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Chemistry

Chemistry. Session objectives. General trends of group 17 elements Chemical properties of group 17 elements Oxoacids of group 17 elements General properties of group 18 elements Compounds of xenon. Occurrence. The halogens are very reactive, and donot occur in free state.

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Chemistry

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  1. Chemistry

  2. Session objectives General trends of group 17 elements Chemical properties of group 17 elements Oxoacids of group 17 elements General properties of group 18 elements Compounds of xenon

  3. Occurrence The halogens are very reactive, and donot occur in free state. All except At are found in combined form in the earth’s crust. Astatine is radioactive and has a short half life.

  4. Flourine Flourine is found naturally in many minerals, such as flourspar. There is no element that can be used to oxidize F- to F2 Flourine is produced by electrolysing a molten mixture of KF and HF at a carbon anode.

  5. Difficulties in obtaining flourine HF is corrosive, and etches glass forming SiF6- ions and also causes very painful skin wounds. Wounds are slow to heal because F- ions remove Ca2+ ions from the tissues. Gaseous HF is very toxic (3 ppm) compared with HCN (10 ppm). HF is poor conductor of electricity. Thus a mixture of KF and HF is electrolysed to increase the conductivity. Flourine oxidises water to dioxygen, so water should be excluded. It catches fire with the traces of greases or with crystalline silicon. Anode should be graphite free carbon otherwise flourine invade into the sheet of the graphite causes an increase in the distance between sheets. It reduces the conductivity of the graphite and loss of electricity as thermal effect. Eventually a explosion may occur.

  6. Properties and uses of flourine Fluorine is a pale yellow, extremely reactive, corrosive gas. It has a characteristic pungent odour. It is used to produce the volatile UF6, used in processing nuclear fuel. To make SF6; which is very inert gas ,used as dielectric for high voltage equipment. In making various of freons, which are chemically inert, not oxidised by air, hot nitric acid or by concentrated sulfuric acid. Traces of flouride ions in drinking water (~1 ppm) greatly reduce the incidence of dental caries making an enamel on teeth.

  7. Chlorine Prepared by electrolysis of molten salt or brine. Reacts directly with nearly all elements (exceptions: C, N, O and the noble gases) Oxidizes metals to high oxidation state. For example, iron reacts vigorously in Cl2 to form FeCl3, not FeCl2. Uses : Manufacturing of plastics, solvents, pesticides. As bleach in paper and textile industries. As disinfectant in water treatment

  8. Bromine Br2 is obtained from sea water and brine lakes. First H2SO4 is added to adjust the pH to about 3.5. Then Cl2 gas is passed through the solution to oxidize the Br– to Br2. Br2 is removed by a stream of air, because Br2 is quite volatile. The gas is passed through a solution of Na2CO3, when Br2 is absorbed, forming a mixture of NaBr and NaBrO3. Finally, the solution is acidified and distilled to give pure bromine.

  9. Bromine is corrosive, red brown fuming liquid . Bromine compounds are used as a fire retardant in textiles and as pesticides. AgBr is used in photographic emulsions. ZnBr2(aq) has a very high density and is used to control the escape of oil from wells.

  10. Iodine Can be obtained from seaweed It is blue black lustrous solid; sublimes easily to purple vapour. Uses (1) Added to iodised salt. (2) dissolves in alcohol to form tincture of iodine, an antiseptic. (3) serves as indicator in presence of starch

  11. From left, Iodine dissolved in CCl4 Iodine dissolved in water. Iodine dissolved in KI. With a little starch added.

  12. Illustrative Problem Explain why fluorine does not undergo disproportionation reaction but other halogens do. Solution : Fluorine being the most electronegative element undergoes only reduction but not oxidation. As a result, it shows only –1 oxidation state while other halogens show both negative (–1) and positive (+1, +3, +5, +7) oxidation states. Thus, F does not show disproportionation reactions while other halogens do.

  13. Electron affinity Electron affinity values are high in case of halogens. As we move down the group, the electron affinities decrease as the size of the halogen increases. Electron affinity of fluorine is lower than that of chlorine. This is due to small size of fluorine atom. As a result of which strong electron-electron repulsions are present in the relatively compact 2p-orbitals of fluorine.Therefore, the electron affinities decrease in the order: Cl > F > Br > I

  14. Stability of X-X bond The bond energy in the X2 molecules would decrease as the atoms become larger, since increased size results in less effective overlap of orbitals. The F — F bond energy is smaller than that of Cl — Cl. Due to small size of fluorine atoms, there are high interelectronic repulsions between non-bonding electrons in 2p-orbitals of fluorine. As a result, F — F bond is weaker in comparison to Cl — Cl and Br — Br bonds.

