Sulfur trioxide (British spelling: Sulphur trioxide) has the chemical formula SO₃.

Sulfur trioxide is liquid, density 1.92 g/cm³, boiling point +43.7°C, melting point +16.8°C. 3 electron pairs for VSEPR consideration.

When mixed with water, it produces sulfuric acid (H₂SO₄):

SO₃ + H₂O = H₂SO₄     (+ 88 kJ)

which, upon heating, boils at ~290°C at atmospheric pressure, and begins to decompose back into sulfur trioxide and water at 340°C.

Sulfur trioxide can be prepared by the two-stage pyrolysis of sodium bisulfate:

2NaHSO₄ → Na₂S₂O₇ + H₂O

Na₂S₂O₇ → Na₂SO₄ + SO₃

This method will work for other metal bisulfates, the controlling factor being the stability of the intermediate pyrosulfate salt.

Industrially it is made by the contact process. sulfur dioxide generally made by the burning of sulfur or pyrites (a sulfide ore of iron) is first purified by electrostatic precipitation and this is then oxidised by atmospheric oxygen at between 400°C and 600°C over a catalyst consisting of vanadium pentoxide V₂O₅ activiated with potassium oxide K₂O on kieselguhr or silica support. Platinum also works very well but is simply too expensive and is poisoned (rendered ineffective) much more easily by impurities.

The majority of sulphur trioxide made in this way is converted into sulfuric acid not by the direct addition of water, with which it forms a fine mist, but by absorption in concentrated sulfuric acid and dilution with water of the produced oleum.

Sources

• WebElements
• NIST Standard Reference Database
• European Chemicals Bureau

See also

• Hypervalent molecule

External links

Oxides | Sulfur compounds | Acid anhydrides

Oxid sírový | Schwefeltrioxid | Trióxido de azufre | Trioxyde de soufre | Zwaveltrioxide | 三酸化硫黄 | Tlenek siarki(VI) | Svaveltrioxid | Триоксид сірки