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
3D model (JSmol)
ECHA InfoCard 100.029.069
EC Number 231-976-8
MeSH Pyrosulfuric+acid
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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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]