Oxygen Family

The p-Block Elements Oxygen Family ELECTRONIC CONFIGURATION : * The general outer electronic configuration of VI A group is ns2np4. Hence these elements belong to p– * block * Element Configuration * Oxygen [O8] [He] 2s22p4 Sulphur [S16] [Ne] 3s23p4 Selenium [Se34] [Ar] 3d104s24p4 Tellurium [Te52] [Kr] 4d105s25p4 Polonium [Po84] [Xe] 4f14 5d10 6s26p4 O, S, Se, Te element are collectively called chalcogens (ore forming). Most of the metal ores are oxides or sulphides. * * ANOMALOUS BEHAVIOUR OF OXYGEN : * O, S, Se are non metals, Te is metalloid and Po is metal. * O resembles N and F. It forms strong p–p bond. It also forms H–bond like N and F. * Oxygen, the most abudnt element (21% is present in atmosphere, constitute 46.6% of earth’s crust). 1. * Sulphur constitute about 0.05% of earth’s crust. * S, Se, Te can make use of d–orbital and show a covalency of six and can exhibit an octahedral geometry. * Polonium, the most metallic element is radioactive. 2. 3. 4. HYDRIDES OF GROUP 16 ELEMENTS : * Form hydrides of the formula H2X [H2O, H2S, H2Se, H2Te and H2Po] * Bond angle of hydrides decreases from H2O to H2Po * The volatility of hydrides increases from H2O to H2S then decrease. High b.p. of H2O is due to the presence of intermolecular H–bonding. * The acidic strength increases from H2O to H2Te. This is due to increase of size of the anion in the group. The thermal stability of hydrides increases in the order : H2O > H2S > H2Se > H2Te > H2Po. The covalent character of hydrides increases in going from O to Po. H2S, H2Se, H2Te and H2Po burn in atmosphere of oxygen with blue flame forming dioxides e.g., 2H2S + 3O2  2H2O + 2SO2. Except H2O all hydrides of this group are reductant. This is due to weakening of M–H bond the bond length increase with increases of size of M–atom. Except H2O (liquid), all hydrides are gases at room temperature. 5. 6. The anomalous behaviour of oxygen is due to : (a) Small size (b) High electronegativity and (c) absence of d–orbitals. Point of difference are : Oxygen is a gas while other elements are solids at ordinary temperature. Oxygen is diatomic molecule while others are polyatomic (S, Se etc. are octaatomic ||.) Compounds of oxygen are more ionic than those of other elements. Oxygen forms strong hydrogen bonds whereas other elements of this group from no H–bonding. Oxygen is paramagnetic (molecular orbital theory) while other elements are diamagnetic. Oxygen generally shows O.S. of –2, while other members exhibit o.s. of +2, +4, +6 in addition to – 2. OXYGEN [PRIESTLY AND SCHEELE] : * * * Oxygen constitutes 21% by volume of the atmosphere (23% by mass) It’s abundance in earth’s crust is 46%. It is present to the extent of 89% by weight in water. The p-Block Elements 1. 3. Preparations : (i) On heating suitable compounds such as oxides, Nb(NO3)2, CaOCl2, KMnO4 etc. (a) Physical characteristic : * It is colourless, odourless and tasteless gas.   2HgO   2Hg + O2, 2BaO2  2BaO + O2, * Slightly heavier than air.  2Ag2O   4Ag + O2, * Slightly soluble in water.   3MnO2   Mn3O4 + O2, 2PbO2  2PbO + O2, * Boiling point is – 183ºC and Freezing point is – 219ºC.  2Pb(NO3)2   2PbO + O2 + 4NO2, * Liquid oxygen is pale blue in colour.   2CaCl2 + O2, 2CaOCl2  * Paramagnetic (on the basis of molecular orbital theory).  