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Concept
Perxenate
Summary
In chemistry, perxenates are salts of the yellow xenon-containing anion XeO64-. This anion has octahedral molecular geometry, as determined by Raman spectroscopy, having O–Xe–O bond angles varying between 87° and 93°. The Xe–O bond length was determined by X-ray crystallography to be 1.875 Å.
Perxenates are synthesized by the disproportionation of xenon trioxide when dissolved in strong alkali:
2 XeO3 ( ()) + 4 OH− ( ()) → Xe ( ()) + XeO64- ( ()) + O2 ( ()) + 2 H2O ( ())
When Ba(OH)2 is used as the alkali, barium perxenate can be crystallized from the resulting solution.
Perxenic acid is the unstable conjugate acid of the perxenate anion, formed by the solution of xenon tetroxide in water. It has not been isolated as a free acid, because under acidic conditions it rapidly decomposes into xenon trioxide and oxygen gas:
Its extrapolated formula, H4XeO6, is inferred from the octahedral geometry of the perxenate ion (XeO64-) in its alkali metal salts.
The pKa of aqueous perxenic acid has been indirectly calculated to be below 0, making it an extremely strong acid. Its first ionization yields the anion H3XeO6-, which has a pKa value of 4.29, still relatively acidic. The twice deprotonated species H2XeO62- has a pKa value of 10.81. Due to its rapid decomposition under acidic conditions as described above, however, it is most commonly known as perxenate salts, bearing the anion XeO64-.
Perxenic acid and the anion XeO64- are both strong oxidizing agents, capable of oxidising silver(I) to silver(III), copper(II) to copper(III), and manganese(II) to permanganate. The perxenate anion is unstable in acidic solutions, being almost instantaneously reduced to HXeO4-.
The sodium, potassium, and barium salts are soluble. Barium perxenate solution is used as the starting material for the synthesis of xenon tetroxide (XeO4) by mixing it with concentrated sulfuric acid:
Ba2XeO6 (s) + 2 H2SO4 (l) → XeO4 (g) + 2 BaSO4 (s) + 2 H2O (l)
Most metal perxenates are stable, except silver perxenate, which decomposes violently.
Official source
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