Sulphur trioxide, SO

The moiecule SO3 has zero dipole moment and e.d. shows it to be a symmetrical planar molecule (S-0, 143 A, 0—S-0, 120 ). ° Like P2O5 SO3 has a number of polymorphs, and as in the case of P2OS the forms differ appreciably in their stability towards moisture. 

The orthorhombic modification consists of trimers (Fig. 16.5(a)). The bond length in the ring is 1 -62 A, and there is an unexpected (and unexplained) difference between the lengths of the axial and equatorial bonds, 1-37 A and 1-43 A respectively. The 0 bond angle is 121-5°. 

In the asbestos-like form the tetrahedral SO4 groups are joined to form infinite chain molecules (Fig. 16.5(b)), with S—0 in the chain 1-61 A and to the unshared 0, 1-41 A. In addition to these two well-defined forms of known structure the existence of others has been postulated to account for the unusual physical properties of solid and liquid sulphur trioxide. The properties of the liquid are still incompletely understood and the system is obviously complex—compare sulphur itself, to which SO3 is topologically similar in that both can form chain and ring polymers, and units being -S- and -0(802)0—. 

Other oxides of sulphur which have been described include the blue-green 8203 (formed by the reaction of 8 with liquid 8O3), and 8O4, produced by the action of an electric discharge on a mbcture of O2 or O3 with 8O2 or 8O3. The existence of 82O7 seems less certain. 

Pure SeOs may be prepared by heating K2Se04 with SO3, which gives a liquid mixture of SeOa and SO3 from which the more volatile SO3 may be distilled. Like SO3 it is at least dimorphic. Although the cyclic modification forms mixed 

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Selenium Sulphoxide or Sulphur - Selenium Sesquioxide, SeS03.—This has already been mentioned in connection with the solubility of selenium in sulphuric acid (pp. 291, 300) as the probable cause of the green colour produced. It is also obtainable, similarly to sulphur sesquioxide, from fluid sulphur trioxide by the addition of powdered amorphous selenium, the sulphoxide being insoluble in the sulphur trioxide, so that the latter can be decanted off ... 

Hence the strength of the acid goes up as sulphur trioxide is dissolved in it. The acidity of pure and fuming sulphuric acids is not so apparent as in ordinary aqueous acids because it is masked by the oxidising and other properties moreover, the conductivity... 

Lead will resist chlorine up to about 100°C , is used for dry bromine at lower temperatures and is fairly resistant to fluorine . Hydrofluoric acid does not passivate lead, so lead should not be used in this environment. Lead is very resistant to sulphur dioxide and fairly resistant to sulphur trioxide, wet or dry, over a wide temperature range . ... 

Sulphur Trioxide (SO2 -I- O2) Linear reaction rates are observed due to phase boundary control by adsorption of the reactant, SO3. Maximum rates of reaction occur at a SO2/O2 ratio of 2 1 where the SO3 partial pressure is also at a maximum. With increasing 02 S02 ratio the kinetics change from linear to parabolic and ultimately, of course, approach the behaviour of the Ni/NiO system. At constant gas composition and pressure, the reaction also reaches a maximum with increasing temperature due to the decreasing SO3 partial pressure with increasing temperature, so that NiS04 formation is no longer possible and the reaction rate falls. 

If nitration acid is made up using oleum in place of the 98 per cent sulphuric acid, there will be four distinct chemical species sulphuric acid, sulphur trioxide, nitric acid, water. The sulphur trioxide will react with the water producing sulphuric acid so there are only three independent components... 

The copolymer produced in this way has been found to be insoluble and infusible but is still chemically reactive. On heating the polymer with a mixture of sulphur trioxide and sulphuric acid, the sulphonic acid group gets introduced into the aromatic rings. The resin so formed is known as a cation exchanger. 

So far, no reference has been made to the presence of more than one phase in the reactor. Many important chemicals are manufactured by processes in which gases react on the surface of solid catalysts. Examples include ammonia synthesis, the oxidation of sulphur dioxide to sulphur trioxide, the oxidation of naphthalene to phthalic anhydride and the manufacture of methanol from carbon monoxide and hydrogen. These reactions, and many others, are carried out in tubular reactors containing a fixed bed of catalyst which may be either a single deep bed or a number of parallel tubes packed with catalyst pellets. The latter arrangement is used, for exjimple, in the oxidation of ethene to oxiran (ethylene oxide)... 

Thus if we insert the values m = 1 and w = 2 in the first case and m = 2, K = 1 in the second, so that the hydrogen atom is held on by one elementary space and the hydrogen molecule by two elementary spaces, the conception of a uni- and divalent union is readily realised. In this way a valency of two for carbon monoxide, sulphur trioxide and ammonia on platinum two for the oxygen molecule and one for carbon monoxide on quartz can be established. 

Sulphonation Pyrrole, furan and thiophene undergo sulphonation with the pyridine-sulphur trioxide complex (CsHsN+SOs ). 

The limit of concentration attainable by any of the foregoing methods is about 98 per cent. If acid of higher concentration than this is required, acid of 97 to 98 per cent, strength can be partially frozen,2 when the colourless prisms which separate contain 99-5 to 100 per cent. H2S04 and constitute the frequently so-called, but misnamed, monohydrate. Owing to the success of the contact process for the preparation of sulphur trioxide and of fuming sulphuric acid, the production of highly concentrated sulphuric acid has been rendered so simple a matter that concentration of the lead chamber acid is of diminished importance. The main value of the chamber process lies in its economical production of a not necessarily very concentrated acid. There are indications, however, that by a suitable combination of the two processes, an even more economical production of concentrated acid may be obtainable (see p. 162). 

Arsenic pentoxide catalyses the reaction between sulphur dioxide and oxygen,9 the amount of sulphur trioxide formed reaching 54 per cent, at 660° C. The reaction consists in the alternate reduction of the pentoxide to arsenious oxide by the sulphur dioxide and reoxidation to the pentoxide, so that arsenious oxide acts similarly. The catalytic activity is less than that of ferric oxide, but the latter is activated by addition of arsenic pentoxide the maximum amount of conversion increases from 69-5 to 78-5 per cent, and occurs at a temperature 63° lower than is required in the absence of the promoter. Arsenic pentoxide does not activate catalysts which act rapidly, such as vanadium pentoxide. Platinum and silver catalysts are poisoned by arsenic pentoxide.10... 

This reaction is just similar to the synthesis of ammonia described above. So, the effect of pressure, temperature and concentration will be the same as mentioned in the synthesis of ammonia. Low temperature, high pressure and increased concentrations of S02 and 02 will favour the formation of sulphur trioxide. 

However, many species which are acids under Lewis definition cannot be termed so according to Bronsted definition. A few examples are sulphur trioxide and halides of boron, aluminium, iron (ferric) and zinc. The central atom in each is able to accept a pair of electrons to complete is octet. 

Gas phase oxidation of sulphur dioxide is kinetically inhibited and virtually impossible without a catalyst at any temperature. At ordinary temperatures the reaction is so slow that, in practical terms, it does not occur at all. Increasing the temperature increases the rate of reaction, but simultaneously the position of the equilibrium shifts unfavourably -away from sulphur trioxide and towards sulfur dioxide and oxygen. Without a catalyst, the temperature needed to make the system react at a practical speed is so high that a very poor conversion [i.e. very little SO3 production] is obtained."... 

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