2KMnO4   K2MnO4 + MnO2 + O2, 16 8O   2KCl + 3O2 (Lab. method) 2KClO3    4K2CrO4 + 2Cr2O3 + 3O2 4K2CrO4  (ii) Reaction of water or acid with certain suitable compounds : 2. Properties : : 8O17 : 8O18 : : 10,000 : 1:8 (b) Chemical characteristics : Important chemical properties of oxygen are given below : Reactions of Na, Ca, Al, Fe are : 2Na2O2 + 2H2O  4NaOH + O2 4Na + O2  2Na2O ; 2Ca + O2  2CaO 4KMnO4 + 6H2SO4  2K2SO4 + 4MnSO4 + 6H2O + 5O2 Reactions of C, P4, S and H2 : 4Al + 3O2  2Al2 O3 ; 3Fe + 2O2  Fe3O4 2K2Cr2O7 + 8H2SO4  2K2SO4 + 2Cr2(SO4)3 + 8H2O + 3O2 C + O2  CO2 ; P4 + 5O2  2P2O5 2MnO2 + 2H2SO4  2MnSO4 + 2H2O + O2 2H2 + O2  2H2O Manufacture : Reactions of NH3 and HCl : Oxygen is obtained on commercial scale by following methods. Pt 4NH 3 + 5O 2  4NO + 6H 2 O ; 4HCl + Cu Cl 2 2  2H O + 2Cl O2  2 2 Pt S + O2  SO2 ; 2SO2 + O2  2SO3 (a) By Brin’s process. Reactions of organic compounds : (b) From liquid air by fractional distillation (Claude’s process) C2H6 + 3.5O2  2CO2 + 3H2O ; C2H2 + 2.5O2  2CO2 + H2O (c) By electrolysis of acidic or alkaline water. 2CH3OH + O2  2HCHO + 2H2O ; C6H12O6 + 6O2  6CO2 + 6H2O Electrolysis of acidic and alkaline water : H2SO4 2H+ + SO42– At cathode : 2H+ + 2e At anode : 2H2O + 2SO42–  4H+ + 2SO42– + O2 + 4e H2 NaOH  Na+ + OH– At cathode : 2Na+ + 2H2O + 2e  2Na+ + 2OH– + H2 At anode : 4OH–  2H2O + O2 + 4e 4. Uses of oxygen : * It is used in artificial respiration (O2 + He) * For the combustion process. * Liquid oxygen is used as a fuel in the rockets. * For oxyacetylene (3300ºC) and oxyhydrogen (2800ºC) flames, employed in cutting and welding purposes. * As an oxidant in several reactions. * In the preparation of several important oxides. The p-Block Elements SULPHUR : OXIDES OF SULPHUR : * Constitute about 0.1% earth’s crust. 1. Extraction : (a) Main source of sulphur are : Sicily and Louisiana the processes employed are called : Sicilian process and Louisiana (or Frasch) process : (b) Sulphur from alkali wastes, spent oxides of coal gas and iron pyrites : Alkali wastes : Contain CaS. CaS + H2O + CO2  CaCO3 + H2S , 2H2S + O2  2H2O + S Spent oxides of coal gas : Contain Fe2S3 2Fe2S3 + 3O2  2Fe2O3 + 6S Iron pyrites : distillation 3FeS2   Fe3S4 + 2S   Fe3S4 + 3O2 + 3S 3FeS2   2. 3. * * * * Name Formula Nature Sulphur suboxide S2O Colourless gas Sulphur monoxide SO Colourless gas Sulphur sesquioxide S2 O3 Green crystalline solid Sulphur dioxide SO2 Colourless gas Sulphur trioxide SO3 Volatile liquid Sulphur heptoxide S2 O 7 Liquid Sulphur tetroxide SO4 White solid. Here we shall discuss SO2 and SO3 being common and important. SULPHUR DIOXIDE SO2 1. FeS + CO2  FeO + CO + S Flowers of sulphur : In the purification process, vapours of boiling sulphur (444ºC) are condensed on the cold walls of the vessel as a light yellow powder called flowers of sulphur. Chemical characteristics : Reactions with H2, Cl2, C, As the Fe : H2 + S  H2S, Cl2 + 2S  S2Cl2 C + 2S  CS2, 2As + 3S  As2S3 ; Fe + S  FeS Reaction with conc.. HNO3, conc. H2SO4 and NaOH S + HNO3  H2SO4 + 6NO2 + 2H2O S + 2H2SO4  3SO2 + 2H2O 2. 4S + 6NaOH  2Na2S + Na2S2O3 + 3H2O * Reactions with K2S : K2S + 4S  K2S5 Uses of sulphur : In the manufacturer of SO2, H2SO4, CS2, matches, gun powder. Used for destroying bacteria, fungi, insects, etc. Used for Vulcanizing rubber and in the manufacture of sulphur dyes. Used in medicines. Methods of preparation : Laboratory methods : S + 2H2SO4  H2O + 3SO2 Cu + 2H2SO4 (conc.)  CuSO4 + 2H2O + S2 2Ag + 2H2SO4 (conc.)  Ag2SO4 + 2H2O + SO2 Na2SO3 + 2HCl (dil.)  2NaCl + H2O + SO2 NaHSO4 + HCl (dil.)  NaCl + H2O + SO2 Industrial method : S + O2  SO2 2ZnS + 3O2  2ZnO + 2SO2 4FeS2 + 11O2  2Fe2O3 + 8SO2 2CaSO4 + C  2CaO + 2SO2 + CO2 Chemical characteristics : Aqueous solutions of SO2 acts as a reducing agent : H2O + SO2  H2SO3 ; H2SO3 + H2O  H2SO4 + [H] (nascent hydrogen) 2KMnO4 + 5SO2 + 2H2O  K2SO4 + 2MnSO4 (colourless) + 2H2SO4 K 2 Cr 2 O 7 + 3SO 2 + H 2 SO 4  K 2 SO 4 + Cr2(SO4)3 (green) + H2O * Acts as an oxidising agent : Mg + SO2  2MgO + MgS 4K + 3SO2 K2SO3 + K2SO4 2SnCl2 + SO2 + 4HCl  SnCl4 + 2H2O + S 2Hg2Cl2 + SO2 + 4HCl  4HgCl2 + 2H2O + S * Acts as bleaching agent (due to reducing nature) SO2 + 2H2O  H2SO4 + 2H Coloured matter + H  Colourless matter * Acidic nature (SO2) is an acidic oxide) SO2 + H2O  H2SO3 The p-Block Elements Mechanism may be described as : 2NO + O2  2NO2 NO2 + SO2 + H2O  H2SO4 + NO The unreacted gases (NO, NO2, O2 and N2) are absorbed in H2SO4 and nitrosyl sulphuric acid. The product on decomposition gives H2SO4. 