CID 24682
SO3
Sulfur trioxide
- S 1
- O 3
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- IUPAC name
- sulfur trioxide
- Molar mass
- 80.057(23) g/mol
- Atoms
- 4
- SMILES
- O=S(=O)=O
- InChIKey
- AKEJUJNQAAGONA-UHFFFAOYSA-N
- PubChem CID
- 24682
In air at room temperature, pure sulfur trioxide fumes, as liquid or as solid. HSDB, the Hazardous Substances Data Bank, calls the pure liquid colourless and likens the crystals of its alpha form to ice needles. Each unit joins one sulfur atom to three oxygen atoms, for a molar mass of 80.06 g/mol. The liquid boils at about 45 °C, and OSHA, the US Occupational Safety and Health Administration, records its other name, sulfuric anhydride.
The solid exists in three forms, named alpha, beta and gamma, and only alpha is stable. Molten sulfur trioxide takes the gamma form and tends towards alpha as it solidifies. The forms differ in melting point and in vapour pressure, the pressure of the vapour each gives off.
Structure and bonding
The model holds four atoms: a yellow sulfur at the centre and three red oxygens around it, each joined to the sulfur by a double bond. In Ball-and-stick the oxygens mark the corners of a triangle, with all three O–S–O angles at 120° in this computed model. Every atom lies in one plane, and turned edge-on the model flattens into a single line. Stick leaves only three doubled rods meeting at the sulfur; Space-filling swells the atoms into a flat, rounded triangle of overlapping spheres. Labels prints S at the centre and O on each corner.
That flat triangle is the shape VSEPR (valence-shell electron-pair repulsion) theory predicts for the AX3 case, a central atom bonded to three others and holding no lone pair. Stephen Lower’s Chem1 text names it trigonal planar. The model shows one SO3 unit, but HSDB describes the pure gas and liquid as single units and the trimer S3O9 in a steady balance. Traces of moisture turn the liquid into polymer: first the low-melting beta form, then, as beta cross-links, the high-melting alpha form, both looking like asbestos.
Where it turns up
Sulfur trioxide is usually generated in the plant that will use it, according to HSDB. When it does travel, the hazard database CAMEO Chemicals notes that an inhibitor often goes with it to prevent polymerisation. A product sold under the trademark Sulfan carries a stabiliser for the same purpose, keeping the liquid from turning into its higher-melting forms. Water changes how it treats metal: absolutely dry, it leaves metals uncorroded, according to HSDB, but a solution of it in water corrodes them, on the International Chemical Safety Card.
It also travels dissolved in sulfuric acid. One US Department of Transportation shipping name covers fuming sulfuric acid with 30 per cent or more free sulfur trioxide. The route that makes it belongs with sulfuric acid, and its part in polluted air with sulfur dioxide.
Why it matters
Sulfur trioxide goes into detergents and into sulfonated oils, oils treated with it, HSDB records. In 1983 its main jobs were making detergents and serving as the starting material for chlorosulfuric acid. It is also an intermediate in the manufacture of sulfuric acid itself. Under the 2006 Inventory Update Rule, US national production was reported in the band from 500 million to under 1 billion pounds.
Coal-burning power stations have found a use for it outside chemical making. At many of them it has been added to the cooled combustion gases, to help electrostatic precipitators, the dust collectors in the gas stream, take out more dust. It also serves as an oxidising agent, a powerful one but seldom a selective one. OSHA lists it as a highly hazardous chemical, one whose toxicity or reactivity could lead to a catastrophic event where large quantities are held.
Properties
| Property | Value | Unit | Source |
|---|---|---|---|
| Molecular formula (Hill order) | O3S | ||
| Molar mass | 80.057(23) g/mol | g/mol | IUPAC/CIAAW 2024 |
| Sulfur atoms | 1 | ||
| Oxygen atoms | 3 | ||
| Bonds | 3 | ||
| PubChem molecular weight | 80.07 g/mol | g/mol | PubChem |
Safety
Sulfur trioxide reacts violently with water and with moist air, giving off heat that brings a risk of fire or explosion. Its classification under the Globally Harmonized System (GHS) carries the signal word Danger. Most notifications say it causes severe skin burns and eye damage, and a smaller share add suspected of causing cancer. Exposure limits exist, and the PubChem Laboratory Chemical Safety Summary for sulfur trioxide gathers them with the full classification.
Questions
What shape is a sulfur trioxide molecule?
The 3D model is flat: it centres a sulfur atom in a triangle of oxygens and joins it to each by a double bond. The computed structure puts 120° between neighbouring bonds. VSEPR (valence-shell electron-pair repulsion) theory predicts that trigonal planar shape for the AX3 case, a central atom bonded to three others with no lone pair. One molecule is not the whole substance: the Hazardous Substances Data Bank (HSDB) describes the gas and liquid as also holding S3O9 trimers.
Why does sulfur trioxide have more than one melting point?
Its solid comes in three forms. Haz-Map gives 62.3 °C for alpha, 32.5 °C for beta and 16.8 °C for gamma. HSDB also gives 30.5 °C for beta, and the International Chemical Safety Card gives 17 °C. Only alpha is stable, and HSDB adds that changes between the solid forms are usually slow, so one sample may be a mixture whose melting point is not constant.
Does sulfur trioxide burn?
Hardly: HSDB says the material itself does not burn, or burns only with difficulty. Its fire danger lies in what it does to other substances. The International Chemical Safety Card calls it a strong oxidant, violent in its reactions with organic compounds and with materials that burn or act as reducing agents. CAMEO Chemicals, a hazard database, adds that it chars most organic substances.
Is sulfur trioxide dangerous to breathe?
Yes. The International Chemical Safety Card rates it very corrosive to the respiratory tract as well as the eyes and skin, and notes that its vapour is heavier than air. Under the Globally Harmonized System, most notifications warn of respiratory irritation and a smaller share class it as fatal if inhaled. Repeated breathing of its aerosol risks eroding the teeth, and Haz-Map treats its health effects as basically those of sulfuric acid.
Made of the same elements
- SO2 Sulfur dioxide 3 atoms, 2 double bonds.