Sulfur trioxide
Other names	Sulfuric anhydride Sulfan® Sulphur trioxide Sulfur 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, 62.4 °F
Boiling point	45 °C, 113 °F
Critical Point	218.3 °C at 8.47 MPa
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
Related compounds
Related compounds	SO2 H2SO4 SO2Cl2
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Sulfur trioxide (British spelling: Sulphur trioxide) is the chemical compound with the formula SO₃. In the gaseous form, this species is an important pollutant, being the primary agent in acid rain. It is prepared on massive scale as a precursor to sulfuric acid. Image File history File links Size of this preview: 780 × 600 pixel Image in higher resolution (1100 × 846 pixel, file size: 56 KB, MIME type: image/png) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Sulfur trioxide ... Image File history File links Size of this preview: 616 × 599 pixel Image in higher resolution (1100 × 1070 pixel, file size: 156 KB, MIME type: image/png) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Sulfur trioxide ... A chemical formula (also called molecular formula) is a concise way of expressing information about the atoms that constitute a particular chemical compound. ... Molar mass is the mass of one mole of a chemical element or chemical compound. ... CAS registry numbers are unique numerical identifiers for chemical compounds, polymers, biological sequences and alloys. ... In physics, density is defined as mass m per unit volume V. For the common case of a homogeneous substance, it is expressed as: where, in SI units: ρ (rho) is the density of the substance, measured in kg·m-3 m is the mass of the substance, measured in kg... Solubility refers to the ability for a given substance, the solute, to dissolve in a solvent. ... This article describes water from a scientific and technical perspective. ... The melting point of a crystalline solid is the temperature at which it changes state from solid to liquid. ... The boiling point of a substance is the temperature at which it can change its state from a liquid to a gas throughout the bulk of the liquid at a given pressure. ... In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions (temperature, pressure) at which the liquid state of the matter ceases to exist. ... The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states (the most stable form of the element at 1 atmosphere... In chemistry, the standard molar entropy is the entropy content of one mole of substance, under conditions of standard temperature and pressure. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Council Directive 67/548/EEC of 27 June 1967 on the approximation of laws, regulations and administrative provisions relating to the classification, packaging and labelling of dangerous substances (as amended) is the main source of European Union law concerning chemical safety. ... Corrosion is the destructive reaction of a metal with another material, e. ... R-phrases are defined in Annex III of European Union Directive 67/548/EEC: Nature of special risks attributed to dangerous substances and preparations. ... S-phrases are defined in Annex IV of European Union Directive 67/548/EEC: Safety advice concerning dangerous substances and preparations. ... Sulfur dioxide (or Sulphur dioxide) has the chemical formula SO2. ... Sulfuric acid, H2SO4, is a strong mineral acid. ... Sulfuryl chloride is SO2Cl2, a compound composed of sulfur, oxygen, and chlorine. ... In chemistry, the standard state of a material is its state at 1 bar (100 kilopascals) and 25 degrees Celsius (298. ... SO3 may refer to: Sulfur trioxide, a chemical compound of sulfur and oxygen. ... The term acid rain is commonly used to mean the deposition of acidic components in rain, snow, food, dew, or dry particles. ... Sulfuric acid, H2SO4, is a strong mineral acid. ...

Structure and bonding

Gaseous SO₃ is a trigonal planar molecule of D_3h symmetry, as predicted by VSEPR theory. ǃǁɚɵ A generic trigonal planar molecule showing ideal bond angle. ... Valence shell electron pair repulsion theory (VSEPR) (1957) is a model in chemistry that aims to generally represent the shapes of individual molecules [1] . To achieve this, it is necessary to construct a valid Lewis structure that shows all of the bonds within the molecule and the locations of lone...

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. In quantum chemistry, molecular orbitals are the statistical states electrons can have within molecules. ... A hypervalent molecule is a molecule that contains one or more typical elements (group 1, 2, 13-18) formally bearing more than eight electrons in their valence shells. ...

Chemical reactions

SO₃ is the acid anhydride of H₂SO₄. Thus, the following reaction occurs: Acid anhydrides are chemical compounds that look like, and sometimes are, the product resulting from dehydration of an acid. ...

SO₃(l) + H₂O(l) → H₂SO₄(l) (+88 kJ mol⁻¹)

The reaction occurs both rapidly and endothermically. At or above ~340 °C, sulfuric acid, sulfur trioxide, and water coexist in significant equilibrium concentrations. A kilojoule (abbreviation: kJ) is a unit of energy equal to 1000 joules. ... The mole (symbol: mol) is the SI base unit that measures an amount of substance. ...