2H2SO4 + NO + NO2  2NO+H SO4– + H2O 2NOHS4 + H2O  2H2SO4 + NO + NO2 (b) Contact process : * It involves the oxidation of SO2 by air in presence of a catalyst BaO + SO2  BaSO3 * Addition reaction : SO2 + Cl2  SO2Cl2 (Sulphurly chloride) Fe2O3 or   2SO 2SO2 + O2  Pt or V2O5 3 * PbO2 + SO2  PbSO4 * Thermal decomposition : 1200ºC 3SO2  2SO3 + S * * Reaction with burning Mg : 3Mg + SO2  2MgO + MgS 4. Uses of SO2 * Used in the manufacture of H2SO4, paper (from wood pulp) * Used as disinfectant, antechoir, bleaching agent. * As a refrigerant (liquid SO2) * Used in refining petroleum and sugar. 2. * * * * SULPHURIC ACID [H2SO4] : * It is a king of chemicals * It is also known as oil of vitriol. * The three step process for the preparation of H2SO4 is described as : * * SO3 is dissolved in 98% sulphuric acid resulting in the formation of oleum. H2SO4 + SO3  H2S2O7 (oleum) On dilution, H2SO4 of required concentration can be obtained. H2S2O7 + H2O  2H2SO4 Properties : Physical characteristics : H2SO4 is colourless, only liquid. Freezing point and boiling point are 10.5ºC and 340ºC respectively. Forms constant boiling mixture (at 370ºC) containing 98.33% of the acid. Density is 1.84 gm cm–3. Forms hydrate with the evolution of heat : H 2 SO 4 .H 2 O [monohydrate], H 2 SO 4 .2H 2 O [dihydrate], H2SO4.3H2O [tri hydrate] Conductor of heat and electricity High b.p. and viscosity of H2SO4 is dut to H– bonding. S  SO2  SO3  H2SO4 1. Manufacture : (a) Lead chamber process (b) Contact process (a) Lead chamber process : Here oxidation of SO2 is affected catalytically by means of oxides of nitrogen in the presence of water. 2SO2 + O2(air) + 2H2O + [NO] (catalyst)  2H2SO4 + [NO] * Chemical characteristics : Acid character (dibasic acid) H2SO4  H+ + HSO4–, HSO4–  H+ + SO42– Forms two type of salts : NaOH + H2 SO4  NaH SO4 + H2O NaHSO4 + NaOH  Na2SO4 + H2O The p-Block Elements * Dehydrating agent [due to high affinity for water] 3. Uses of H2SO4 : conc. H2SO4 C12H22O11   12C + 11H2O conc. H2SO4 ; HCOOH   CO + H2O * Used as a laboratory reagent. * Used in the manufacture of acids (HNO3, HCl, H3PO4), dyes, drugs, disinfectants, alum, ferrous sulfate for ink etc. C + 2H2SO4  CO2 + 2SO2 + 2H2O * Used as a dehydrating and oxidizing agents. 2HBr + H2SO4  2H2O + SO2 + Br2 * Used in the textile, paper and dyeing industries. H2S + H2SO4  2H2O + SO2 + S * Used in leather industry for tanning. Displaces more volatile acids : * Used in the refining of petroleum. 2NaCl + H2SO4  Na2SO4 + 2HCl * Used in the manufacture of explosives such as nitroglycerine gun coating, TNT, picric acid acid. * Used in lead storage batteries. * Used for cleansing metals before electroplating, enameling, galvanizing etc. conc. H2SO4 * * 3C2H5OH  C2H4 + (C2H5)2O Acts as an oxidizing agent 2NaNO3 + H2SO4  Na2SO4 + 2HNO3 FeS + H2SO4  FeSO4 + H2S Ca3(PO4)2 + 3H2SO4  3CaSO4 + 2H3PO4 * Reaction with metals : Zn, Mg, Fe gives hydrogen : Zn + H2SO4(dil)  ZnSO4 + H2 Cu gives SO2. Cu + 2H2SO4 (conc.)  CuSO4 + 2H2O + SO2 * * Formation of insoluble sulphates : TYPICAL EXAMPLE : (i) Number of water of crystallization associated with Na2S2O3 are : Five BaCl2 + H2SO4  BaSO4 + 2HCl (ii) Catalyst used in lead chamber process for the manufacture of H2SO4 is : Pb(NO3)2 H2SO4  PbSO4 + 2HNO3 NO Reaction with PCl5 and KClO3 : (iii) Of the two processes for the manufacture of H2SO4 which one is cheaper. PCl5 + H2SO4  ClSO2OH + POCl3 + HCl 3KClO 3 + 3H 2SO 4  3KHSO 4 + HClO 3 + 2ClO2 + H2O Contact process (iv) Give two example of amphoteric oxide Al2O3, ZnO.