Disulfuric acid

Disulfuric acid

150px
Ball and stick model of the disulfuric acid molecule
Names
IUPAC name Disulfuric acid
Other names Pyrosulfuric acid, Oleum or pyrosulfuric acid
Identifiers
CAS Number	7783-05-3 Y
ChEBI	CHEBI:29211 Y
ChemSpider	56433 Y
EC Number	231-976-8
Jmol 3D model	Interactive image Interactive image Interactive image
MeSH	Pyrosulfuric+acid
PubChem	62682
InChI InChI=1S/H2O7S2/c1-8(2,3)7-9(4,5)6/h(H,1,2,3)(H,4,5,6) Y Key: VFNGKCDDZUSWLR-UHFFFAOYSA-N Y InChI=1/H2O7S2/c1-8(2,3)7-9(4,5)6/h(H,1,2,3)(H,4,5,6) Key: VFNGKCDDZUSWLR-UHFFFAOYAZ
SMILES OS(=O)(=O)OS(O)(=O)=O OS(=O)(=O)OS(=O)(=O)O O=S(=O)(O)OS(=O)(=O)O
Properties
Chemical formula	H2O7S2
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 (or pyrosulfuric 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

• Disulfurous acid
• Oleum
• Sulfuric acid
• Sulfur oxoacid