2<\/sub> comprises 78% by volume of the atmosphere. N and P are essential constituents of animals and plants. N – Present in proteins, P – Present in bones.<\/p>\nCharacteristics:
\n1. Atomic radii increases with increase in Atomic Number.<\/p>\n
2. Ionisation Enthalpy decreases down the group due to gradual increase in atomic size. Because of the extra stable half-filled p orbitals electronic configuration and smaller size, the ionisation enthalpy is less than that of group 14 elements in the corresponding periods.<\/p>\n
3. Electronegativity decreases down the group.<\/p>\n
Physical Properties:
\nAll are polyatomic, metallic character increases from N to Bi, density increases from N to Bi, M.P. and B.P. increases down the group, except N all other elements show allotropy.<\/p>\n
Chemical Properties:
\nOxidisation states and trends in chemical reactivity:
\nThe common oxidation states of 15 group elements are (-3), (+3) and (+5). The stability of +5 oxidisation state decreases and that of +3 state increases down the group due to inert pair effect. Nitrogen exhibits +1, +2, +4 oxidation states also when it react with O2<\/sub>.<\/p>\nThe maximum covalence of N restricted to 4 since only 4 orbitals (one S and three P) are available for bonding.<\/p>\n
Anomalous Properties of Nitrogen:
\nIt is due to its small size, high electronegativity, high ionisation enthalpy and non-availability of \u2018d\u2019 orbitals. Nitrogen has unique ability to form p\u03c0 – p\u03c0 multiple bond. It cannot form d\u03c0 – p\u03c0 bond. P and A scan form d\u03c0 – d\u03c0 bond.<\/p>\n
(i) Reactivity towards hydrogen:
\nEH3<\/sub> hydrides, the central atom is sp3<\/sup> hybridised, molecules assume trigonal pyramidal geometry with a lone pair on the central atom. Stability-decreases from NH3<\/sub> to BiH3<\/sub>.<\/p>\nThis is because, down the group the E-H bond dissociation enthalpy decreases due to increase in size of the central atom. Consequently, reducing character increases from NH3<\/sub> to BiH3<\/sub>. The basicity decreases in the order NH3<\/sub> > PH3<\/sub> > AsH3<\/sub> > SbH3<\/sub>> BiH3<\/sub>.<\/p>\nAs the electronegativity of the central atom decreases on moving down the group, the bond pair-bond pair repulsion decreases. Hence the bond angle decreases in the order NH3<\/sub> > PH3<\/sub> > AsH3<\/sub>.<\/p>\n(ii) Reactivity towards oxygen:
\nThey form E2<\/sub>O3<\/sub> & E2<\/sub>O5<\/sub> type oxides. The oxide in the higher oxidisation state of the element is more acidic than that in lower oxidation state.<\/p>\n(iii) Reactivity towards halogens:
\nThey form EX3<\/sub> and EX5<\/sub> type halides. Nitrogen does not form pentahalide due to non-availability of d-orbital.<\/p>\n(iv) Reactivity towards Metals:
\nThey react with some metals exhibiting – 3 oxidation state, e.g. Calcium nitrate (Ca3<\/sub>N2<\/sub>), Calcium phosphide (Ca3<\/sub>P2<\/sub>), Sodium arsenide (Na3<\/sub>As2<\/sub>).<\/p>\nDinitrogen (N2<\/sub>):<\/span>
\nIt is produced commercially by the liquefaction and fractional distillation of air.
\nIn laboratory, N2<\/sub> is prepared by<\/p>\nNH4<\/sub>Cl(aq) + NaNO2<\/sub>(aq) \u2192 N2<\/sub>(g) + 2 H2<\/sub>O(l)+ NaCl(aq)
\n
\nProperties:
\nColourless, odourless, non-toxic gas; inert at room temperature because of high bond enthalpy of N \u2261 N.<\/p>\nUses:
\nManufacture of NH3<\/sub>, liquid N2<\/sub> is used as refrigerant to preserve biological materials, food items and in cryosurgery.<\/p>\nAmmonia:<\/span>
\nLaboratory preparation:
\n2NH4<\/sub>Cl + Ca(OH)2<\/sub> \u2192 2NH3<\/sub> + 2H2<\/sub>O\u00a0+ CaCl2<\/sub>
\n(NH4<\/sub>)2<\/sub>SO4<\/sub> + 2NaOH \u2192 2NH3<\/sub> + 2H2<\/sub>O\u00a0+ Na2<\/sub>SO4<\/sub><\/p>\nIndustrial (large scale) preparation by Haber\u2019s process:
\nN2<\/sub>(g) + 3H(g) \u21cc NH3<\/sub>(g); \u0394f<\/sub>H\u24bd<\/sup> = -46.1 kJ\/mol-1<\/sup> Catalyst used earlier- spongy iron with molybdenum promoter. Catalyst used now – iron oxide with small amounts of K2<\/sub>O and Al2<\/sub>O3<\/sub>.<\/p>\nHigh pressure and low temperature will favour the formation of NH3<\/sub> as the forward reaction is exothermic and is accompanied by decrease in number of moles (Le Chatelier\u2019s principle). Hence, a pressure of 200 \u00d7 105<\/sup> Pa (about 200 atm) and a temperature of ~ 700 K are employed to increase the yield of NH3<\/sub>.<\/p>\nProperties:
\nColourless, pungent smelling gas, trigonal pyramidal geometry, highly soluble in water.
