JEE Chemistry The P Block Elements Notes Set 07

Group 15: Properties and Trends

1. The group 15 elements are N, P, As, Sb and Bi with the configuration ns²np³. Nitrogen and phosphorus are non-metals, arsenic and antimony are metalloids and bismuth is a metal.
2. Common oxidation states are −3, +3 and +5. Because of the inert pair effect, the +3 state becomes more stable down the group and Bi(III) is much more stable than Bi(V).
3. Nitrogen differs from the rest because of its small size, high electronegativity, absence of d orbitals and ability to form pπ-pπ multiple bonds. It exists as N₂ with a triple bond, while the other elements are tetra-atomic or polymeric solids.
4. Reactivity towards hydrogen: all form EH₃. Stability, basic character and the bond angle fall from NH₃ to BiH₃, while reducing character rises. Bond angles are NH₃ 107, PH₃ 93.5, AsH₃ 91, SbH₃ 91 degrees.
5. Nitrogen cannot form pentahalides because it has no d orbitals, but P, As and Sb do (PCl₅ is trigonal bipyramidal).

Nitrogen and Ammonia

1. Nitrogen is made in the laboratory by heating ammonium nitrite (NH₄NO₂ → N₂ + 2H₂O) or ammonium dichromate. Very pure N₂ comes from heating sodium or barium azide: Ba(N₃)₂ → Ba + 3N₂.
2. N₂ is inert at room temperature because of the strong N≡N bond (946 kJ per mole). At high temperature it forms nitrides, for example CaC₂ + N₂ → CaCN₂ + C (nitrolim). Liquid nitrogen is a cryogenic refrigerant.
3. Ammonia is made in the laboratory from NH₄Cl and Ca(OH)₂, and industrially by the Haber process N₂ + 3H₂ ⇌ 2NH₃ at about 200 atm and 700 K with an iron catalyst promoted by Mo.
4. Ammonia is a pyramidal, polar, basic gas with a pungent smell. It is very soluble in water, forms NH₄Cl fumes with HCl, gives a deep blue complex [Cu(NH₃)₄]²⁺ with Cu²⁺, and gives a reddish brown precipitate or colour with Nessler's reagent (alkaline K₂HgI₄).
5. NH₃ is dried with quicklime (CaO) and not with conc. H₂SO₄, P₄O₁₀ or anhydrous CaCl₂. Ammonia is oxidised by O₂ over Pt at 1100 K to give NO (Ostwald's process).
6. With chlorine: 8NH₃ + 3Cl₂ → 6NH₄Cl + N₂ (excess NH₃), and NH₃ + 3Cl₂ → NCl₃ + 3HCl (excess Cl₂).

Oxides of Nitrogen and Nitric Acid

1. N₂O (+1, laughing gas, neutral, used as an anaesthetic), NO (+2, neutral, colourless), N₂O₃ (+3, blue solid, anhydride of HNO₂), NO₂ (+4, brown, odd-electron, dimerises to N₂O₄), N₂O₅ (+5, solid, anhydride of HNO₃).
2. NO₂ is paramagnetic as a gas (odd electron) and the dimer N₂O₄ is diamagnetic. The equilibrium 2NO₂ ⇌ N₂O₄ is exothermic, so heating deepens the brown colour.
3. Nitric acid is made by the Ostwald process: 4NH₃ + 5O₂ → 4NO + 6H₂O (Pt, 1100 K), 2NO + O₂ → 2NO₂ and 3NO₂ + H₂O → 2HNO₃ + NO.
4. Reactions of HNO₃: Cu + 4HNO₃(conc.) → Cu(NO₃)₂ + 2NO₂ + 2H₂O, and 3Cu + 8HNO₃(dil.) → 3Cu(NO₃)₂ + 2NO + 4H₂O. Zn gives N₂O with 6 percent HNO₃ and NH₄NO₃ with very dilute acid.
5. Non-metals are oxidised by conc. HNO₃ to the highest oxyacids: I₂ → HIO₃, S₈ → H₂SO₄, P₄ → H₃PO₄, C → CO₂ and B → H₃BO₃, with NO₂ formed.
6. Fe, Al and Cr are made passive by conc. HNO₃ because of a protective oxide film, and Au and Pt dissolve only in aqua regia (3 parts HCl to 1 part HNO₃).

Phosphorus and Its Compounds

1. White phosphorus is P₄ with bond angle 60 degrees and large ring strain. It glows in the dark due to slow oxidation, ignites at about 303 K and is stored under water. Heating at 573 K in an inert atmosphere gives red phosphorus, and heating under pressure gives black phosphorus.
2. Phosphine PH₃ is made by PH₄I + KOH → PH₃ + KI + H₂O or by boiling white P with NaOH: P₄ + 3NaOH + 3H₂O → PH₃ + 3NaH₂PO₂. It has a smell like rotten fish, is poisonous and a weaker base than NH₃, and impure PH₃ contains P₂H₄, which makes it ignite spontaneously (smoky rings of P₂O₅).
3. Oxides: P₄O₆ (burning P in limited O₂) and P₄O₁₀ (excess O₂). In P₄O₁₀ each P is sp³ with a terminal P=O, there are six bridging P–O–P linkages (bent), and it is a powerful dehydrating agent.
4. PCl₃ is prepared from white P and dry Cl₂ (P₄ + 6Cl₂ → 4PCl₃) and PCl₅ with excess Cl₂. PCl₃ is pyramidal and PCl₅ is trigonal bipyramidal in the gas phase but ionic [PCl₄]⁺[PCl₆]⁻ in the solid.
5. Phosphorus oxyacids: H₃PO₂ (hypophosphorous, monobasic, strong reducing agent), H₃PO₃ (phosphorous, dibasic, +3), H₃PO₄ (orthophosphoric, tribasic, +5), H₄P₂O₇ (pyrophosphoric, +5), HPO₃ (metaphosphoric, +5, cyclic trimer (HPO₃)₃), H₄P₂O₆ (hypophosphoric, +4) and H₄P₂O₅ (pyrophosphorous, +3).