Difference between revisions of "Sulfur trioxide"

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|  OtherNames = Sulfuric anhydride<br/>Sulfan
 
|  OtherNames = Sulfuric anhydride<br/>Sulfan
 
| Section1 = {{Chembox Identifiers
 
| Section1 = {{Chembox Identifiers
 +
|  InChI = 1/O3S/c1-4(2)3
 +
|  StdInChI = 1S/O3S/c1-4(2)3
 +
|  InChIKey = AKEJUJNQAAGONA-UHFFFAOYAX
 +
|  StdInChIKey = AKEJUJNQAAGONA-UHFFFAOYSA-N
 
|  CASNo = 7446-11-9
 
|  CASNo = 7446-11-9
 
|    CASNo_Ref = {{cascite}}
 
|    CASNo_Ref = {{cascite}}
 +
|  EC-number = 231-197-3
 +
|  ChemSpiderID = 23080
 
|  UNNumber = 1829
 
|  UNNumber = 1829
 
|  RTECS = WT4830000
 
|  RTECS = WT4830000
Line 21: Line 27:
 
|  MeltingPt = 16.9 °C
 
|  MeltingPt = 16.9 °C
 
|  BoilingPt = 45 °C
 
|  BoilingPt = 45 °C
|  Solubility = hydrolyses
+
|  Solubility = reacts violently
 
   }}
 
   }}
 
| Section4 = {{Chembox Thermochemistry
 
| Section4 = {{Chembox Thermochemistry
|  DeltaHf = &minus;397.77 kJ/mol
+
|  DeltaHf = –397.77 kJ/mol
|  Entropy = 256.77 J&thinsp;K<sup>&minus;1</sup>&thinsp;mol<sup>&minus;1</sup>}}
+
|  Entropy = 256.77 J&thinsp;K<sup>–1</sup>&thinsp;mol<sup>–1</sup>}}
 
| Section7 = {{Chembox Hazards
 
| Section7 = {{Chembox Hazards
ExternalMSDS = [http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc12/icsc1202.htm ICSC 1202]
+
Reference = <ref>{{CLP Regulation|index=016-019-00-2|page=400}}</ref>
|   EUIndex = 016-019-00-2
+
ExternalMSDS = {{ICSC-small|12|02}}
|   EUClass = [[Corrosive]] ('''C''')
+
EUIndex = 016-019-00-2 <!-- as oleum -->
RPhrases = {{R14}}, {{R35}}, {{R37}}
+
|  GHSPictograms = {{GHS05|Skin Corr. 1A}}{{GHS07|STOT SE 3}}
SPhrases = {{S1/2}}, {{S26}}, {{S30}}, {{S45}}
+
GHSSignalWord = DANGER
NFPA-H = 3
+
HPhrases = {{H-phrases|314|335}} <!-- also EUH014 in the European Union -->
NFPA-F = 0
 
|   NFPA-R = 2
 
|  NFPA-O = W
 
 
|  FlashPt = Non-flammable
 
|  FlashPt = Non-flammable
|  LD50 =
 
|  PEL =
 
 
   }}
 
   }}
 
| Section8 = {{Chembox Related
 
| Section8 = {{Chembox Related
|  OtherAnions =
 
 
|  OtherCations = [[Selenium trioxide]]<br/>[[Tellurium trioxide]]
 
|  OtherCations = [[Selenium trioxide]]<br/>[[Tellurium trioxide]]
 
|  OtherFunctn = [[Sulfur monoxide]]<br/>[[Sulfur dioxide]]
 
|  OtherFunctn = [[Sulfur monoxide]]<br/>[[Sulfur dioxide]]
 
|    Function = [[sulfur]] [[oxide]]s
 
|    Function = [[sulfur]] [[oxide]]s
|  OtherCpds = [[Sulfuric acid]]
+
|  OtherCpds = [[Sulfuric acid]]<br/>[[Disulfuric acid]]
 
   }}
 
   }}
 
}}
 
}}
'''[[Sulfur]] trioxide''' (also spelled '''sulphur trioxide''') is the chemical compound with the formula SO<sub>3</sub>. In the gaseous form, this species is a significant pollutant, being the primary agent in [[acid rain]].  It is prepared on massive scales as a precursor to [[sulfuric acid]].
+
'''[[Sulfur]] trioxide''' (also spelled '''sulphur trioxide''') is the chemical compound with the formula SO<sub>3</sub>. It is prepared industrially on massive scales as a precursor to [[sulfuric acid]].
  
 
==Structure and bonding==
 
==Structure and bonding==
Line 54: Line 54:
  
 
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.
 
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.
 
Sulfur trioxide also exhibits [[hybridization (chemistry)|hybridization]].
 
  
 
==Chemical reactions==
 
==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> is the [[anhydride]] of H<sub>2</sub>SO<sub>4</sub>.  Thus, the following reaction occurs:
 +
:SO<sub>3</sub> + H<sub>2</sub>O → H<sub>2</sub>SO<sub>4</sub> (–88 kJ/mol)
  
: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, too violently to be used in large-scale manufacturing. Above 340&nbsp;°C, sulfuric acid, sulfur trioxide, and water coexist in significant equilibrium concentrations.
 
