G-15 contains 5 elements: Group 15 contains nitrogen, phosphorus, arsenic, antimony and bismuth. The electronic configurations are given in table :- Element Symbol Elec. Conf. Nitrogen 7N [He]2s22p3 Phosphorous 15P [Ne]3s23p3 Arsenic 33As [Ar]3d104s24p3 Antimony 51Sb [Kr]4d105s25p3 Bismuth 83Bi [Xe]4f145d106s26p3 General properties of G-15 are listed below: - 1. Electronic Configuration:-The valence shell electronic configuration of these elements is ns2np3. The s orbital in these elements is completely filled and p orbitals are half-filled, making their electronic configuration extra stable. 2. Atomic and Ionic Radii: - Covalent and ionic (in a particular state) radii increase in size down the group. 3. Ionisation Enthalpy: - Ionisation enthalpy decreases down the group due to gradual increase in atomic size. 4. Electro negativity: - The electro negativity value, in general, decreases down the group with increasing atomic size. 5. 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 metalloids and bismuth is a metal. 6. Chemical Properties:-The common oxidation states of these elements are –3, +3 and +5. 7. Anomalous properties of nitrogen Nitrogen differs from the rest of the members of this group due to its smaller size, high electro negativity, high ionisation enthalpy and non-availability of d orbitals. 8. Reactivity towards hydrogen: All the elements of Group 15 form hydrides of the type EH3 where E = N, P, As, Sb or Bi. Some of the properties of these hydrides are shown in Table Prop NH3 PH3 AsH3 SbH3 BiH3 M.p/K 195.2 139.5 156.7 185 – B p/K 238.5 185.5 210.6 254.6 290 (E–H) Distance/pm 101.7 141.9 151.9 170.7 – HEH angle (°) 107.8 93.6 91.8 91.3 – Δf HV/kJ mol–1 –46.1 13.4 66.4 145.1 278 ΔdissHV (E–H)/kJ mol–1 389 322 297 255 – Note:- Thermal stability increases from NH3 to BiH3 Bond strength decreases from NH3 to BiH3 Bond energy decreases from NH3 to BiH3 Reducing properties increases fromNH3 to BiH3 Bond angle decreases from NH3 to BiH3 BP increases from PH3 to BiH3 9. Reactivity towards oxygen:- All these elements form two types of oxides: E2O3 and E2O5. The oxide in the higher oxidation state of the element is more acidic than that of lower oxidation state. Their acidic character decreases down the group. The oxides of the type E2O3 of nitrogen and phosphorus are purely acidic, that of arsenic and antimony amphoteric and those of bismuth is predominantly basic. 10. Reactivity towards halogens:-These elements react to form two series of halides: EX3 and EX5. Nitrogen does not form pentahalide due to non-availability of the d orbitals in its valence shell. Pentahalides are more covalent than trihalides. All the trihalides of these elements except those of nitrogen are stable. In case of nitrogen, only NF3 is known to be stable. Trihalides except BiF3 are predominantly covalent in nature. 11. Reactivity towards metals:-All these elements react with metals to form their binary compounds exhibiting –3 oxidation state, such as, Ca3N2 (calcium nitride) Ca3P2 (calcium phosphide), Na3As2 (sodium arsenide), Zn3Sb2 (zinc antimonide) and Mg3Bi2 (magnesium bismuthide). Dinitrogen (N2) Methods of preparation of N2:- 1).Laboratory method: - In the laboratory, dinitrogen is prepared by treating an aqueous solution of ammonium chloride with sodium nitrite. NH4CI (aq) + NaNO2 (aq) → N2 (g) + 2H2O (l) + NaCl (aq) 