HALOGEN OXIDES AND OXYACIDS

In spite of their relevance to air pollution and to the combustion of sulphur-contaminated fuels, kinetic data for the homogeneous decomposition of S03 are practically non-existent. The thermal decomposition is dominated by a heterogeneous component, even in clean silica vessels390 reactions of S03 at a glass surface are evident even at room temperature380. The kinetics of approach to the equilibrium 

S03 is decomposed to S02 and 02 upon photolysis. Absorption, which commences at 3000 A, consists of broad, diffuse bands superimposed on a continuous background392. The relative contributions of the reactions 

The halogens form a number of stable oxides, but information on their thermal and photochemical decompositions in the gas phase is limited to two oxides of fluorine and several oxides of chlorine. Little research has been done on the decompositions of bromine, iodine or mixed-halogen oxides. 

The oxyacids of iodine are solids and will not be discussed here. Perchloric acid is the only oxyacid of bromine or chlorine which is stable in the pure state. There have been a few studies of its pyrolysis in the vapour phase. 

Only two stable fluorine oxides are known. Except for an early investigation of the decomposition of F202, studies of decomposition kinetics have been confined to F20. There still remains considerable doubt concerning the mechanisms by which these two compounds decompose to their elements. Unlike its chlorine analogue, FO has defied spectroscopic detection in the fluid phases, although it is considered an important intermediate in many of the reactions of F20. 

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The halogens and their oxyacids, particularly chlorine and hypochlorous acid, are widely used as oxidizing and bleaching agents. These properties are related to the variations of redox potentials.17... 

The halogens, except fluorine, can be oxidized to positive oxidation states. Most commonly you will encounter these positive oxidation states in a set of compounds called halogen oxyacids and their ions. 

The oxidation of arsine may be accomplished by means of the halogen oxyacids and their salts,8 although not so readily as with the halogens themselves. Hypochlorites and hypobromites cause complete oxidation to arsenic acid, but side reactions are liable to occur, especially if the gas is present in excess. Chloric add slowly oxidises arsine to arsenious acid a trace of silver nitrate catalyses the reaction. Chlorates are quite inactive. More complete oxidation results with solutions of bromic acid and bromates, iodic acid and iodates, especially in the presence of catalysts. The reactions are of the type represented by the equation8... 

The oxidation states of the halogens vary from -1 to +7. Identify compounds of chlorine that have 1, -1-1, -1-3, -1-5, and -1-7 oxidation states. How does the oxyacid strength of the halogens vary as the number of oxygens in the formula increases ... 

All of the halogens form oxyacids and salts of these acids. The names of these compounds are determined by the oxidation state of the halogen. 

The halogens and their oxyacids probably are the most important oxidants used in the carbohydrate field. They are widely used as bleaching agents, but the mechanism of this action remains to be clarified. As reagents for preparatory purposes (particularly for aldonic acids and lactones) and for analytical procedures, they are very important. Periodic acid, discussed in a later section, has an important application for the elucidation of structures of carbohydrates. A number of valuable commercial products are made by treatment of polysaccharides with halogens, particularly chlorine or hypochlorous acid, but the nature of these actions, such as the modification of starch, has not been clarified. 

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