  15. Illustrative Problem Explain why F2 is a stronger oxidizing agent than Cl2 while electron affinity of fluorine is less than that of chlorine.

  16. Solution F2 is a stronger oxidizing agent than Cl2 because F2 has higher oxidation potential than Cl2. Electron affinity is the energy released when a gaseous atom accepts an electron to form a gaseous anion while oxidation potential is the sum of energy changes taking place in the following steps:

  17. Some Important Inorganic Compounds of Fluorine

  18. Chemical properties Hydrides : HX HF is liquid HCl, HBr and HI are gases. Thermal stability : HF > HCl > HBr > HI Reducing character: HF < HCl < HBr < HI Acid strength : HF < HCl < HBr < HI H — X bond strength : HF > HCl > HBr > HI

  19. Illustrative Problem A greenish yellow gas reacts with an alkali metal hydroxide to form a halate which can be used in fire works & safety matches.what will be the gas & halate? Solution: The greenish yellow gas is Cl2 & halate used in fire works & safety matches is KClO3

  20. Oxo acids Hypohalous acid, HOX Halous acid, HXO2 Halic acid, HXO3 Acid strength: HClO3 < HBrO3 < HIO3 HClO4 > HClO3 > HClO2 > HOCl

  21. Oxoanions of Chlorine

  22. Illustrative Example When NaBr is heated with Conc. H2SO4, Br2 is produced but when NaCl is heated with Conc. H2SO4, HCl is produced. Why? Solution : When NaBr is heated with Conc. H2SO4, HBr is first produced which being a reducing agent reduces H2SO4 to SO2 while itself gets oxidised to Br2. As a result, only Br2 is produced. Similarly, NaCl reacts with Conc. H2SO4 to form HCl but since HCl does not act as a reducing agent it does not get oxidised to Cl2. As a result, only HCl is evolved.

  23. Pseudo halogens Pseudohalides Pseudohalogens CN–   (CN)2 cyanogen SCN–   (SCN)2 thiocynogen OCN–   (OCN)2 oxy cyanogens

  24. Illustrative example Iodine is liberated when KI is added to solution of Cu2+ ions but Cl2 is not liberated when KCl is added to a solution of Cu2+ ions. Why? Solution : The I– ion, being a strong reducing agent, reduces Cu2+ to Cu+ and itself gets oxidised to I2. The Cl– ion is a week reducing agent. Thus, it does not reduce Cu2+ ion.

  25. Group 18 elements Ionization energies are high but decrease down the group. The research leading to the first noble-gas compound was prompted by the observation that xenon lost an electron as easily as oxygen.

  26. Illustrative example Liquids flow from a higher to a lower level .Give the name of the liquid that can climb up the walls of the glass vessel in which it is placed. Solution: HeII is liquid form of helium which is obtained on cooling gaseous He to2.2K at 1 atm .It is able to flowuphill like a gas.

  27. Xenon and Fluorine All three binary Xe-F compounds are crystalline solids All are molecular in gas phase. Solid XeF6 is ionic, being composed of XeF5+ cations and F- anions. The binary Xe-F compound are powerful flourinating agents.

  28. Compounds of xenon

  29. Illustrative example Explain the following: (i) Why do helium and neon not form compounds with fluorine? (ii) Neon is generally used for warning signal illumination.

  30. Solution (a) He and Ne do not contain d-orbitals in their valence shell and hence their electrons cannot be promoted to higher energy levels like that in Xe to form bonds. \ He and Ne do not form compounds with fluorine. (b) Neon lights are visible from long distances. The lights are even visible during fog and mist conditions.

  31. Illustrative example Which mixture of gases is used by sea divers to go deep in the sea water ? Solution: Mixture of O2 &He (1:4) is used by deep sea divers for repiration as He does not dissolve in blood at high deep sea pressure

  32. Illustrative example Which gas is filled in electric bulbs/tubes? Solution : Argon is filled in electric bulbs/tubes.

  33. Thank you

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