Noble gases, Group 18

Noble Gases
Introduction, isolation of Helium
from Natural gas, applications of
Noble gases. Preparation,
properties and structures of
fluorides and oxides of Xenon
(XeF2 , XeF4 , XeF6 , XeO3, XeO4 ).
Group 18 of the periodic table comprises helium(He), neon(Ne), argon (Ar), krypton (Kr),xenon (Xe), radon (Rn) and oganesson (Og).

1. Syllabus :- Noble Gases Introduction, isolation of Helium from Natural gas, applications of Noble gases. Preparation, properties and structures of fluorides and oxides of Xenon (XeF 2 , XeF 4 , XeF 6 , XeO 3, XeO 4 ). Presented by G SMITHA Assistant professor MES COLLEGE OF ARTS, C OMMERCE AND SCIENCE NOBLE GASES

2. Introduction Group 18 of the periodic table comprises helium(He), neon(Ne), argon ( Ar ), krypton (Kr),xenon ( Xe ), radon ( Rn ) and oganesson (Og). The noble gases all have stable electron configurations. They behave just like noble people. They not react at all, whatever you do. They got their name ‘noble’ because they don’t interact with “common” elements.

3. Properties The electronic configuration of He is 1s 2 , while that of remaining noble gases is ns 2 np 6 . Name of the elements Atomic number Electronic configuration Helium 2 1s 2 Neon 10 [He]2s 2 2p 6 Argon 18 [Ne]3s 2 3p 6 Krypton 36 [ Ar ]3d 10 4s 2 4p 6 Xenon 54 [Kr]4d 10 5s 2 5p 6 Radon 86 [ Xe ]4f 14 5d 10 6s 2 6p 6 Oganesson 118 [ Rn ]5f 14 6d 10 7s 2 7p 6

4. In any case, these elements have a complete valence shell, and this is responsible for their non-reactive nature. The electron affinity of Group 18 members is zero and their ionization energies are very high. In their natural states, these elements are colourless, odourless and tasteless monoatomic gases. The only source of attraction between the atoms of a particular noble gas are weak Vander waals forces. Hence the melting and boiling points of Group elements increase from He to Og. The noble gases are able to diffuse through material such as rubber, polyvinyl chloride and even metals

5. Applications Helium : 1. It is light, inert and non-flammable. Hence it is used in filling air ships and ballons . 2. Atmosphere of helium is used during welding of metals which readily combine w ith oxygen or nitrogen of air during welding.

6. 3. A mixture of helium and oxygen is used by deep sea divers for respiration.If air is used, nitrogen of air dissolves in blood under high pressure of water felt under deep sea. Dissolved nitrogen leads to complication. 4. It is used in the treatment of asthma.

7. b. Neon : Under low pressure in electrical discharge tubes, neon gives orange red glow. So it is used in neon signs boards. Light of various shades can be obtained by mixing neon with other gases.

8. c. Argon : 1. A mixture of argon and nitrogen is used in electric bulb to increase the life of the filament. 2. It is used to maintain inert atmosphere during welding.

9. 3. Mixed with neon, it is used in neon sigh boards. 4. It is used in radio valves and tubes.

10. d. Kryptons : It is used in filling electric bulbs to increase the life of the filament. It is used in discharge tubes. Kr 85 (radio isotope) is used in the measurement of thickness of sheets of metals and plastics.

11. e. Xenon : 1. It is used in discharge tube and in quick photography. 2. A mixture of krypton and xenon is used for examining respiratory organs.

12. Radon : Being radioactive, it is used in radioactive research. It is also used in radiotherapy for treatment of cancer.

13. Isolation of helium from natural gas Natural gas : Natural gas (also called fossil gas ) is a naturally occurring hydrocarbon gas mixture consisting primarily of methane, but commonly including varying amounts of other higher alkanes , and sometimes a small percentage of carbon dioxide, nitrogen, hydrogen sulfide , and helium .

