Chemistry

1. Chemistry

2. Session Objectives • Chemical properties of group 14 elements • Extraction and properties of silicon • Extraction of tin • Extraction of lead • Silicates and silicones • Glass • General properties of group 14 elements • Oxoacids of phosphorous

3. 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. Tin dissolves slowly in dilute HCl but readily in Conc. HCl. Lead dissolves in Conc. HCl formig chloro-plumbous acid.

4. 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.

5. Oxides of group 14 elements Carbon forms the oxides CO, CO2, C3O2, Si forms SiO2, which is solid at room temperature because it exists in the form of a three-dimensional network due to lack of formation of p bonds with oxygen. Three crystalline modifications of SiO2are quartz, cristobalite and tridymite Pb forms a number of oxides like PbO, PbO2, Pb2O3, Pb3O4(red lead). Pb3O4 is actually 2PbO.PbO2 2Pb(NO3)2 -2PbO + 4NO2 + O2 6PbO + O2 2Pb3O4 Pb3O4 + 4HNO3 Pb(NO3)2 + PbO2 + 2H2O GeO2, SnO2 etc are also network solids. SnO2 is used as a polishing powder and also in the manufacture of glass and pottery

6. PbBr4 and is PbI4 do not exist because is a strong oxidant and Br- and I- are strong reductants. Halides of group 14 elements React with halogens directly to form tetrahedral and covalent halides except C. CCl4 does not undergo hydrolysis due to non availability of d orbital. SiCl4 and the halides of heavier metals can undergo hydrolysis due to availability of vacant d orbitals. SiCl4 on hydrolysis gives silicic acid (H4SiO4). Apart from tetrahalides, germanium, tin and lead form dihalides MX2. The stability of the dihalides increases steadily in the sequence CX2 < SiX2 < GeX2 < SnX2 < PbX2

7. Illustrative Example Explain why PbCl4 is less stable than SnCl4? Solution In the 14th group, the stability of +4 oxidation state decreases down the group so Pb4+ is less stable than Sn4+.This is actually due to the inert pair effect as s-electrons do not participate in bond formation.

8. Extraction and properties of Si By reduction of sand SiO2 with coke in an electric furnace(96-98% pure). Semiconductor grade silicon is prepared mainly by the reduction of SiCl4/SiHCl3 with H2 or by the pyrolysis of SiH4 At room temperature Si is unreactive towards all elements except flourine. Combines with halogens, N2 and O2 at high temperature. Forms carborundum(SiC) with carbon; extremely hard; used as abrasive and refractory material. With hot aqueous alkali liberates hydrogen.

9. Extraction of tin Tin is commonly available as the mineral cassiterite, SnO2. Ore is crushed and washed with water to remove impurities such as arsenic and sulphur as volatile oxides. The roasted ore is heated with coal in a reverberatory furnace at 1500 K. SnO2 + 2C  Sn + 2CO Tin is remelted on inclined surface to remove the impurities having higher melting point.

10. 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 SnO2. 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.

11. Lead 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 + 3O2  2PbO + 2SO2 (ii) PbO + C Pb + CO (iii) PbO + CO Pb + CO2 (iv) PbS + 2PbO3Pb + SO2 Lead is mostly used in storage batteries, in alloy making and pigments/chemicals. PbCrO4 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.

12. Pb3(OH)2(CO3)2 Toxicity of Lead 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.

13. Silicates Orthosilicates :contain single discrete unit of SiO44– tetrahedra Pyrosilicates Basic unit is (Si2O7)-6

14. Cyclic structure Basic unit is (Si6O18)-12Example is beryl, Be3Al2Si6O18

15. Linear silicate chain Continuous single chain units of tetrahedra each sharing 2 oxygens.Basic unit is(SiO3)-2 or (Si2O6)-4.e.g., pyroxenes; MgCaSi2O6.

16. Amphiboles Continuous double chain units of tetrahedra each sharing 2 and 3 oxygens alternately. Basic unit is (Si4O11)-6 or (Si8O22)-12e.g., asbestos; [Mg3(Si2O5)(OH)4]

17. Phyllosilicates Continuous sheet units of tetrahedra each sharing 3 oxygens Basic unit (Si2O5)-2 e.g.Mica

18. 3 D framework Continuous framework of tetrahedra each sharing all 4 oxygen atoms. Basic units can be (SiO2) e.g. zeolites,feldspar

19. Silanes and Silicones The hydrides of silicon are called silanes having general formula SinH2n+2 Polymeric organo-silicon compounds containing Si-O-Si bonds are called silicones. These have the general formula (R2SiO)n. Where R is CH3 group (majority cases) or C6H5 group.

