View source for Sulfur trioxide

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{{Chembox new
|   Name = Sulfur trioxide
|   ImageFileL1 = Sulfur-trioxide-2D.svg
|   ImageSizeL1 = 120px
|   ImageFileR1 = Sulfur-trioxide-3D-vdW.png
|   ImageSizeR1 = 120px
|   ImageName = Sulfur trioxide
|   OtherNames = Sulfuric anhydride<br />Sulfan®<br />Sulphur trioxide<br />Sulfur trioxide<br /><!--
| [[Molecular formula]]
| SO<sub>3</sub>
|-
 -->
| Section1 = {{Chembox Identifiers
|   CASNo = [7446-11-9]
  }}
| Section2 = {{Chembox Properties
|   MolarMass = 80.06 g mol<sup>−1</sup>
|   Density = 1.92 g cm<sup>−3</sup>
|   Solvent = other solvents
|   SolubleOther = Hydrolysis
|   MeltingPt = 16.9 °C, 62.4 °F
|   BoilingPt = 45 °C, 113 °F
  }}
| Section4 = {{Chembox Thermochemistry
|   DeltaHf = −397.77 kJ/mol
|   Entropy = 256.77 J.K<sup>−1</sup>.mol<sup>−1</sup>
  }}
| Section7 = {{Chembox Hazards
|   EUClass = [[Corrosive]] ('''C''')
|   RPhrases = R14, R35, R37
|   SPhrases = S1/2, S26, S30, S45
  }}
| Section8 = {{Chembox Related
|   OtherCpds = [[Sulfur dioxide|SO<sub>2</sub>]]<br />[[Sulfuric acid|H<sub>2</sub>SO<sub>4</sub>]]<br />[[Sulfuryl chloride|SO<sub>2</sub>Cl<sub>2</sub>]]}}
}}
{{redirect|SO3}}
{{SI}}
{{CMG}}

==Overview==
'''[[Sulfur]] trioxide''' (also spelled '''sulphur trioxide''') is the chemical compound with the formula SO<sub>3</sub>.  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]].

==Structure and bonding==
Gaseous SO<sub>3</sub> is a [[trigonal planar]] molecule of D<sub>3h</sub> 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 molecule]]s.

==Chemical reactions==
SO<sub>3</sub> is the [[anhydride]] of H<sub>2</sub>SO<sub>4</sub>.  Thus, the following reaction occurs:
:SO<sub>3</sub>(l) + H<sub>2</sub>O(l) → H<sub>2</sub>SO<sub>4</sub>(l) (-88 [[kilojoule|kJ]] [[mole (unit)|mol<sup>−1</sup>]])
The reaction occurs both rapidly and  exothermically.  At or 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]].

:SO<sub>3</sub> + SCl<sub>2</sub> → SOCl<sub>2</sub> + SO<sub>2</sub>

SO<sub>3</sub> is a strong Lewis acid readily forming crystalline complexes with [[sulfur trioxide pyridine complex| pyridine]] , [[dioxane]]  and [[trimethylamine]] which can be used as sulfonating agents<ref>{{Cotton&Wilkinson6th}}</ref>.

==Preparation==
Sulfur trioxide can be prepared in the laboratory by the two-stage [[pyrolysis]] of [[sodium bisulfate]]:
;1) dehydration:
: 2NaHSO<sub>4</sub>  →  Na<sub>2</sub>S<sub>2</sub>O<sub>7</sub> + H<sub>2</sub>O&nbsp; @ 315°C
;2) cracking:
: Na<sub>2</sub>S<sub>2</sub>O<sub>7</sub>  →  Na<sub>2</sub>SO<sub>4</sub> + SO<sub>3</sub>&nbsp; @ 460°C
This method will work for other metal bisulfates, the controlling factor being the stability of the intermediate pyrosulfate salt.

Industrially SO<sub>3</sub> 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 SO<sub>2</sub> is then oxidised by atmospheric [[oxygen]] at between 400 and 600 °C over a catalyst consisting of [[vanadium]] [[oxide|pentoxide]] [[Vanadium(V) oxide|V<sub>2</sub>O<sub>5</sub>]] activated with potassium oxide K<sub>2</sub>O on [[Diatomaceous earth|kieselguhr]] or [[Silicon dioxide|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 SO<sub>3</sub>==
[[Image:Sulfur-trioxide-trimer-3D-balls.png|thumb|right|200px|[[Ball-and-stick model]] of the ''γ''-SO<sub>3</sub> molecule]]
The nature of solid SO<sub>3</sub> is a surprisingly complex area because of structural changes caused by traces of water.<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref>  Upon condensation of the gas, absolutely pure SO<sub>3</sub> condenses into a trimer, which is often called ''γ''-SO<sub>3</sub>.  This molecular form is a colorless solid with a melting point of 16.8 °C. It adopts a cyclic structure described as [S(=O)<sub>2</sub>(''μ''-O)]<sub>3</sub><ref name="cw">Advanced Inorganic Chemistry by Cotton and Wilkinson, 2nd ed p543</ref>.

If SO<sub>3</sub> is condensed above 27 °C, then ''α''-"SO<sub>3</sub>" forms, which has a melting point of 62.3°C. ''α''-SO<sub>3</sub> is fibrous in appearance, like asbestos (with which it has no chemical relationship).  Structurally, it is the [[polymer]]  [S(=O)<sub>2</sub>(''μ''-O)]<sub>''n''</sub>.  Each end of the polymer is terminated with OH groups (hence ''α''-"SO<sub>3</sub>" is not really a form of SO<sub>3</sub>).  ''β''-SO<sub>3</sub>, 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<ref name="Merck">Merck Index of Chemicals and Drugs, 9th ed. monograph 8775</ref>.

Relative vapor pressures of solid SO<sub>3</sub> are alpha< beta< gamma at identical temperatures, indicative of their relative [[molecular weight]]s. Liquid sulfur trioxide has vapor pressure consistent with the gamma form. Thus heating a crystal of  ''α''-SO<sub>3</sub> 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."<ref name="Merck"/>

SO<sub>3</sub> is aggressively [[hygroscopic]].  In fact, the heat of hydration is sufficient that mixtures of SO<sub>3</sub> and wood or cotton can ignite. In such cases, SO<sub>3</sub> dehydrates these [[carbohydrate]]s.<ref name="Merck"/>

==Sources==
* [http://www.webelements.com/ WebElements]
* [http://webbook.nist.gov/chemistry/ NIST Standard Reference Database]
* [http://ecb.jrc.it/ European Chemicals Bureau]

==See also==
*[[Hypervalent molecule]]

==References==
{{Reflist|2}}

[[Category:Sulfur oxides]]
[[Category:Acidic oxides]]
[[Category:Hazardous air pollutants]]

[[ar:ثلاثي أكسيد الكبريت]]
[[cs:Oxid sírový]]
[[de:Schwefeltrioxid]]
[[es:Trióxido de azufre]]
[[fr:Trioxyde de soufre]]
[[it:Triossido di zolfo]]
[[lt:Sieros trioksidas]]
[[nl:Zwaveltrioxide]]
[[ja:三酸化硫黄]]
[[pl:Tlenek siarki(VI)]]
[[pt:Óxido sulfúrico]]
[[ru:Оксид серы(VI)]]
[[sk:Oxid sírový]]
[[sv:Svaveltrioxid]]
[[vi:SO3]]
[[uk:Триоксид сірки]]
[[zh:三氧化硫]]