School curriculum· 30 min
Learn first: Non-metals and their compounds
The Group VA elements (pnictogens) are nitrogen N, phosphorus P, arsenic As, antimony Sb and bismuth Bi. Going down the group, non-metallic character weakens and metallic character grows: nitrogen and phosphorus are typical non-metals, while bismuth is a metal. Nitrogen makes up 78 % of air by volume and is part of proteins and nucleic acids. Phosphorus is needed for bones and teeth (calcium phosphate), ATP and DNA. Compounds of both elements are the basis of the fertiliser industry: in Uzbekistan, plants in Chirchiq, Navoiy and Olmaliq produce nitrogen and phosphate fertilisers. Exam problems on nitric acid with metals, the decomposition of nitrates and the salts of phosphoric acid are especially common.
Valence electrons ns² np³; nitrogen’s oxidation states run from −3 to +5: NH₃ (−3), N₂H₄ (−2), N₂ (0), N₂O (+1), NO (+2), HNO₂ (+3), NO₂ (+4), HNO₃ (+5)
Nitrogen has no d-orbitals, so its valency cannot exceed IV (in HNO₃ nitrogen has valency IV but oxidation state +5). Phosphorus has empty 3d-orbitals and can be pentavalent.
The N₂ molecule has a triple bond (N≡N: one σ- and two π-bonds), which is very strong (about 945 kJ/mol), so nitrogen is inert under ordinary conditions and reacts only with lithium (6Li + N₂ → 2Li₃N). When heated it forms nitrides with magnesium and calcium and ammonia with hydrogen, and in an electric discharge it forms NO with oxygen (N₂ + O₂ ⇌ 2NO, as in lightning).
Preparation: industrially, by fractional distillation of liquid air; in the laboratory, by heating ammonium nitrite: NH₄NO₂ → N₂ + 2H₂O.
Synthesis of ammonia (Haber process):
N₂ + 3H₂ ⇌ 2NH₃ + Q (Fe catalyst, ≈ 450 °C, 200–300 atm)
The reaction is exothermic and the volume of gas decreases, so by Le Chatelier’s principle high pressure shifts the equilibrium towards ammonia. At too low a temperature the reaction is slow, so a compromise temperature is chosen. Hydrogen comes from natural gas: CH₄ + H₂O → CO + 3H₂.
Properties of ammonia: a colourless gas with a sharp smell, less dense than air and extremely soluble in water (about 700 volumes in 1 volume of water). Nitrogen has a lone pair, so ammonia is a base:
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻; NH₃ + HCl → NH₄Cl (white smoke)
Nitrogen is in the −3 state, so ammonia is a reducing agent: 4NH₃ + 3O₂ → 2N₂ + 6H₂O (combustion), and catalytic oxidation 4NH₃ + 5O₂ → 4NO + 6H₂O (Pt) is the first stage in making nitric acid.
In the laboratory ammonia is made by heating an ammonium salt with an alkali: 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2NH₃ + 2H₂O. This is also the test for the ammonium ion: the smell of ammonia, and damp red litmus turns blue. Ammonium salts decompose on heating: NH₄Cl → NH₃ + HCl, NH₄NO₃ → N₂O + 2H₂O.
10.7 g of ammonium chloride was heated with excess calcium hydroxide. What volume of ammonia (STP) is released?
N₂O and NO are neutral (non-salt-forming) oxides; NO turns at once into brown NO₂ in air: 2NO + O₂ → 2NO₂. NO₂ is a toxic brown gas that gives nitric acid with water and oxygen.
Industrial nitric acid: NH₃ → NO → NO₂ → HNO₃ (4NO₂ + O₂ + 2H₂O → 4HNO₃).
Nitric acid is a strong acid and a strong oxidising agent (nitrogen +5). With metals it never releases `H₂`: the oxidising agent is the nitrogen, not the hydrogen. The product depends on the acid’s concentration and the metal’s reactivity:
Conc. HNO₃ + less reactive metal → NO₂ (brown gas): Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O
Dilute + less reactive metal → NO: 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 4H₂O
Very dilute + reactive metal → NH₄NO₃: 4Zn + 10HNO₃ → 4Zn(NO₃)₂ + NH₄NO₃ + 3H₂O
Cold conc. HNO₃ passivates Fe, Al and Cr (a dense oxide film forms), so it is transported in iron or aluminium tanks. Au and Pt dissolve only in aqua regia (HNO₃ : HCl = 1 : 3 by volume). When writing equations remember that some of the acid acts as the oxidising agent and some forms the salt: making NO from copper, only 2 of the 8 mol of acid are reduced.
12.8 g of copper dissolved in dilute nitric acid. What volume of NO (STP) is released?
On heating, nitrates decompose in three ways depending on the metal’s position in the reactivity series:
Ammonium nitrate is a special case: NH₄NO₃ → N₂O + 2H₂O. Because nitrates release oxygen on heating, they are strong oxidising agents; potassium nitrate is part of black gunpowder.
Allotropes of phosphorus: white phosphorus (P₄ molecules, highly toxic, catches fire spontaneously in air, glows in the dark, stored under water), red phosphorus (polymeric, stable, non-toxic, used on matchboxes) and black phosphorus (graphite-like, a semiconductor). Preparation: Ca₃(PO₄)₂ + 3SiO₂ + 5C → 3CaSiO₃ + 2P + 5CO (electric furnace).
4P + 5O₂ → 2P₂O₅; P₂O₅ + 3H₂O → 2H₃PO₄; 2P + 3Ca → Ca₃P₂ (a phosphide)
P₂O₅ is a powerful drying agent. H₃PO₄ is a moderately strong, tribasic acid with no oxidising properties; it forms three series of salts:
NaOH : H₃PO₄ = 1 : 1 → NaH₂PO₄; 2 : 1 → Na₂HPO₄; 3 : 1 → Na₃PO₄
At intermediate ratios a mixture of two salts forms. The test for the phosphate ion: 3Ag⁺ + PO₄³⁻ → Ag₃PO₄↓ (yellow precipitate).
In what mole ratio must KOH and H₃PO₄ react so that the salt formed has an equivalent mass of 87?
Fertiliser quality is usually stated as the mass fraction of N, P₂O₅ or K₂O respectively.
4.96 g of phosphorus was burnt and the P₂O₅ absorbed in a solution containing 0.48 mol of NaOH. Which salt forms?