Oxides of selenium and tellurium

Selenium and tellurium dioxides are white solids obtained by direct combination of the elements. The polymorph of Te02 so formed is 0t-TeO2, whereas P-Te02 occurs naturally as the mineral tellurite. Both forms of Te02 contain structural units 15.43 which are connected by shared O 

The eruption of a volcano is accompanied by emissions of water vapour ( 70% of the volcanic gases), CO2 and SO2 plus lower levels of CO, sulfur vapour and CI2. Carbon dioxide contributes to the greenhouse effect, and it has been estimated that volcanic eruptions produce 112 million tonnes of CO2 per year. Levels of CO2 in the plume of a volcano can be monitored by IR spectroscopy. Sulfur dioxide emissions are particularly damaging to the environment, since they result in the formation of acid rain. Sulfuric acid aerosols persist as suspensions in the atmosphere for long periods after an eruption. The Mount St Helens eruption occurred in May 1980. Towards the end of the eruption, the level of SO2 in the volcanic plume was 2800 tonnes per day, and an emission rate of p 1600 tonnes per day was measured in July 1980. Emissions of SO2 (diminishing with time after the major eruption) continued for over two years, being boosted periodically by further volcanic activity. 

Casadevall, W. Rose, T. Gerlach, L.P. Greenland, J. Ewert, R. Wunderman and R. Symonds (1983) Science, vol. 221, p. 1383 - Gas emissions and eruptions of Mount St. Helens through 1982 . 

Malinconico, Jr (1979) Nature, vol. 278, p. 43 - Fluctuations in SO2 emission during recent eruptions of Etna . 

Symonds, T.M. Gerlach and M.H. Reed (2001) Journal of Volcanology and Geothermal Research, vol. 108, p. 303 - Magmatic gas scrubbing Implications for volcano monitoring . 

Explosive eruption of Mount St Helens, Washington, US on 22 July 1980. 

Oppenheimer (2004) in Treatise on Geochemistry, eds H.D. Holland and K.K. Turekian, Elsevier, Oxford, vol. 3, p. 123 - Volcanic degassing . 

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Sulfation Roasting. Acid roasting technology (Fig. 2) rehes on differences in the volatiUty of the tetravalent oxides of selenium and tellurium at roasting temperatures of 500—600°C to selectively volatilise selenium from slimes. Acid roasting uses sulfuric acid as the oxidant for the conversion of selenium/selenides and tellurium/teUurides to their respective tetravalent oxides. Typical oxidation reactions are as foUow ... 

CAS/JER] Castro, A., Jerez, A., Pico, C., Veiga, M. L., Preparation and characterization of mixed oxides of selenium and tellurium, J. Chem. Soc. Dalton Trans., (1982), 733-736. Cited on pages 185, 186. 

Dehydrogenation activities of the oxides of selenium and tellurium were observed by the pulse technique.Thus, the zirconium oxides which are loaded with selenic acid or telluric acid and calcined in air can dehydrogenate 2-propanol to acetone and hexane to benzene.In a typical reaction of 2-propanol, the conversion into acetone decreases continuously after the third pulse, probably owing to a decrease in the amount of oxygen on the catalyst surface. Poisoning experiments with injection of CO2, H2O or butylamine at 523 K before reaction had no effect on the yield of acetone. Thus this dehydrogenation process appears to be an oxidative dehydrogenation. No studies on the acidic or basic character of oxides of selenium and tellurium oxides have been reported. 

The controlled-potential oxidation of selenium and tellurium has been studied extensively by Lingane and Niedrach (222). Both selenium and tellurium (—II) undergo well defined two-electron oxidation processes at all pH values. For selenium (—II) the primary anodic reaction seems to involve oxidation of the mercury electrode followed by formation of HgSe while tellurium (—II) seems to be directly oxidized to tellurium metal. In strongly alkaline solution tellurium (—II) appears to undergo an additional incomplete oxidation to TeOs-... 

C. Determination of mixtures of selenium and tellurium Procedure. Dissolve the mixed oxides (not exceeding 0.25 g of each) in 100 mL of concentrated hydrochloric acid, and add with constant stirring 50 mL cool concentrated hydrochloric acid which has been saturated with sulphur dioxide at the ordinary temperature. Allow the solution to stand until the red selenium has settled, filter through a weighed filtering crucible (sintered-glass or porcelain) and complete... 

The possibility of preparation of selenium and tellurium containing dications by direct oxidation with triflic anhydride was investigated only recently.119 An example is provided by the synthesis of dication 110 from mixed bis-chalco-genide 109 (Scheme 42). 

Shock-tube experiments on the decomposition of hydrogen sulphide have been performed but were unsuccessful because traces of oxygen and other oxidizers could not be removed from the reactant24. No data are available on the homogeneous decomposition of hydrogen polysulphides, nor have the kinetics of pyrolysis of selenium and tellurium hydrides been studied. 

Two-electron oxidations and reductions of selenium and tellurium compounds 80... 

Although there are similarities between the chemistry of the chalcogenide elements, the properties of selenium and tellurium clearly lie between those of non-metallic sulfur and metallic polonium. The enhancement in metallic character as the group is descended is illustrated in the emergence of cationic properties by polonium, and marginally by tellurium, which are reflected in the ionic lattices of polonium(IV) oxide and tellurium(IV) oxide and the formation of salts with strong acids. 

The principal oxidation states of sulfur are —2, 0, +4, and +6. These states are represented by many important substances, including those given in the diagram below. The important compounds of selenium and tellurium have similar formulas. 

Give the names and formulas of the oxides and oxygen acids of selenium and tellurium. 

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