Tellurium trioxide

Tellurium trioxide, TeOa, is an orange yellow powder made by thermal decomposition of telluric(VI) acid Te(OH)g. It is a strong oxidising agent which will, like H2Se04, oxidise hydrogen chloride to chlorine. It dissolves in hot water to give telluric(VI) acid. This is a weak acid and quite different from sulphuric and selenic acids. Two series of salts are known. 

Tellurium Trioxide, Te03.—This, like so many of the tellurium compounds, was discovered by Berzelius. It is formed when telluric acid is heated a little above 360° C. Care is necessary, since overheating induces decomposition of the yellow trioxide into the colourless dioxide. Any dioxide that forms can be removed by treating with concentrated hydrochloric acid, in which the trioxide is only sparingly soluble.6... 

Tellurium trioxide is an orange-yellow solid, of density 5T0 at the ordinary temperature.7 Its heat of formation per gram-molecular weight is 83-6 Calorics,8 and according to Mixter the decomposition of the trioxide into the dioxide is an exothermic reaction (cf. p. 380). As might therefore be expected, this decomposition occurs readily and at such a temperature that the resulting dioxide is unfused. 

The trioxide is insoluble in water, hot or cold, in nitric acid and in cold hydrochloric acid. Concentrated hydrochloric acid when heated attacks it with liberation of chlorine and formation of tellurium tetrachloride in the solution. The caustic alkalis only dissolve tellurium trioxide in hot concentrated solution, with formation of tellurates. 

Tellurium trioxide gives rise to several hydration products which can all be regarded as telluric acids, but which are more conveniently considered as products of the further hydration of telluric acid, HaTe04, the compound H2Te04.2H20 (possibly an ortho-55 telluric acid, H6Te06, see p. 387) being the most stable. 

By very cautious heating at 140° C. the dihydrate very gradually loses a bimolccular proportion of water, forming allotelluric acid, H2Te04, which is a loose powder of density 3-43 at 19° C. 3 on further heating it passes successively into tellurium trioxide and tellurium dioxide. 

Tellurium and Oxygen Tellurium Monoxide—Tellurium Dioxide and the Tolluritos—Tellurium Trioxide, Telluric Acids and the Tollurates. Tellurium and Sulphur —Tellurium Disulphide, Tellurium Sulplioxide, Tellurium Sulphates, Telluropentathionie Acid. 

In Group VIA (16) the same phenomenon is encountered. Selenium trioxide is thermodynamically unstable relative to sulfur trioxide and tellurium trioxide. The enthalpies of formation of SF6, SeF6, and TeFft are -1210. — 1117, and — 1320 kj mol 1, respectively. This indicates comparable bond energies for S—F and Te—F bonds (317 and 330 kj mol-1, respectively), which are more stable than Se—F bonds (285 kj mol 1). 

Monomeric selenium trioxide (Se03) and tellurium trioxide (Te03) have a trigonal planar structure in the gas phase. In the solid state, Se03 forms cyclic tetramers (Se03)4, in which each Se atom connects two bridging O atoms and two terminal O atoms, with Se-Ob 177 pm and Se-Ot 155 pm (AX4 type). 

Tellurium trioxide is made by dehydration of Te(OH)6. This orange compound reacts only slowly with water but dissolves rapidly in bases to give tellurates. 

TeOg, tellurium trioxide HgTeO(j, telluric acid TeFg, tellurium hexafluoride (gas)... 

Tellurium trioxide, TeOs, is made by carefully heating H2TeO< to a red heat. It is an orange-yellow crystalline substance, sparingly soluble in water, and easily decomposed by heat, forming the dioxide and oxygen. 

The trioxides, SeOg and TeOg, are also known, but they are not obtained by oxidation of the dioxide as is the case with SO3. The selenium and tellurium trioxides are usually prepared by dehydration of their respective oxoacids, H2Se04 and Te(OH)6 or (HgTeOe). 

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