Sulfur trioxide also reacts with sulfur dichloride to yield the useful reagent, thionyl chloride. Sulphur Dichloride Sulpur dichloide is a cherry-red liquid at room temperature that is produced from the chlorination of [S2Cl2] (disulphur dichloride). ... A reagent or reactant is any substance used in a chemical reaction. ... R-phrases , , , S-phrases , , , Flash point non flammable Supplementary data page Structure and properties n, εr, etc. ...

SO₃ + SCl₂ → SOCl₂ + SO₂

Preparation

Sulfur trioxide can be prepared in the laboratory by the two-stage pyrolysis of sodium bisulfate: Simple sketch of pyrolysis chemistry Pyrolysis usually means the chemical decomposition of organic materials by heating in the absence of oxygen or any other reagents, except possibly steam. ... Sodium bisulfate, also sodium hydrogen sulfate, has formula NaHSO4. ...

1) dehydration

2NaHSO₄ → Na₂S₂O₇ + H₂O  @ 315°C

2) cracking

Na₂S₂O₇ → Na₂SO₄ + SO₃  @ 460°C

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 activiated 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. This article or section does not cite its references or sources. ... Sulfur dioxide (or Sulphur dioxide) has the chemical formula SO2. ... General Name, Symbol, Number sulfur, S, 16 Chemical series nonmetals Group, Period, Block 16, 3, p Appearance lemon yellow Standard atomic weight 32. ... The mineral pyrite, or iron pyrite, is iron disulfide, FeS2. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Standard atomic weight 15. ... General Name, Symbol, Number vanadium, V, 23 Chemical series transition metals Group, Period, Block 5, 4, d Appearance silver-grey metal Atomic mass 50. ... An oxide is a chemical compound containing an oxygen atom and other elements. ... Vanadium(V) oxide (V2O5), commonly known as vanadium pentoxide, is the most important compound of vanadium. ... A sample of diatomaceous earth Diatomaceous earth (IPA: , also known as DE, diatomite, diahydro, kieselguhr, kieselgur and Celite) is a naturally occurring, soft, chalk-like sedimentary rock that is easily crumbled into a fine white to off-white powder. ... R-phrases R42 R43 R49 S-phrases S22 S36 S37 S45 S53 Flash point non-flammable Supplementary data page Structure and properties n, εr, etc. ...

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. Sulfuric acid, H2SO4, is a strong mineral acid. ... Oleum refers to a solution of sulfur trioxide in sulfuric acid or sometimes more specifically to pyrosulfuric acid, disulfuric acid. ...

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.^[1] 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^[2]. Image File history File links Size of this preview: 598 × 600 pixel Image in higher resolution (1097 × 1100 pixel, file size: 180 KB, MIME type: image/png) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Sulfur trioxide ... Image File history File links Size of this preview: 598 × 600 pixel Image in higher resolution (1097 × 1100 pixel, file size: 180 KB, MIME type: image/png) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Sulfur trioxide ... armchair conformational isomerism of Cyclohexane. ...

If SO₃ is condensed above 27 °C, then α-"SO₃" forms, which has a melting point of 62.3°C. α-SO₃ is fibrous in appearance, like asbestos (with which it has no chemical relationship). Structurally, it is the polymer [S(=O)₂(μ-O)]_n. Each end of the polymer is terminated with OH groups (hence α-"SO₃" is not really a form of SO₃). β-SO₃, like the alpha form, is fibrous but of different molecular weight, consisting of an hydroxyl-capped polymer, but melts at 32.5 °C. Both the gamma and the beta forms are metastable, eventually converting to the stable alpha form if left standing for sufficient time. This conversion is caused by traces of water^[3]. A polymer is a substance composed of molecules with large molecular mass consisting of repeating structural units, or monomers, connected by covalent chemical bonds. ...

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."^[3] The molecular mass of a substance (less accurately called molecular weight and abbreviated as MW) is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon-12). ...

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.^[3] A hygroscopic substance is a substance that absorbs water readily from its surroundings. ... Lactose is a disaccharide found in milk. ...

Sources

• WebElements
• NIST Standard Reference Database
• European Chemicals Bureau

See also

• Hypervalent molecule

A hypervalent molecule is a molecule that contains one or more typical elements (group 1, 2, 13-18) formally bearing more than eight electrons in their valence shells. ...

Notes

1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. ^ Advanced Inorganic Chemistry by Cotton and Wilkinson, 2nd ed p543
3. ^ ^{a b c} Merck Index of Chemicals and Drugs, 9th ed. monograph 8775

External links

• Links to external chemical sources