2). It can also be obtained by the thermal decomposition of ammonium dichromate. (NH4)2Cr2O7 → N2 + 4H2O + Cr2O3 3).Very pure nitrogen can be obtained by the thermal decomposition of sodium or barium azide. Ba (N3)2 → Ba + 3N2 Properties Dinitrogen is a colourless, odourless, tasteless and non-toxic gas. Dinitrogen is rather inert at room temperature because of the high bond enthalpy of N≡N bond. Reactivity, At higher temperatures, it directly combines with some metals to form ionic nitrides and with non-metals, covalent nitrides. A few typical reactions are: 6Li + N2 → 2Li3N 3Mg + N2 → Mg3N2 It combines with hydrogen at about 773 K in the presence of a catalyst (Haber’s Process) to form ammonia: N2 (g) + 3H2 (g) ⇌2NH3 (g); ΔfH = – 46.1 kJ mol–1 Dinitrogen combines with dioxygen only at very high temperature (at about 2000 K) to form nitric oxide, N2 (g) + O2 (g)⇌ 2NO (g) Ammonia (NH3) Methods of preparation of NH3:- Ammonia is prepared by decay of Urea. NH2 CONH2 2H2 O → 2NH3 + H2O 2).On small scale ammonia is obtained from ammonium salts which decompose when treated with caustic soda or lime. 2NH4Cl + Ca (OH) 2 → 2NH3 + 2H2O + CaCl2 (NH4)2 SO4 + 2NaOH → 2NH3 + 2H2O + Na2SO4 3). On a large scale, ammonia is manufactured by Haber’s process. N2 (g) + 3H2 (g) ⇌ 2NH3 (g); Δf H0 = – 46.1 kJ mol–1 In accordance with Le Chatelier’s principle, high pressure would favour the formation of ammonia. The optimum conditions for the production of ammonia are a pressure of 200 × 105 Pa (about 200 atm), a temperature of ~ 700 K and the use of a catalyst such as iron oxide with small amounts of K2O and Al2O3 to increase the rate of attainment of equilibrium. The flow chart for the production of ammonia is shown in Fig. Oxides of Nitrogen Nitrogen forms a number of oxides in different oxidation states. The names, formulas, preparation and physical appearance of these oxides are listed below- (i) Dinitrogen oxide/ Nitrogen (I) oxide/Nitrous oxide/Laughing gas (N2O):- it is colourless, neutral gas and is prepared by as: - NH4NO3 → N2O + 2H2O (ii) Nitrogen monoxide/Nitrogen (II) oxide/Nitric oxide (NO):- it is colourless, neutral gas and is prepared by as: - 2NaNO2 + 2FeSO4 + 3H2SO4 → Fe2 (SO4)3 + 2NaHSO4 + 2H2O + 2NO (iii) Dinitrogen tri oxide/ Nitrogen (III) oxide (N2O3): - It is blue acidic solid and is prepared as: 2NO + N2O4 → 2N2O3 (iv) Nitrogen dioxide/ Nitrogen (IV) oxide (NO2):-it is brown acidic gas and is prepared by as: - 2Pb (NO3)2 → 4NO2 + 2PbO (v) Dinitrogen tetra-oxide/Nitrogen (II) oxide/Nitric oxide (N2O4):- it is colourless, acidic gas and is prepared by as: - 2NO2 ⇌ N2O4 (vi) Dinitrogen pentoxide/ Nitrogen (v) oxide: - It is coloueless acidic solid and is prepared as: 4HNO3 + P4O10 → 4HPO3 +2N2O5 Nitric Acid Methods of preparation:- 1). Laboratory method: - In the laboratory, nitric acid is prepared by heating KNO3 or NaNO3 and concentrated H2SO4 2).Ostwald’s process: - On a large scale it is prepared mainly by Ostwald’s process in following steps:- Step-I Ammonia undergoes catalytic oxidation to get nitric oxide. 4NH3 + 5O2 → 4NO + 6H2O Step –II Nitric oxide thus formed combines with oxygen giving NO2. 2NO + O2 → 2NO2 Step –III Nitrogen dioxide so formed, dissolves in water to give HNO3. 3NO2 + H2O → 3HNO3 + NO
Posted on: Fri, 05 Jul 2013 03:34:06 +0000
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