Disulfuric acid

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Disulfuric acid
Structural formula of disulfuric acid Ball and stick model of disulfuric acid
CAS number 7783-05-3 YesY
PubChem 62682
ChemSpider 56433 YesY
EC number 231-976-8
MeSH Pyrosulfuric+acid
ChEBI CHEBI:29211 YesY
Jmol-3D images Image 1
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Molecular formula H2O7S2
Molar mass 178.14 g mol−1
Melting point 36 °C (97 °F; 309 K)
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

Disulfuric acid (or pyrosulfuric acid) is an oxoacid of sulfur. It is a major constituent of fuming sulfuric acid, oleum, and this is how most chemists encounter it. It is also a minor constituent of liquid anhydrous sulfuric acid due to the equilibria:

H2SO4 ⇌ H2O + SO3
SO3 + H2SO4 ⇌ H2S2O7

The acid is prepared by reacting excess SO3 with sulfuric acid:

H2SO4 + SO3 → H2S2O7

Disulfuric acid can be seen as the sulfuric acid analogue of an acid anhydride. The mutual electron-withdrawing effects of each sulfuric acid unit on its neighbour causes a marked increase in acidity. Disulfuric acid is strong enough to protonate "normal" sulfuric acid in the (anhydrous) sulfuric acid solvent system. There are salts of disulfuric acid, commonly called pyrosulfates, e.g. potassium pyrosulfate.

There are other related acids with the general formula H2O·(SO3)x though none are isolable.

See also[edit]