\nNH3<\/sub>(g) + H2<\/sub>O(l) \\(\\rightleftharpoons\\) NH+<\/sup>4<\/sub>(aq) + OH–<\/sup>(aq)
\nLewis base – due to the presence of a lone pair of electrons on N. It can form complex compounds with metal ions. This finds application in the detection of
\nCu2+<\/sup> and Ag+<\/sup>.<\/p>\nUses:
\nTo produce various nitrogeneous fertilizers, manufacture of inorganic nitrogen compounds (e.g. HNO3<\/sub>), liquid NH3<\/sub> is used as a refrigerant.<\/p>\nOxides of Nitrogen:<\/span><\/p>\n\n- Dinitrogen oxide (N2<\/sub>O) or laughing gas – Oxdation state (+1) – Colourless gas, neutral.<\/li>\n
- Nitrogen monoxide(NO) – Oxdation state (+2) colourless gas, neutral.<\/li>\n
- Dinitrogen Trioxide(N2<\/sub>O3<\/sub>) – Oxdation state (+3), blue solid, acidic in nature.<\/li>\n
- Nitrogen dioxide(NO2<\/sub>) – Oxdation state (+4) brown gas, acidic. It contains odd number of valence electrons. On dimerisation, it is converted to stable N2<\/sub>O4<\/sub> molecule with even number of electrons.<\/li>\n
- Dinitrogen tetroxide(N2<\/sub>O4<\/sub>) – Dimer of NO2<\/sub> – Oxdation state (+4), colourless solid\/liquid, acidic.<\/li>\n
- Dinitrogen pentoxide (N2<\/sub>O5<\/sub>) – Oxdation state (+5), colourless solid, acidic.<\/li>\n<\/ol>\n
<\/p>\n
Nitric Acid:<\/span>
\nIt is the most important oxoacid of N.
\nLaboratory preparation:
\nKNO3<\/sub>\/NaNO3<\/sub> + H2<\/sub>SO4<\/sub>(conc.) \u2192 KHSO4<\/sub>\/NaHSO4<\/sub> + HNO3<\/sub>
\nIndustrial preparation – Ostwald\u2019s process:
\n(1) NH3<\/sub> oxidised to NO by air.
\n<\/p>\n(2) NO is converted to NO2<\/sub>
\n2NO(g) + O2<\/sub>(g) \u21c4 2NO2<\/sub>(g)<\/p>\n(3) NO2<\/sub> dissolved in water to give HNO3<\/sub>
\n3NO2<\/sub>(g) + H2<\/sub>O(l) \u2192 2HNO3<\/sub> (aq) + NO(g)<\/p>\nProperties:
\nColourless liquid, strong acid in aqueous solution. Concentrated HNO3<\/sub> is a strong oxidising agent and attacks most metals except noble metals like Au and Pt. The products of oxidation depend upon the concentration of the acid, temperature and the nature of the material undergoing oxidation, e.g.<\/p>\n\n- 3Cu + 8HNO3<\/sub>(dilute) \u2192 3Cu(NO3<\/sub>)2<\/sub> + 2NO + 4H2<\/sub>O<\/li>\n
- Cu + 4HNO3<\/sub>(conc.) \u2192 CuCu(NO3<\/sub>)2<\/sub> + 2NO2<\/sub> + 2H2<\/sub>O<\/li>\n
- 4Zn + 10HNO3<\/sub>(dilute) \u2192 4Zn(NO3<\/sub>)2<\/sub> + 5H2<\/sub>O + N2<\/sub>O<\/li>\n
- Zn + 4HNO3<\/sub>(conc.) \u2192 Zn(NO3<\/sub>)2<\/sub> + 2H2<\/sub>O + 2N2<\/sub>O<\/li>\n<\/ul>\n
Some metals (e.g., Cr, Al) do not dissolve in concentrated nitric acid because of the formation of a passive film of oxide on the surface.<\/p>\n
Structure:
\nIn the gaseous state, HNO3<\/sub>\u00a0exists as a planar molecule.
