Sulfur trioxide

Sulfur trioxide
IUPAC name	Sulfur trioxide
Other names	Sulfuric anhydride Sulfan
Identifiers
InChI	InChI=1/O3S/c1-4(2)3
InChIKey	AKEJUJNQAAGONA-UHFFFAOYAX
Standard InChI	InChI=1S/O3S/c1-4(2)3
Standard InChIKey	AKEJUJNQAAGONA-UHFFFAOYSA-N
CAS number	[7446-11-9]
EC number	231-197-3
UN number	1829
RTECS	WT4830000
ChemSpider	23080
Properties
Chemical formula	SO3
Molar mass	80.06 g/mol
Density	1.92 g/cm3, liquid
Melting point	16.9 °C
Boiling point	45 °C
Solubility in water	reacts violently
Thermochemistry
Std enthalpy of formation ΔfHo298	–397.77 kJ/mol
Standard molar entropy So298	256.77 J K–1 mol–1
Hazards
Material safety data sheet (MSDS)	ICSC 1202
EU index number	016-019-00-2
GHS pictograms
GHS signal word	DANGER
GHS hazard statements	H314, H335
Flash point	Non-flammable
Related compounds
Other cations	Selenium trioxide Tellurium trioxide
Other sulfur oxides	Sulfur monoxide Sulfur dioxide
Other compounds	Sulfuric acid Disulfuric acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Sulfur trioxide (also spelled sulphur trioxide) is the chemical compound with the formula SO₃. It is prepared industrially on massive scales as a precursor to sulfuric acid.

Structure and bonding

Gaseous SO₃ is a trigonal planar molecule of D_3h symmetry, as predicted by VSEPR theory.

In terms of electron-counting formalisms, the sulfur atom has an oxidation state of +6, a formal charge of 0, and is surrounded by 6 electron pairs. From the perspective of molecular orbital theory, most of these electron pairs are non-bonding in character, as is typical for hypervalent molecules.

Chemical reactions

SO₃ is the anhydride of H₂SO₄. Thus, the following reaction occurs:

SO₃ + H₂O → H₂SO₄ (–88 kJ/mol)

The reaction occurs both rapidly and exothermically, too violently to be used in large-scale manufacturing. Above 340 °C, sulfuric acid, sulfur trioxide, and water coexist in significant equilibrium concentrations.

Sulfur trioxide also reacts with sulfur dichloride to yield the useful reagent, thionyl chloride.

SO₃ + SCl₂ → SOCl₂ + SO₂

SO₃ is a strong Lewis acid readily forming crystalline complexes with pyridine, dioxane and trimethylamine, which can be used as sulfonating agents.

Preparation

Sulfur trioxide can be prepared in the laboratory by the two-stage pyrolysis of sodium bisulfate. Sodium pyrosulfate is an intermediate product:

1. Dehydration at 315°C:

   2 NaHSO₄ → Na₂S₂O₇ + H₂O

2. Cracking at 460°C:

   Na₂S₂O₇ → Na₂SO₄ + SO₃

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

Industrially SO₃ is made by the contact process. Sulfur dioxide, generally made by the burning of sulfur or iron pyrite (a sulfide ore of iron), is first purified by electrostatic precipitation. The purified SO₂ is then oxidised by atmospheric oxygen at between 400 and 600 °C over a catalyst consisting of vanadium pentoxide (V₂O₅) activated with potassium oxide K₂O on kieselguhr or silica support. Platinum also works very well but is 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.

Structure of solid SO₃

Ball-and-stick model of the γ-SO₃ molecule

The nature of solid SO₃ is a surprisingly complex area because of structural changes caused by traces of water. Upon condensation of the gas, absolutely pure SO₃ condenses into a trimer, which is often called γ-SO₃. This molecular form is a colorless solid with a melting point of 16.8 °C. It adopts a cyclic structure described as [S(=O)₂(μ-O)]₃.

If SO₃ is condensed above 27 °C in the presence of traces of water (amount fraction x = 10^–5), then fibrous α-SO₃ forms, which has a melting point of 62.3 °C. Structurally, it is the polymer [S(=O)₂(μ-O)]_n, with each end of the polymer terminated by –OH groups. β-SO₃ is also fibrous, but of different molecular weight, also consisting of a hydroxyl-capped polymer and melting at 32.5 °C. Both the gamma- and the beta-forms are metastable, eventually converting to the more stable alpha-form if left standing for sufficient time in the presence of traces of water.

Relative vapor pressures of solid SO₃ are alpha < beta < gamma at identical temperatures, indicative of their relative molecular weights. Liquid sulfur trioxide has vapor pressure consistent with the gamma form. Thus heating a crystal of α-SO₃ to its melting point results in a sudden increase in vapor pressure, which can be forceful enough to shatter a glass vessel in which it is heated. This effect is known as the "alpha explosion."

SO₃ is aggressively hygroscopic. In fact, the heat of hydration is sufficient that mixtures of SO₃ and wood or cotton can ignite. In such cases, SO₃ dehydrates these carbohydrates.