Sulfur trioxide
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| Sulfur trioxide | |
|---|---|
| Other names | Sulfuric anhydride Sulfan® Sulphur trioxide |
| Molecular formula | SO3 |
| Molar mass | 80.06 g/mol |
| CAS number | [7446-11-9] |
| Density | 1.92 g/cm3 |
| Solubility (water) | Hydrolysis |
| Melting point | 16.9°C |
| Boiling point | 45°C |
| Thermodynamic data | |
| Standard enthalpy of formation ΔfH°gas | -397.77 kJ/mol |
| Standard molar entropy S°gas | 256.77 J.K–1.mol–1 |
| Heat capacity Cp,gas | 24.02 J.K–1.mol–1 |
| Safety data | |
| EU classification | Corrosive (C) |
| R-phrases | R14, R35, R37 |
| S-phrases | S1/2, S26, S30, S45 |
| Disclaimer and references |
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 suphide 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
See also
Template:Inorganic-compound-stubcs:Oxid sírový de:Schwefeltrioxid es:Trióxido de azufre fr:Trioxyde de soufre nl:Zwaveltrioxide ja:三酸化硫黄 pl:Tlenek siarki(VI) uk:Триоксид сірки