Halogen compounds.pptx

introduction- isolation of fluorine-manufacturing methods

1. Halogen compounds- unit I Paper name: inorganic and analytical chemistry By Dr.S.Manimekalai E.M.G.Yadava Women’s college, Madurai-14

2. Position of halogens in periodic table F ,Cl, Br, I and At are present in  VII A or seventeen group of periodic table. These are p-block elements. The last element At is radio-active. These elements are halogens because their salts such as NaCl , KCl , KI, MgCl 2   etc are present in sea water . [Halos: sea water, Gens: born ]. Cl 2 is a gas Br 2 is a liquid I 2 is a solid

3. Electronic configuration: These elements have seven electrons in their outer shell The electronic configuration of the valence shell in the ground state is ns2np5 The last electron during configuration occupies p-orbital, hence these elements are p-block elements Element Symbol Electronic configuration Fluorine F [He]2s22p5 Chlorine Cl [Ne]3s23p5 Bromine Br [ Ar ]3d104s24p5 Iodine I [Kr]4d105s25p5 Astatine At [ Xe ]4f145d106s26p5

4. b) Diatomic nature: All the halogens exist as covalent diatomic molecules in the gaseous, liquid and crystalline states These discrete diatomic molecules are held together by weak vanderwaals forces At high temperatures, diatomic molecules dissociate into x atoms The stability of diatomic molecules towards heat decreases from F 2 to I 2 No dissociation occurs in case of F 2 gas Cl 2 is slightly dissociated above 1100 o C Br 2 about 6% while I2 quite extensively above 1100 o C

5. c) Oxidising property a substance that has a tendency to accept one or more electrons is said to show oxidising property is said to show oxidising property. The halogens due to high electron affinity values, have a great tendency to accept an electron and hence act as strong agent The oxidising property of a halogen molecule(X 2 ) is represented by ½ X 2 ( s,l,g ) + e - + aq → X - ( aq ) + E (release) the release of energy, which is made up of many energy terms like heat of fusion, vaporisation etc The values of E are decreasing from F 2 to I 2 , the oxidising power of halogens is in the order F 2 >Cl 2 >Br 2 >I 2

6. F2 is the strongest oxidising agent in the series, it will oxidise other halide ions to halogens in solution F2 + 2X- → 2F- + X2 (x= Cl,Br,I ) Cl2 will displace Br- and I- ions from their solutions and Br2 will displace I- ions from their solutions Cl2 +2X- → 2Cl- + X2 Br2 + 2I- → 2 Br- + I2 Electronegativity Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. As the halogen atoms get bigger, any bonding pair gets further and further away from the halogen nucleus, and so is less strongly attracted towards it. In other words, as you go down the Group, the elements become less electronegative. It decreases on moving down the group because size of atom increases. Decreasing order of electro-negativity of halogens is F, Cl, Br, I.  Fluorine has highest electronegativity among all the known elements .

7.   Electron Affinity : The electron affinity is a measure of the attraction between the incoming electron and the nucleus. The higher the attraction, the higher the electron affinity . As the atom gets bigger, the incoming electron is further from the nucleus and so feels less attraction. The electron affinity therefore falls as you go down the Group . It decreases on moving down the group because electronegativity decreases & metallic character increases.

8. Why Fluorine electron affinity is bigger than chlorine's ? In the case of fluorine, because the atom is very small, the existing electron density is very high. That means that the extra repulsion is particularly great and lessens the attraction from the nucleus enough to lower the electron affinity below that of chlorine.

9. Peculiarities of fluorine Fluorine is the most reactive and the most electronegative of all the elements . Fluorine is a pale yellow, diatomic, highly corrosive, flammable gas, with a pungent odor. It reacts violently with water to produce oxygen and the extremely corrosive hydrofluoric acid . Electropositive character of iodine It is a lustrous solid element with dark grey colour under normal conditions . Iodine is the most electropositive halogen group member.  The electropositive characteristic of iodine causes it to lose electrons during chemical reactions converting the element to positive ion. 

10. Difficulties in the discovery and isolation of fluorine The main difficulties are, Extreme reactivity of fluorine Fluorine is reacts with the material of all the vessels Ex: F2 attacks carbon and silicon present in the glass vessels with the formation of CF4 and SiF4 F2 also attacks the electrodes and vessels made of Pt with the formation of chocolate coloured powder PtF4 Greater stability and non-conducting nature of hydrofluoric acid Hydrofluoric acid is very stable, highly poisonous and corrosive in nature. It is so stable that all attempts to isolate F2 from hydrofluoric acid by oxidising agents failed Electrolysis of aq. Solution of hydrofluoric acid gives H2 and ozone mixed O2 while anhydrous hydrofluoric acid is non-conductor

11. isolation of fluorine Different methods employed for the isolation of F are Dennis method Whytlaw Gray’s method Modern method Dennis method This is devised by Dennis,Veeder and Rochow in 1931. Fluorine is prepared by the electrolysis of fused Na and KHF 2 . Electrolysis is carried out between graphite electrodes in a V-shaped electrically heated copper tube. The ends of the tube are covered with copper caps into which the graphite electrodes are fixed with Bakelite cement .

12. The copper tube is thickly lagged to prevent loss of heat Fluorine liberated at the anode is passed through the U tube containing sodium fluoride. This removes the hydrogen fluoride vapours coming with fluoride. NaF + HF → NaHF 2

13. Dennis method

14. Whytlaw Gray’s method In this method electrolysis of fused KHF 2 is carried out in an electrically heated copper cell which serves as cathode also. a graphite electrode serves as anode and it surrounded by a copper diaphram perforated at the bottom. The diaphram prevents mixing of hydrogen and fluorine which would otherwise react explosively H & F are collected as described above. This method is superior in as much as there is no frothing or chance of an explosion due to the mixing up of F and H

15. Whytlaw Gray’s Method

16. Modern method the electrolysis of a fused mixture of KF with 2 to 3 moles of HF, at about 343-373K in a rectangular steel vessel. The steel vessel serves as the cathode while the anode was earlier made up of a special graphitic form of carbon. Atomic fluorine liberated at the anode enters the crystal lattice to form an interstitial compound which approximates to CF. anodes of petroleum coke, impregnated with copper are preferred. Nickel anodes are used in the beginning when the electrolyte is not completely anhydrous.

17. The valves are made up of nickel with teflon packing. The electrolytic bath is electrically heated by resistance coil lagged externally. A metal cylinder attached to the lid acts as a diaphragm and separates the hydrogen evolved at the cathode from the fluorine liberated at the anode. The two gases thus led out through separate tubes at the top. The current used is 1,500 amperes at 8.5 to 11 volts. The gas is now sold compressed in steel cylinders.

18. Modern method