Disulfuric acid

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Disulfuric acid
Dischwefelsäure - Disulfuric acid.svg
Ball and stick model of the disulfuric acid molecule
IUPAC name
Disulfuric acid [1]
Other names
Pyrosulfuric acid, Oleum
7783-05-3 YesY
ChEBI CHEBI:29211 YesY
ChemSpider 56433 YesY
ECHA InfoCard 100.029.069
EC Number 231-976-8
Jmol 3D model Interactive image
Interactive image
Interactive image
MeSH Pyrosulfuric+acid
PubChem 62682
Molar mass 178.13 g·mol−1
Melting point 36 °C (97 °F; 309 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Disulfuric acid (alternative spelling disulphuric acid) or pyrosulfuric acid (alternative spelling pyrosulphuric acid), also named oleum is an oxyacid 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]