Presentation on theme: "Chemistry. Session Objectives 1.Chemical properties of group 14 elements 2.Extraction and properties of silicon 3.Extraction of tin 4.Extraction of lead."— Presentation transcript:
Session Objectives 1.Chemical properties of group 14 elements 2.Extraction and properties of silicon 3.Extraction of tin 4.Extraction of lead 5.Silicates and silicones 6.Glass 7.General properties of group 14 elements 8.Oxoacids of phosphorous
Action of Acids on group 14 elements Non-oxidising acids do not attack carbon and silicon. Ge is not attacked by dilute HCl. However, when metal is heated in a stream of HCl gas, germanium chloroform is formed. Lead dissolves in Conc. HCl formig chloro-plumbous acid. Tin dissolves slowly in dilute HCl but readily in Conc. HCl.
Action of Alkalies Carbon is unaffected by alkalies. Silicon reacts slowly with cold aq. NaOH and readily with hot solution to form silicate. Sn and Pb are slowly attacked by cold alkali but readily by hot alkali giving stannates and plumbates.
Oxides of group 14 elements Carbon forms the oxides CO, CO 2, C 3 O 2, Si forms SiO 2, which is solid at room temperature because it exists in the form of a three-dimensional network due to lack of formation of bonds with oxygen. Three crystalline modifications of SiO 2 are quartz, cristobalite and tridymite Pb forms a number of oxides like PbO, PbO 2, Pb 2 O 3, Pb 3 O 4 (red lead). Pb 3 O 4 is actually 2PbO.PbO 2 GeO 2, SnO 2 etc are also network solids. SnO 2 is used as a polishing powder and also in the manufacture of glass and pottery 2Pb(NO 3 ) 2 - 2PbO + 4NO 2 + O 2 6PbO + O 2 2Pb 3 O 4 Pb 3 O 4 + 4HNO 3 Pb(NO 3 ) 2 + PbO 2 + 2H 2 O
Halides of group 14 elements React with halogens directly to form tetrahedral and covalent halides except C. CCl 4 does not undergo hydrolysis due to non availability of d orbital. SiCl 4 and the halides of heavier metals can undergo hydrolysis due to availability of vacant d orbitals. SiCl 4 on hydrolysis gives silicic acid (H 4 SiO 4 ). Apart from tetrahalides, germanium, tin and lead form dihalides MX 2. The stability of the dihalides increases steadily in the sequence CX 2 < SiX 2 < GeX 2 < SnX 2 < PbX 2 PbBr 4 and is PbI 4 do not exist because is a strong oxidant and Br - and I - are strong reductants.
Illustrative Example Explain why PbCl 4 is less stable than SnCl 4 ? In the 14 th group, the stability of +4 oxidation state decreases down the group so Pb 4+ is less stable than Sn 4+. This is actually due to the inert pair effect as s-electrons do not participate in bond formation. Solution
Extraction and properties of Si By reduction of sand SiO 2 with coke in an electric furnace(96-98% pure). Semiconductor grade silicon is prepared mainly by the reduction of SiCl 4 /SiHCl 3 with H 2 or by the pyrolysis of SiH 4 At room temperature Si is unreactive towards all elements except flourine. Combines with halogens, N 2 and O 2 at high temperature. Forms carborundum(SiC) with carbon; extremely hard; used as abrasive and refractory material. With hot aqueous alkali liberates hydrogen.
Extraction of tin Tin is commonly available as the mineral cassiterite, SnO 2. SnO 2 + 2C Sn + 2CO The roasted ore is heated with coal in a reverberatory furnace at 1500 K. Ore is crushed and washed with water to remove impurities such as arsenic and sulphur as volatile oxides. Tin is remelted on inclined surface to remove the impurities having higher melting point.
Properties of tin Tin is a soft, silvery white metal. It is ductile and can be rolled into thin foils. Tin is not attacked by air or water at ordinary temperatures: Heating with air or oxygen results in the formation of SnO 2. Tin is used as a coating on metals and in making various alloys like solder, bronze. It is also used for electroplating steel to make tin-plate. Tin –plate is extensively used for making cans for food and drinks.
Lead Lead is mostly used in storage batteries, in alloy making and pigments/chemicals. PbCrO 4 is used as a strong yellow pigment for road signs and markings. Lead compounds are also included in crown glass and cut glass, and in ceramic glazes. The ore is concentrated by froth-floatation and then roasted in a limited supply of air to give PbO which is reduced to the metal by heating with coke and limestone in a blast furnace. The molten lead is tapped from the bottom of the furnace. (i)2PbS + 3O 2 2PbO + 2SO 2 (ii)PbO + C Pb + CO (iii)PbO + CO Pb + CO 2 (iv)PbS + 2PbO 3Pb + SO 2
Toxicity of Lead Pb 3 (OH) 2 (CO 3 ) 2 Large amounts of lead in a child's blood can cause brain damage, mental retardation, behavior problems, anemia, liver and kidney damage, hearing loss, hyperactivity, developmental delays, other physical and mental problems, and in extreme cases, death.
Silicates Orthosilicates : contain single discrete unit of SiO 4 4– tetrahedra Pyrosilicates Basic unit is (Si 2 O 7 ) -6
Cyclic structure Basic unit is (Si 6 O 18 ) -12 Example is beryl, Be 3 Al 2 Si 6 O 18
Linear silicate chain Continuous single chain units of tetrahedra each sharing 2 oxygens. Basic unit is (SiO 3 ) -2 or (Si 2 O 6 ) -4. e.g., pyroxenes; MgCaSi 2 O 6.
