Sulphur hexafluoride

Sulphur Hexafluoride. As a result of a chemical laser study it has been proposed that collision-induced dissociation (78) rather than abstraction (79) is the principal path in the reaction of atoms of energy up to 430 kJ mol with SF . 

This compound also possesses a comparatively large ionisation potential (15.3 eV)163,164, and one of the largest known cross-sections for the capture of thermal electrons. The latter process has been studied in considerable detail by beam, swarm and microwave techniques104 165-170. The initial attachment gives rise to a vibrationally excited ion169,17°, viz. 

When formed from thermal electrons at room temperature its lifetime with respect to auto-ionisation is about 25 psec170. Consequently few of these ions could escape collisional stabilization 

The cross-section for electron attachment shows an inverse dependence on electron velocity170, and for this reason there has been a marked inconsistency in the cross-sections obtained by different methods. Mahan and Young104 have reported a capture rate coefficient for thermal electrons of 2x 1014 l.mole-1.sec-1. This was obtained by a microwave technique in the presence of helium as a moderating gas. 

The comparatively high ionisation potential of sulphur hexafluoride and its inertness toward attack by thermal hydrogen atoms have lead to its use as a specific scavenger for electrons in several irradiated systems. This has already been illustrated in section 1.7.2. The ionisation processes in SF6 have been studied by beam techniques171, but to date there has been no investigation of its radiolysis per se. Such a study would be well worthwhile. 

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Sulphur hexafluoride, SF, m.p. — 5LC. Formed S plus Fj. Very inert material used as an inert dielectric. S2F10 (toxic) is also formed from S plus F2 and there is an extensive chemistry of SFj derivatives (e.g. SF5CI, CIF plus SFJ. 

Both selenium hexafluoride and tellurium hexafluoride are more reactive than sulphur hexafluoride. Tellurium hexafluoride is slowly hydrolysed by water to telluric) VI) acid and on heating it decomposes to fluorine and the tetrafluoride. 

If a molecule has a centre of inversion (or centre of symmetry), i, reflection of each nucleus through the centre of the molecule to an equal distance on the opposite side of the centre produces a configuration indistinguishable from the initial one. Figure 4.4 shows s-trans-buta-1,3-diene (the x refers to trans about a nominally single bond) and sulphur hexafluoride, both of which have inversion centres. 

Figure 4.4 Inversion centre, i, in (a) s-traws-buta-f, 3-diene and (b) sulphur hexafluoride...

Figure 5.1 Principal inertial axes of (a) hydrogen cyanide, (b) methyl iodide, (c) benzene, (d) methane, (e) sulphur hexafluoride, (f) formaldehyde, (g) s-lraws-acrolein and (h) pyrazine...

With the availability of 3.3 and 6.6 kV vacuum contactors the control of HT motors up to 6.6 kV systems has now become easier and economical, compact and even more reliable. For 11 kV. systems, vacuum as well as SF (Sulphur hexafluoride) breakers can be used. The HT motor s switching and protection through a vacuum contactor provides a replica of an LT system. The earlier practice of using an HT OCB, MOCB, or an air blast circuit breaker for the interruption of an HT circuit is now a concept of the past. 

An extreme example of hybidization is the structure proposed for sulphur hexafluoride, SFe. The six S-F bonds are dhected to the apices of a regular octahedron. An aiTangement which would satisfy this number of covalent bonds is sp d hybridization. The ground state of the sulphur atom is s p° and... 

Whereas some atoms have only one valency, others have several, e.g. sulphur has valencies of two, four and six and can form compounds as diverse as hydrogen sulphide, H2S (valency two), sulphur dioxide, SO2 (valency four) and sulphur hexafluoride, SF6 (valency six). Clearly some compounds comprise more than two different elements. Thus hydrogen, sulphur and oxygen can combine to produce sulphuric acid, H2SO4. From the structure it can be seen that hydrogen maintains its valency of one, oxygen two and sulphur is in a six valency state. 

Sulfometuron methyl Sulphur dioxide Sulphur hexafluoride Sulphuric acid -... 

Sulphur hexafluoride steel for moist gas Most common metals. ... 

Sulphur hexafluoride Sulphuric acid Sulphur monochioride Suiphuryl fluoride 2, 4, 5-T Tantalum TEDP... 

Sulphur dioxide Sulphur hexafluoride C T O Most common metals for dry gas. Lead, carbon, aluminium and stainless steel for moist gas Most common metals. Copper, stainless steel and aluminium are resistant to the decomposition products at 150°C Zinc... 

Johnson KA, Westberg HH, Lamb BK, Kincaid RL. The use of sulphur hexafluoride for measuring methane emissions from farm animals. In Proceedings of the 1st International Conference on Greenhouse Gases and Animal Agriculture, Obihiro, Hokkaido, Japan. 2001. pp. 72-81. 

Two mole per cent sulphur hexafluoride caused only a slight reduction in G(H2). By contrast, in gaseous HC1 2 x 10-2 mole % sufficed to halve the yield of hydrogen from electron reactions. The difference is readily understood since the HC1 concentration in the liquid is such that the reaction... 

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