 
The reaction occurs both rapidly and exothermically, too violently to be used in large-scale manufacturing. 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]].
 
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> + 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>
+
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==
 
==Preparation==
Sulfur trioxide can be prepared in the laboratory by the two-stage [[pyrolysis]] of [[sodium bisulfate]]. [[Sodium pyrosulfate]] is an intermediate product:
+
Sulfur trioxide can be prepared in the laboratory by the two-stage [[pyrolysis]] of [[sodium bisulfate]]. [[Sodium pyrosulfate]] is an intermediate product:
 
 
 
<ol>
 
<ol>
 
<li>Dehydration at 315°C:
 
<li>Dehydration at 315°C:
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This method will work for other metal bisulfates, the controlling factor being the stability of the intermediate pyrosulfate salt.
 
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(V) oxide|vanadium pentoxide]] (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.
+
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&nbsp;°C over a catalyst consisting of [[Vanadium(V) oxide|vanadium pentoxide]] (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]].
 
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]].
Line 88: Line 83:
 
==Structure of solid SO<sub>3</sub>==
 
==Structure of solid SO<sub>3</sub>==
 
[[File:Sulfur-trioxide-trimer-from-xtal-1967-3D-balls-B.png|thumb|right|200px|[[Ball-and-stick model]] of the ''γ''-SO<sub>3</sub> molecule]]
 
[[File:Sulfur-trioxide-trimer-from-xtal-1967-3D-balls-B.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&Wiberg}}</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>
+
The nature of solid SO<sub>3</sub> is a surprisingly complex area because of structural changes caused by traces of water.<ref>{{Holleman&Wiberg}}.</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="G&E">{{Greenwood&Earnshaw1st|pages=832–33}}.</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>
+
If SO<sub>3</sub> is condensed above 27&nbsp;°C in the presence of traces of water ([[amount fraction]] ''x''&nbsp;= 10<sup>–5</sup>), then fibrous α-SO<sub>3</sub> forms, which has a melting point of 62.3&nbsp;°C. Structurally, it is the [[polymer]]  [S(=O)<sub>2</sub>(''μ''-O)]<sub>''n''</sub>, with each end of the polymer terminated by –OH groups. β-SO<sub>3</sub> is also fibrous, but of different molecular weight, also consisting of a hydroxyl-capped polymer and melting at 32.5&nbsp;°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.<ref name="Merck">{{Merck9th|8775}}.</ref>
  
 
Relative vapor pressures of solid SO<sub>3</sub> are alpha &lt; beta &lt; 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"/>
 
Relative vapor pressures of solid SO<sub>3</sub> are alpha &lt; beta &lt; 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"/>
 
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"/>
 
==Application==
 
In process plant environment, SO<sub>3</sub> gas is mixed into flue gas from combustion to make the ashes charged up before flowing through electrostatic precipitators. The electrostatic precipitators will then trap the ashes, making cleaner process emission possible.
 
 
==In popular culture==
 
Sulfur trioxide is mentioned in the ''[[Emergency!]]'' episode "The Old Engine Cram" which aired in the [[United States]] on 20 September 1975.
 
 
==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]]
 
*[[Sulfur trioxide pyridine complex]]
 
  
 
==References==
 
==References==
<references/>
+
{{reflist}}
  
[[Category:Sulfur oxides]]
+
[[Category:Sulfur compounds]]
[[Category:Acid anhydrides]]
+
[[Category:Oxides]]
 
[[Category:Acidic oxides]]
 
[[Category:Acidic oxides]]
  
 
{{Imported from Wikipedia|name=Sulfur trioxide|id=303199049}}
 
{{Imported from Wikipedia|name=Sulfur trioxide|id=303199049}}

Latest revision as of 17:44, 24 August 2009

Sulfur trioxide
Sulfur trioxide bonding
Sulfur trioxide space filling
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[1]
Material safety data sheet (MSDS) ICSC 1202
EU index number 016-019-00-2
GHS pictograms Skin Corr. 1ASTOT SE 3
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 D3h 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.[2]

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.[3] 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.[4]

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.[5]

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."[5]

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.[5]

References

  1. Index no. 016-019-00-2 of Annex VI, Part 3, to Regulation (EC) No 1272/2008 of the European Parliament and of the Council of 16 December 2008 on classification, labelling and packaging of substances and mixtures, amending and repealing Directives 67/548/EEC and 1999/45/EC, and amending Regulation (EC) No 1907/2006. OJEU L353, 31.12.2008, pp 1–1355 at p 400.
  2. Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred Advanced Inorganic Chemistry, 6th ed.; Wiley-Interscience: New York, 1999. ISBN 0-471-19957-5
  3. Holleman, A. F.; Wiberg, E. Inorganic Chemistry; Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  4. Greenwood, Norman N.; Earnshaw, A. Chemistry of the Elements; Pergamon: Oxford, 1984; pp 832–33. ISBN 0-08-022057-6.
  5. 5.0 5.1 5.2 The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals, 9th ed.; Merck, 8775.
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