HNO₃ is a strong oxidizing agent: Cu + 4HNO₃(conc.) → Cu(NO₃)₂ + 2NO₂ + 2H₂O.
• Cold concentrated HNO₃ passivates Al, Fe, Cr.

Phosphorus

Allotropes: white, red and black.

• White phosphorus is stored under water. It is poisonous and ignites spontaneously in air.
• P₂O₅ + 3H₂O → 2H₃PO₄.
• Qualitative test for PO₄³⁻: a yellow precipitate of Ag₃PO₄ with AgNO₃.

Fertilizers

• Nitrogen: NH₄NO₃, urea (NH₂)₂CO (≈46% N).
• Phosphorus: superphosphate Ca(H₂PO₄)₂.
• Potassium: KCl.