20. Small amounts of impurities impart beautiful colours. Fe2O3 green CoO blue Soda lime glass or Soft glass. Focus On Glassmaking Glass is a mixture of sodium and calcium silicates. Glass is not a true solid and don’t have definite melting point. Adding B2O3 gives, borosilicate glass (Pyrex) having low coefficient of thermal expansion and used in making laboratory glasswares. Lead-potash glass has high refractive index and used in lenses.

21. Group 15 elements Nitrogen N [He] 2s2p3 Phosphorus P [Ne] 3s23p3 Arsenic As [Ar]3d104s24p3 Antimony Sb [Kr]4d105s25p3 Bismuth Bi [Xe]4f145d106s26p3

22. 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. Ionisation energy 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.

23. Illustrative Problem Why nitrogen exists as N2 whereas phosphorous exists as P4 Solution : Because d orbitals are not available in nitrogen.

24. Oxidation state Stability of +3 oxidation state increases down the group while that of +5 oxidation state decreases down the group. Bi5+ salts are very rare and good oxidising agents As3+ salts are good reducing agents.

25. Oxidation Number Nitrogen Compound Phosphorus Compound 0 N2 P4 +3 HNO2 (nitrous acid) H3PO3 (phosphorous acid) +3 N2O3 P4O6 +5 HNO3 (nitric acid) H3PO4 (phosphoric acid) +5 NaNO3 (sodium nitrate) Na3PO4 (sodium phosphate) +5 N2O5 P4O10

26. Hydrides: MH3 NH3 > PH3 > AsH3 > SbH3 > BiH3 107° 48’ 93° 48’ 91° 48’ 91° 18’ 90° Down the group, covalent character,basicity and thermal stabilitydecrease while reducing character increases.

27. Illustrative Problem Explain why NH3 is a stronger base than PH3? Solution Since phosphorus is bigger in size as compared to nitrogen so, availabilty of lone pair is less. Thus PH3 is a weaker base than NH3

28. Illustrative Problem Give the order of basicity and reducing character and stability for the following hydrides: NH3, PH3, AsH3, SbH3 Solution : Basicity NH3 > PH3 > AsH3 > SbH3 Stability NH3 > PH3 > AsH3 > SbH3 Reducing character NH3 < PH3 < AsH3 < SbH3

29. Oxides of nitrogen

30. Oxides of nitrogen

31. Illustrative Problem Which oxide of nitrogen is coloured ? Solution : NO2 has unpaired electrons,so it is coloured.

32. P, As, Sb and Bi form two types of oxides: M2O3 and M2O5 and exists as dimer due to reluctance for bonding. Oxides Oxides of phosphorous — P4O6 and P4O10 Both P4O6 and P4O10 are acidic oxides which dissolve in water to give phosphonic acid and phosphoric acid respectively.

33. Halides Forms two series of halides; MX3 (pyramidal) MX5 (trigonal bipyramidal) Trihalides readily hydrolyse with water. PCl5 is molecular in gas and liquid phases but exists as [PCl4]+[PCl6]- in the solid state .

34. Illustrative Example Explain why PCl5 exists but NCl5 does not? Solution NCl5 is not formed because nitrogen does not have d-orbitals.

35. Illustrative Example Solid phosphorous-pentachloride exhibits some ionic character, why? Solution This is because PCl5 exists as [PCl4]+ [PCl6]- in solid phase and hence exhibits ionic character.

36. Allotropy of P White phosphorus: Waxy solid, insoluble in water, highly soluble in CS2 and benzene, highly reactive, highly toxic and glow in dark. Stored in water because ignite spontaneously in air. Consist of discreate molecules P4.

37. Most stable form Allotropy of P Red phosphorus: Amorphous and polymeric structure. Less reactive and nontoxic Black phosohorus : Inert and has layered structure.

38. Oxy-acids of P

39. Phosphatic fertilizers Fertilizers usually contains N,P,K. Generally the ratio of N-P-K is written as 10-10-10 which indicates the percentage of N, P205, and K2O . Most important phosphatic fertilizer is the superphosphate of lime, Ca(H2PO4)2 which is produced from the treatment of phosphatic rock with sulphuric acid. Triple superphosphate Ca(H2PO4)2.H2O

40. Eutrophication Eutrophication is the enrichment of an ecosystem with chemical nutrients, typically compounds containing nitrogen or phosphorus.

41. Thank you