Allotropic Forms of Sulphur

The Element.— Two reviews on the properties of elemental sulphur have been published. The first deals with new allotropic forms of sulphur as prepared by the reaction of polysulphides with either SaCla or SCla, the structures of Se, S7, Sg, Sg, Sio, Sii, Sxa, and Sig, and polysulphides up to S . The second reviews sulphur modifications sulphur rings containing heteroatoms and the structures of sulphur molecular ions, melts, and vapours. 

Van t Hoff then deals with chemical equilibrium on the basis of the law of mass action, and the change of equilibrium constant with temperature, introducing the case of condensed systems in the absence of vapour and a transition point (point de transition). Physical equilibria are special cases of chemical equilibria. Graphical methods with vapour pressure curves (e.g. for the allotropic forms of sulphur) are introduced. The principle of mobile equilibrium is explained for homogeneous and heterogeneous equilibria, and the Thomsen-Berthelot principle criticised (see pp. 614, 620). The last chapter, on affinity ,gives the definition The work of affinity (A) is equal to the heat produced in the transformation (q), divided by the absolute temperature of the transition point (P) and multiplied by the difference between this and the given temperature (P) ... 

The equilibrium in the solid state between such substances as the allotropic forms of tin or sulphur, or between the participants in any other chemical reaction, is governed by principles precisely similar to those which regulate the coexistence of phases such as solid and liquid. 

In 1833 Berzelius used the names empirical (empirische) and rational (rationelle) formulae for those giving the result of analysis (e.g. alcohol C H 0) and those showing the electrochemical division of the atoms (e.g. C H + H 0 or C H + 0), respectively. In 1840 he introduced the name allotropy for the existence of different varieties of an element (sulphur, carbon, silicon) different forms of a compound may contain different allotropic forms of an element, e.g. Sa and Sj3 in pyrites and marcasite. 

Like sulphur, selenium exists in a number of allotropic forms. These include both crystalline, rhombic and monoclinic modifications... 

The presence of the u- and 7r-modifieations of sulphur dissolved in molten A-sulphur naturally causes a depression of the freezing-point of the latter, and from the magnitude of this effect it has been possible to demonstrate the probability of a molecular weight corresponding to S6 for the dark brown p.-allotrope, a result which is of especial interest as correlating this form of liquid sulphur with the hexatomic sulphur believed to occur in sulphur vapour (see before).3... 

The latent heat of fusion of solid sulphur varies with the different allotropic forms and with the temperature it is lowest for octahedral sulphur it is also influenced by the proportion of insoluble sulphur (y- or fjt,-) in the original solid and in the liquid formed.8 By electrical heating at the melting-point, the latent heat of fusion of monoclinie sulphur has been found to be 4 8-85 gm.-cals. per gm. 

Allotropic Forms, (a) Dissolve in a dry test tube a small piece of roll sulphur in 3 to 5 cc. of carbon disulphide. Pour the clear solution on a watch glass and allow it to evaporate spontaneously under the hood. Examine the crystals. This is rhombic sulphur. 

Sulphur exists in several allotropic forms, and it has been found that the basic constituent of them all is a molecule of eight sulphur atoms Sg. The molecule has a ring structure and in it the sulphur atoms are considered to use sp3 hybridised orbitals (from the 3 and 3p orbitals). There is overlap between adjacent sulphur atoms by the two partly filled sp3 hybrid orbitals, and the two fully occupied sp3 hybrid orbitals constitute lone pairs. These... 

Sulphur has various allotropic forms. Below 95.6°C the stable crystd form is rhombic above this temperature the element transforms into a triclinic form. These crystalline forms both contain cyclic Sb molecules. At temperatures just above its melting point, molten sulphur is a yellow liquid containing Ss rings (as in the soiid form). At about 160°C, the sulphur atoms form chains and the iiquid becomes more viscous and dark brown. If the molten sulphur is cooled quickly from this temperature (e.g. by pouring into cold water) a reddish-brown solid known as plastic sulphur is obtained. Above 200°C the viscosity decreases. Sulphur vapour contains a mixture of S2, S4, Sg, and Ss molecules. Flowers of sulphur is a yellow powder obtained by subliming the vapour. It is used as a plant fungicide. The element is also used to produce sulphuric acid and other sulphur compounds. 

There are now doubts as to whether Venus is in fact extremely hostile to life. An audacious theory suggests that the cloud cover in the Venusian atmosphere could have provided a refuge for microbial life forms. As the hot planet lost its oceans, these primitive life forms could have adapted to the dry, acid atmosphere. However, the intensity of the UV radiation is very puzzling. The authors suggest that sulphur allotropes such as Sg act on the one hand as a UV umbrella and on the other as an energy-converting pigment (Schulze-Makuch et al 2004). 

By crystallising sulphur from chloroform solution containing rubber as a thickening agent and a few drops of benzonitrile, two other forms, also claimed to be distinct allotropes, have been obtained,7 designated respectively and 7)-sulphur. Both forms are practically colourless and the crystals are doubly refracting the former crystallises in rhombic plates and the latter in hexagonal plates. 

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