Amphiboles Continuous double chain units of tetrahedra each sharing 2 and 3 oxygens alternately. Basic unit is (Si 4 O 11 ) -6 or (Si 8 O 22 ) -12 e.g., asbestos; [Mg 3 (Si 2 O 5 )(OH) 4 ]
Phyllosilicates Continuous sheet units of tetrahedra each sharing 3 oxygens Basic unit (Si 2 O 5 ) -2 e.g.Mica
3 D framework Continuous framework of tetrahedra each sharing all 4 oxygen atoms. Basic units can be (SiO 2 ) e.g. zeolites,feldspar
Silanes and Silicones The hydrides of silicon are called silanes having general formula Si n H 2n+2 Polymeric organo-silicon compounds containing Si-O-Si bonds are called silicones. These have the general formula (R 2 SiO) n. Where R is CH 3 group (majority cases) or C 6 H 5 group.
Focus On Glassmaking Small amounts of impurities impart beautiful colours. Fe 2 O 3 green CoO blue Glass is a mixture of sodium and calcium silicates. Lead-potash glass has high refractive index and used in lenses. Adding B 2 O 3 gives, borosilicate glass (Pyrex) having low coefficient of thermal expansion and used in making laboratory glasswares. Soda lime glass or Soft glass. Glass is not a true solid and don’t have definite melting point.
General trends of group 15 elements The covalent radius increases down the group. All elements have nearly same and low electronegativity except nitrogen. P, As, Sb and Bi are solids under normal conditions. The value of ionization energy is quite high for the members of group 15 than the corresponding members of group 14. This is due to smaller atomic radii, increased nuclear charge and stable electronic configuration of half filled orbitals. Ionisation energy
Illustrative Problem Why nitrogen exists as N 2 whereas phosphorous exists as P 4 Because d orbitals are not available in nitrogen. Solution :
Oxidation state Stability of +3 oxidation state increases down the group while that of +5 oxidation state decreases down the group. Bi 5+ salts are very rare and good oxidising agents As 3+ salts are good reducing agents.
Oxidation NumberNitrogen CompoundPhosphorus Compound 0N2N2 P4P4 +3HNO 2 (nitrous acid)H 3 PO 3 (phosphorous acid) +3N2O3N2O3 P4O6P4O6 +5HNO 3 (nitric acid)H 3 PO 4 (phosphoric acid) +5NaNO 3 (sodium nitrate)Na 3 PO 4 (sodium phosphate) +5N2O5N2O5 P 4 O 10
Down the group, covalent character, basicity and thermal stability decrease while reducing character increases. NH 3 > PH 3 > AsH 3 > SbH 3 > BiH 3 107° 48’ 93° 48’ 91° 48’ 91° 18’ 90° Hydrides: MH 3
Illustrative Problem Explain why NH 3 is a stronger base than PH 3 ? Solution Since phosphorus is bigger in size as compared to nitrogen so, availabilty of lone pair is less. Thus PH 3 is a weaker base than NH 3
Illustrative Problem Give the order of basicity and reducing character and stability for the following hydrides: NH 3, PH 3, AsH 3, SbH 3 Solution : Basicity NH 3 > PH 3 > AsH 3 > SbH 3 Stability NH 3 > PH 3 > AsH 3 > SbH 3 Reducing character NH 3 < PH 3 < AsH 3 < SbH 3
Oxides of nitrogen
Illustrative Problem Which oxide of nitrogen is coloured ? NO 2 has unpaired electrons,so it is coloured. Solution :
Oxides Oxides of phosphorous — P 4 O 6 and P 4 O 10 P, As, Sb and Bi form two types of oxides: M 2 O 3 and M 2 O 5 and exists as dimer due to reluctance for bonding. Both P 4 O 6 and P 4 O 10 are acidic oxides which dissolve in water to give phosphonic acid and phosphoric acid respectively.
Halides Forms two series of halides; MX 3 (pyramidal) MX 5 (trigonal bipyramidal) Trihalides readily hydrolyse with water. PCl 5 is molecular in gas and liquid phases but exists as [PCl 4 ] + [PCl 6 ] - in the solid state.
Illustrative Example Explain why PCl 5 exists but NCl 5 does not? Solution NCl 5 is not formed because nitrogen does not have d-orbitals.
Illustrative Example Solid phosphorous-pentachloride exhibits some ionic character, why? Solution This is because PCl 5 exists as [PCl 4 ] + [PCl 6 ] - in solid phase and hence exhibits ionic character.
Allotropy of P White phosphorus: Waxy solid, insoluble in water, highly soluble in CS 2 and benzene, highly reactive, highly toxic and glow in dark. Stored in water because ignite spontaneously in air. Consist of discreate molecules P 4.
Allotropy of P Black phosohorus : Inert and has layered structure. Most stable form Red phosphorus: Amorphous and polymeric structure. Less reactive and nontoxic
Oxy-acids of P
Phosphatic fertilizers Fertilizers usually contains N,P,K. Generally the ratio of N-P-K is written as which indicates the percentage of N, P 2 0 5, and K 2 O. Most important phosphatic fertilizer is the superphosphate of lime, Ca(H 2 PO 4 ) 2 which is produced from the treatment of phosphatic rock with sulphuric acid. Triple superphosphate Ca(H 2 PO 4 ) 2.H 2 O
Eutrophication Eutrophication is the enrichment of an ecosystem with chemical nutrients, typically compounds containing nitrogen or phosphorus.