Group 15 Nitrogen Group Group 15—The Nitrogen Group • Nitrogen and phosphorus are required by living things and are used to manufacture various items. • These elements also are parts of the biological materials that store genetic information and energy in living organisms. The nitrogen cycle Ways nitrogen is lost to the cycle ❖ Most of the nitrogen cycle is soil based. Four ways how nitrogen is lost : I. Denitrification Bacteria change nitrate in the soil to atmospheric nitrogen II. Volatilization Turns urea fertilizers and manures on the soil surface into gases III. Runoff Carries the nitrogen in fertilizers and manure and the nitrogen in the soil into our rivers and streams IV. Leaching Carries nitrates so deep into the soil that plants can no longer use them, producing a dual concern — for lost fertility and for water quality, as nitrates enter the groundwater and the wells that provide our drinking water. Physical Properties • All the elements of this group are polyatomic • Dinitrogen is a diatomic gas while all others are solids • Metallic character increases down the group. • Nitrogen and phosphorus are non-metals, arsenic and antimony are metalloids and bismuth is a metal. Due to:✓ (i) decrease in ionisation enthalpy ✓ (ii) increase in atomic size. • The boiling points, in general, increase from top to bottom in the group but the melting point increases up to arsenic and then decreases up to bismuth • Except nitrogen, all the elements show allotropy Preparation of dinitrogen (N2) • In the laboratory, dinitrogen is prepared by treating an aqueous solution of ammonium chloride with sodium nitrite. NH4CI(aq) + NaNO2 (aq) → N2 (g) + 2 H2O (l) + NaCl (aq) • It can be obtained by the thermal decomposition of ammonium dichromate. (NH4)2Cr2O7 (Heat) ⎯⎯⎯→ N2 (g) + 4 H2O (l) + Cr2O3 (s) • Very pure nitrogen can be obtained by the thermal decomposition of sodium or barium azide. Ba(N3)2 → Ba + 3 N2 Air (4 N2 + O2) + C → 4 N2 + CO2 NH3 + 3 O2 → 2 N2 + 6 H2O 2 NH3 + 3 Cl2 → N2 + 6 HCl Reactivity towards hydrogen Reactivity towards oxygen All Group 15 elements form hydrides of the type EH3 where E = N, P, As, Sb or Bi. • All these elements form two types of oxides: E2O3 and E2O5 N2(g) + 3 H2(g) (773 k) ==> 2 NH3(g); ΔH = – 46.1 kJ mol–1 • The oxide in the higher oxidation state of the element is more acidic than that of lower oxidation state. Р4 + 6 Н2 (heat, p) ==> 4 РН3 • The stability of hydrides decreases from NH3 to BiH3 • Reducing character of the hydrides increases. • NH3 is only a mild reducing agent while BiH3 is the strongest reducing agent amongst all the hydrides • Basicity also decreases in the order NH3 > PH3 > AsH3 > SbH3 > BiH3 • Their acidic character decreases down the group. The oxides of the type E2O3 of nitrogen and phosphorus are purely acidic N2 (g) + O2 (g) (heat) ==> 2 NO (g) P4 + 5 O2 (heat) ==> 2 P4O10 Reactivity towards halogens • These elements react to form two series of halides: EX3 and EX5 • Nitrogen does not form pentahalide. Pentahalides are more covalent than trihalides. Reactivity towards metals • All these elements react with metals to form their binary compounds exhibiting –3 oxidation state. E.g. ‒ Ca3N2 (calcium nitride) • All the trihalides of these elements except those of nitrogen are stable ‒ Ca3P2 (calcium phosphide) • In case of nitrogen, only NF3 is known to be stable. ‒ Zn3Sb2 (zinc antimonide) ‒ Na3As2 (sodium arsenide) ‒ Mg3Bi2 (magnesium bismuthide) P4 + 6 Cl2 ==> 4 PCl3 3 PCl5 + 2 P ==> 5 PCl3 3 PCl5 + P2O5 ==> 5 POCl3 3 Mg + N2 ==> Mg3N2 Mg3N2 + 6 H2O ==> 3 Mg(OH)2 + 2 NH3 Ammonia • Ammonia is present in small quantities in air and soil where it is formed by the decay of nitrogenous organic matter e.g., urea. NH2CONH2 + 2 H2O → (NH4)2CO3 → 2 NH3 + H2O + CO2 • On a small scale ammonia is obtained from ammonium salts which decompose when treated with caustic soda or lime. 2 NH4Cl + Ca(OH)2 → 2 NH3 + 2 H2O + CaCl2 (NH4)2SO4 + 2 NaOH → 2 NH3 + 2 H2O + Na2SO4 • On a large scale, ammonia is manufactured by Haber’s process. N2(g) + 3H2(g) → 2NH3(g); ΔH = – 46.1 kJ mol-1 Phosphine • Phosphine is prepared by the reaction of calcium phosphide with water or dilute HCl Ca3P2 + 6 H2O → 3 Ca(OH)2 + 2 PH3 Ca3P2 + 6 HCl → 3 CaCl2 + 2 PH3 • In the laboratory, it is prepared by heating white phosphorus with conc. NaOH solution in an inert atmosphere of CO2. Р4 + 3 КОН + 3 Н2О → РН3 + 3 КН2РО2 PH4I + KOH → KI + H2O + PH3 (phosphonium iodide) Nitrogen oxides Red phosphorus • It is obtained by heating white phosphorus at 573K in an inert atmosphere for several days. When red phosphorus is heated under high pressure, a series of phases of black phosphorus are formed. • Red phosphorus possesses iron grey lustre. It is odourless, non-poisonous and insoluble in water as well as in carbon disulphide. • Chemically, red phosphorus is much less reactive than white phosphorus. It does not glow in the dark. Black phosphorus • It has two forms α-black phosphorus and β-black phosphorus. • α-Black phosphorus is formed when red phosphorus is heated in a sealed tube at 803K. It can be sublimed in air and has opaque monoclinic or rhombohedral crystals. It does not oxidise in air. • β-Black phosphorus is prepared by heating white phosphorus at 473 K under high pressure. It does not burn in air up to 673 K. Applications of nitrogen compounds • As a modified atmosphere, pure or mixed with carbon dioxide, to preserve the freshness of packaged or bulk foods • Nitrogen can be used instead of CO2 to pressurize kegs of some beers, in particular, stouts and British ales, due to the smaller bubbles it produces, which make the dispensed beer smoother and headier • Liquid nitrogen is used in the cryopreservation of blood, reproductive cells (sperm and egg), and other biological samples. It is used in the clinical setting in cryotherapy to remove cysts and warts on the skin. Applications of nitrogen compounds • Nitrous oxide (N2O), "laughing gas“, was discovered early in the 19th century to be a partial anesthetic, though it was not used as a surgical anesthetic until later. Applications of phosphorous compounds • White phosphorus, called "WP" (slang term "Willie Peter") is used in military applications as incendiary bombs, for smoke-screening as smoke pots and smoke bombs, and in tracer ammunition. • The spontaneous combustion of phosphine is technically used in Holme’s signals. Containers containing calcium carbide and calcium phosphide are pierced and thrown in the sea when the gases evolved burn and serve as a signal. • Phosphine is also used in smoke screens.