Telluric acid

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Telluric acid
Telluric acid
Ball-and-stick model of telluric acid
Identifiers
CAS number 7803-68-1 YesY
PubChem 61609
ChemSpider 55517 YesY
ChEBI CHEBI:30463 YesY
Jmol-3D images Image 1
Properties
Molecular formula H6O6Te
Molar mass 229.64 g/mol
Appearance White monoclinic crystals
Density 3.07 g/cm3
Melting point 136°C (409.15 K)
Solubility in water 50.1 g/100 ml at 30°C1
Acidity (pKa) 7.68, 11.0 at 18°C1
Structure
Molecular shape octahedral
Dipole moment 0 D
Hazards
Main hazards corrosive
Related compounds
Other anions hydrotelluric acid
tellurous acid
hydrogen telluride
Related compounds Teflic acid,
Sulfuric acid
Selenic acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Telluric acid is a chemical compound with the formula Te(OH)6. It is a white solid made up of octahedral Te(OH)6 molecules which persist in aqueous solution.2 There are two forms, rhombohedral and monoclinic, and both contain octahedral Te(OH)6 molecules.3 Telluric acid is a weak acid which is dibasic, forming tellurate salts with strong bases and hydrogen tellurate salts with weaker bases or upon hydrolysis of tellurates in water.34

Preparation

Telluric acid is formed by the oxidation of tellurium or tellurium dioxide with a powerful oxidising agent such as hydrogen peroxide, chromium trioxide or sodium peroxide.3

TeO2 + H2O2 + 2H2O → Te(OH)6

Crystallisation of telluric acid solutions below 10°C gives Te(OH)6.4H2O.2 It is oxidizing, as shown by the electrode potential for the reaction below, although it is kinetically slow in its oxidations.3

H6TeO6 + 2H+ + 2e TeO2 + 4H2O Eo = +1.02 V

Chlorine, by comparison, is +1.36V and selenious acid is +0.74V in oxidizing conditions.

Properties and reactions

The anhydrous acid is stable in air at 100°C but above this it dehydrates to form polymetatelluric acid, a white hygroscopic powder (approximate composition (H2TeO4)10), and allotelluric acid, an acid syrup of unknown structure (approximate composition (H2TeO4)3(H2O)4).2
Typical salts of the acid contains the anions [Te(O)(OH)5 and [Te(O)2(OH)42–. The presence of the tellurate ion TeO42– has been confirmed in the solid state structure of Rb6[TeO5[TeO4.5 Strong heating at over 300°C produces the α- crystalline modification of tellurium trioxide, α-TeO3. 4 Reaction with diazomethane gives the hexamethyl ester, Te(OMe)6.2

Telluric acid and its salts mostly contain hexacoordinate tellurium.3 This is true even for salts such as magnesium tellurate, MgTeO4, which is isostructural with magnesium molybdate and contains TeO6 octahedra.3

Other forms of telluric acid

Metatelluric acid, H2TeO4, the tellurium analogue of sulfuric acid, H2SO4, is unknown. Allotelluric acid of approximate composition H2TeO4)3(H2O)4, is not well characterised and may be a mixture of Te(OH)6 and (H2TeO4)n.2

Other tellurium acids

Tellurous acid (H2TeO3), containing tellurium in its +4 oxidation state, is known but not well characterised. Hydrogen telluride is an unstable gas that forms hydrotelluric acid upon addition to water.

References

  1. ^ a b Lide, David R. (1998), Handbook of Chemistry and Physics (87 ed.), Boca Raton, FL: CRC Press, ISBN 0-8493-0594-2 
  2. ^ a b c d e Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419. 
  3. ^ a b c d e f Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5 
  4. ^ a b Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  5. ^ Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 16: The group 16 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 526. ISBN 978-0-13-175553-6. 







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