14. The process of extracting and producing liquefied helium from natural gas consist of six steps. Natural gas processing / pretreatment (removal of hydrogen sulphide, carbon dioxide, water and heavy metals) Natural gas refrigeration (removal of heavier hydrocarbons if any) and liquefaction (production of liquefied natural gas); Nitrogen rejection (removal of nitrogen) / helium recovery from natural gas; 4. Helium upgrading; 5. Helium purification; and Helium liquefaction. 1. Natural gas processing / pretreatment . The pretreatment process is imperative for removing acid gases, water and heavy metals (commonly mercury) prior to entering the refrigeration and liquefaction process.

15. Figure 1 is a block-flow illustration of a general process for helium recovery from natural gas.

16. 1. Acid gas removal : The acid gases, if present, are removed by membrane or amine treating 2. Dehydration : The next step in the gas processing plant is to remove water vapour from the gas using either the regenerable absorption in liquid triethyl glycol(TEG),commonly referred to as glycol dehydration. [note glycol dehydration : Glycol dehydration is a liquid desiccant system for the removal of water from natural gas and natural gas liquids (NGL). It is the most common and economical means of water removal from these streams. Glycols typically seen in industry include triethylene glycol (TEG), diethylene glycol (DEG), ethylene glycol (MEG), and tetraethylene glycol (TREG). TEG is the most commonly used glycol in industry.Contents ] 3. Removal of mercury: Mercury is then removed by using adsorption processes such as activated carbon or regenerable molecular sieves. Off gas : a gas that is produced as a by-product of an industrial process or that is given off by a manufactured object or material 4. nitrogen rejection unit : (NRU) selectively removes nitrogen from a gas. The name can be applied to any system that removes nitrogen from natural gas.

17. 5. Helium upgrade : Gas separation processes are divided into three categories: cryogenic processes(90% helium recovery), pressure swing adsorption (PSA) and membrane separation 6. Helium purification : Final purification of helium, prior to liquefaction, is typically done using either (a) Cryogenic fractionation : recovery > 95% to 99.999%. (b) adsorption based process: activated charcoal absorbers at liquid-nitrogen temperatures and high pressure or (c) pressure-swing adsorption (PSA) processes: Low-temperature adsorption can yield helium purities of 99.9999 percent, while PSA processes recover helium at better than 99.99 percent purity. 7. Liquefaction of helium : At -269 C (super liquid)

18. Preparation, properties and structures of fluorides and oxides of xenon Xenondifluoride (XeF 2 ) : Preparation: 1. Is obtained by heating Xe and F 2 in 2 : 1 volume ratio at 673K In a sealed nickel crucible. Xenon is taken in excess to avoid the simultaneous synthesis of xenon tetrafluoride . 2 : 1 2. It can be prepared by the reaction of xenon with O 2 F 2 at -118 C.

19. Properties : It is colourless crystalline solids. Its Melting pointis 400K. Reaction with hydrogen. Reaction with Iodine in presence of BF 3 . It is a strong oxidising agent. It oxidises HCl to Cl 2 . It reacts with water and liberating oxygen. 7. It is a mild fluorinating agent. It reacts with benzene to give fluorobenzene .

20. Structure : It has linear geometry. It is explained using V.S.E.P R theory. Electronic configuration Xe 54 = [Kr] 4d 10 5s 2 5p 6 Ground state : 5s 5p 4d Excited state : ↑ ↑ 5s 5p 4d Hybridisation : sp 3 d

21. sp 3 d hybrid orbitals have trigonal bipyramidal structure. Three sp 3 d orbitals contain 4 paired electrons. Due to presence of lone pair of electrons, the shape is linear.

22. Xenon tetrafluoride (XeF 4 ) : Preparation : Is prepared by heating xenon and fluorine in a 1: 5 volume ratio at 673 K in a nickel crucible. In this case fluorine is taken in excess to avoid simultaneous formation of XeF 2 1 : 5 Properties : It is a colourless crystalline solid. 2. It is soluble in HF. 3. It melts at 117.1 C 4. It can be stored in pyrex vessel

23. Properties : Complete hydrolysis. Partial hydrolysis. Is a strong fluorinating agent. It fluorinates compounds. 8. It reacts with HCl and NH 4

24. Structure : It has square planar shape. It is explained using V.S.E.P R theory. Electronic configuration Xe 54 = [Kr] 4d 10 5s 2 5p 6 Ground state : 5s 5p 4d Excited state : ↑ ↑ ↑ ↑ 5s 5p 4d Hybridisation : sp 3 d 2

25. sp 3 d 2 hybrid orbitals have octahedral structure. Three sp 3 d 2 orbitals contain 2 paired electrons. Due to presence of lone pair of electrons, the shape is square planar.