\n<\/p>\nUses:
\nManufacture ammonium nitrate (fertilizer), preparation of explosives, preparation of nitroglycerine, pickling of stainless steel, etching of metals, oxidiser in rocket fuels.<\/p>\n
Phosphorus:<\/span>
\nAllotropic forms – White P, red P and black P
\n<\/p>\nWhite Phosphorus:
\nTransient white waxy solid, poisonous, insoluble in water, soluble in CS2<\/sub>, glows in dark (chemiluminescence), kept underwater, less stable and therefore more reactive than other solid phases under normal conditions because of angular strain in discrete tetrahedral P4<\/sub> molecules (angle 60\u00b0), readily catches fire in air and gives dense while fumes of P4<\/sub>O10<\/sub>.
\nP4<\/sub> + 5O2<\/sub> \u2192 P4<\/sub>O10<\/sub><\/p>\nRed Phosphorus:
\nObtained by heating white P at 573 K in an inert atm for several days, possesses iron grey lustre, odourless, non-poisonous, less reactive than white P, does not glow in dark, polymeric consisting of chains of P4<\/sub> tetrahedra.
\n<\/p>\nBlack Phosphorus:
\nObtained when red P is heated under high pressure, two forms \u03b1 – black phosphorus (formed when red P is heated in a sealed tube at 803 K) and \u03b2 – black phosphorus (prepared by heating white P at 473 K under high pressure).<\/p>\n
Phosphine (PH3<\/sub>):<\/span>
\nPrepared by the reaction of calcium phosphide with water or dilute HCl.
\nCa3<\/sub>P2<\/sub> + 6H2<\/sub>O \u2192 3Ca(OH)2<\/sub> + 2PH3<\/sub>
\nCa3<\/sub>P2<\/sub> + 6HCl \u2192 3CaCl2<\/sub> + 2PH3<\/sub><\/p>\nLaboratory preparation:
\nBy heating white P with concentrated NaOH solution in an inert atmosphere of CO2<\/sub>.
\nP4<\/sub> + 3NaOH + 3H2<\/sub>O \u2192 PH3<\/sub> + 3NaH2<\/sub>PO2<\/sub><\/p>\nProperties:
\nColourless gas with a rotten fishy smell, highly poisonous, weakly basic, the structure is similar to NH3<\/sub> and gives phosphonium compounds with
\nacids. PH3<\/sub> + HBr \u2192 PH4<\/sub>Br
\nUses: in Holme\u2019s signals, in smoke screens.<\/p>\nPhosphorus Halides:<\/span>
\nIt forms two types of halids PX3<\/sub> and PX5<\/sub> (X = F, Cl, Br)<\/p>\nPhosphorus Trichloride (PCl3<\/sub>):
\nObtained by passing dry Cl2<\/sub> overheated white P.
\nP4<\/sub> + 6Cl2<\/sub> \u2192 4PCl3<\/sub><\/p>\nOr, by the action of thionyl chloride on white P,
\nP4<\/sub> + 8SOCl2<\/sub> \u2192 4PCl3<\/sub> + 4SO2<\/sub> + 2S2<\/sub>Cl2<\/sub><\/p>\nProperties
\nColourless oily liquid, hydrolyses in the presence of moisture giving fumes of HCl.
\nP4<\/sub> + 3H2<\/sub>O \u2192 H3<\/sub>PO3<\/sub> + 3HCl
\nIt has pyrimidal shape and P is sp3 hybridised.
\n<\/p>\nPhosphorus Pentachloride (PCl5<\/sub>):
\nPreparation:
\nWhite P4<\/sub> + 10Cl2(dry)<\/sub> \u2192 4PCl5<\/sub><\/p>\nProperties:
\nyellowish white powder. In moist air it hydrolysed giving POCl3<\/sub> and finally gets converted to phosphoric acid (H3<\/sub>PO4<\/sub>)
\nPCl5<\/sub> + H2<\/sub>O \u2192 POCl3<\/sub> + 2HCl
\nPOCl3<\/sub> + 3 H