26. Xenon hexafluoride (XeF 6 ) : Preparation : Is prepared by heating Xe and F 2 in 1:20 volume mixture at 550K under 50-60 atm pressure in a nickel vessel. The volume of fluorine in the reaction mixture is so much in excess that XeF 6 Is not contaminated by the difluoride or tetrafluoride . Properties : 1. colourless crystalline solid 2. It is yellow colour in liquid and vapour state. 3. It melts at 322.5 K. 4. XeF 6 can not be handled in glass or quarts apparatus because of stepwise reaction which finally produces the dangerous XeO 3. So it is stored in nickel container.

27. 5. Hydrolysis involves three steps. 6. The sodium and potassium salts are formed directly from sodium fluoride and potassium fluoride. 7. Like other fluorides, it also gets reduced by hydrogen to xenon and hydrogen fluoride. 8. It rapidly react with HCl and ammonia in accordance with the following equations.

28. Structure : It has distorted octahedral shape. It is explained by using V.S.E.P R theory. Electronic configuration Xe 54 = [Kr] 4d 10 5s 2 5p 6 Ground state : 5s 5p 4d Excited state : ↑ ↑ ↑ ↑ ↑ ↑ 5s 5p 4d Hybridisation : sp 3 d 3 ↑

29. sp 3 d 3 hybrid orbitals have octahedral structure. Three sp 3 d 3 orbitals contain 1 paired electrons. Due to presence of lone pair of electrons, the shape is distorted octahedral .

30. Xenon trioxide (XeO 3 ) : Preparation : XeO 3 is prepared by complete hydrolysis of xenon hexafluoride. This reaction proceeds involves three steps. Properties : It is a white non-volatile compound and soluble in water. It is stable in aqueous solution and explosive in dry state. It act as a strong oxidant.

31. 5. In aqueous solution , it behaves as a weak acid. In basic solution, it gets hydrolysed to perxenate . 6. It is explosive in nature. Structure : In excited state, three paired electrons from 5p orbitals are shifted to 5d orbitals . Xenon undergoes sp 3 hybridisation. Out of four SP 3 hybrid orbitals , one hybrid orbital contains lone pair of electrons. Three sp 3 hybrid orbitals form σ (sigma) bonds with three oxygen atoms. Unhybridised d orbitals of Xe form π bonds with oxygen atoms. Due to presence of lone pair of electrons, the molecule has pyramidal shape.

32. Electronic configuration Xe 54 = [Kr] 4d 10 5s 2 5p 6 Ground state : 5s 5p 4d Excited state : ↑ ↑ ↑ ↑ ↑ ↑ 5s 5p 4d Hybridisation : sp 3 ↑

33. Xenon tetroxide (XeO 4 ) : Preparation : Reaction of XeO 3 with an alkali in presence of ozone yields perxenate ions XeO 6 - 2. XeO 4 is prepared in two steps. First barium perxenate is obtained by the reaction of XeF 6 with barium hydroxide. Second barium perxenate reaction with cold concentrated sulphuric acid.

34. Properties : XeO 4 is an yellow coloured volatile substances. It is unstable at room temperature to give xenon and oxygen. XeO 4 dissolves in water to form perxenic acid and in alkali it form perxenate salts. 4. Xenon tetroxide can also react with xenon hexafluoride to give xenon oxyfluorides

35. Structure : . Xenon atom in xenon tetroxide undergoes sp 3 hybridisation. Four SP 3 hybrid orbitalsof xenon overlap with p orbitals of oxygen to form four sigma bonds. There are four d orbitals on xenon containg unpaired electron each. They overlap sidewise with p orbitas of oxygen atoms to form four π bonds. The shape is tetrahedral with bond angle 109.5 .

36. Electronic configuration Xe 54 = [Kr] 4d 10 5s 2 5p 6 Ground state : 5s 5p 4d Excited state : ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ 5s 5p 4d Hybridisation : sp 3 ↑