Sulfur trioxide
Names | |
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Preferred IUPAC name
Sulfur trioxide | |
Systematic IUPAC name
Sulfonylideneoxidane | |
Other names
Sulfuric anhydride, Sulfur(VI) oxide
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Identifiers | |
3D model (
JSmol ) |
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ChEBI | |
ChemSpider | |
ECHA InfoCard
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100.028.361 |
EC Number |
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1448 | |
PubChem CID
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RTECS number
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UNII | |
UN number | UN 1829 |
CompTox Dashboard (EPA)
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Properties | |
SO3 | |
Molar mass | 80.066 g/mol |
Appearance | Colorless to white crystalline solid which will fume in air.[2] Colorless liquid and gas.[3] |
Odor | Varies. Vapor is pungent; like sulfur dioxide.[4] Mist is odorless.[3] |
Density | 1.92 g/cm3, liquid |
Melting point | 16.9 °C (62.4 °F; 290.0 K) |
Boiling point | 45 °C (113 °F; 318 K) |
Reacts to give sulfuric acid | |
Thermochemistry | |
Std molar
entropy (S⦵298) |
256.77 JK−1mol−1 |
Std enthalpy of (ΔfH⦵298)formation |
−395.7 kJ/mol |
Hazards | |
Occupational safety and health (OHS/OSH): | |
Main hazards
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Highly corrosive, extremely strong dehydrating agent |
GHS labelling: | |
Danger | |
H314, H335 | |
P261, P280, P305+P351+P338, P310[5] | |
NFPA 704 (fire diamond) | |
Flash point | Non-flammable |
Lethal dose or concentration (LD, LC): | |
LC50 (median concentration)
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rat, 4 hr 375 mg/m3[citation needed] |
Safety data sheet (SDS) | ICSC 1202 |
Related compounds | |
Other cations
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Selenium trioxide Tellurium trioxide Polonium trioxide |
Sulfur monoxide Sulfur dioxide | |
Related compounds
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Sulfuric acid |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Sulfur trioxide (alternative spelling sulphur trioxide, also known as nisso sulfan) is the chemical compound with the formula SO3. It has been described as "unquestionably the most [economically important]" sulfur oxide.[1] It is prepared on an industrial scale as a precursor to sulfuric acid.
Sulfur trioxide exists in several forms - gaseous monomer, crystalline trimer, and solid polymer. Sulfur trioxide is a solid at just below room temperature with a relatively narrow liquid range. Gaseous SO3 is the primary precursor to acid rain.[6]
Molecular structure and bonding
Monomer
The molecule SO3 is
Trimer
Both liquid and gaseous
Absolutely pure SO3 freezes at 16.8 °C to give the γ-SO3 form, which adopts the cyclic trimer configuration [S(=O)2(μ-O)]3.[10][1]
Polymer
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. Structurally, it is the polymer [S(=O)2(μ-O)]n. Each end of the polymer is terminated with OH groups.[1] 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.[11]
Relative vapor pressures of solid SO3 are alpha < beta < gamma at identical temperatures, indicative of their relative
Chemical reactions
Sulfur trioxide undergoes many reactions.[1]
Hydration and hydrofluorination
SO3 is the
- SO3 + H2O → H2SO4 (ΔfH = −200
Gaseous sulfur trioxide fumes profusely even in a relatively dry atmosphere owing to formation of a sulfuric acid mist. SO3 is aggressively
Akin to the behavior of H2O, hydrogen fluoride adds to give fluorosulfuric acid:
- SO3 + HF → FSO3H
Deoxygenation
SO3 reacts with dinitrogen pentoxide to give the
- 2 SO3 + N2O5 → [NO2]2S2O7
Oxidant
Sulfur trioxide is an oxidant. It oxidizes sulfur dichloride to thionyl chloride.
- SO3 + SCl2 → SOCl2 + SO2
Lewis acid
SO3 is a strong
Sulfonating agent
Sulfur trioxide is a potent sulfonating agent, i.e. it adds SO3 groups to substrates. Often the substrates are organic, as in aromatic sulfonation.[14] For activated substrates, Lewis base adducts of sulfur trioxide are effective sulfonating agents.[15]
Preparation
The direct oxidation of sulfur dioxide to sulfur trioxide in air proceeds very slowly:
- SO2 + 1⁄2O2 → SO3 (ΔH = −198.4 kJ/mol)
Industrial
Industrially SO3 is made by the
Laboratory
Sulfur trioxide can be prepared in the laboratory by the two-stage pyrolysis of sodium bisulfate. Sodium pyrosulfate is an intermediate product:[17]
- Dehydration at 315 °C:
- 2 NaHSO4 → Na2S2O7 + H2O
- Cracking at 460 °C:
- Na2S2O7 → Na2SO4 + SO3
In contrast, KHSO4 does not undergo the same reaction.[17]
Another two step method involving a salt pyrolysis starts with concentrated sulfuric acid and anhydrous tin tetrachloride:
- Reaction between tin tetrachloride and sulfuric acid in a 1:2 molar mixture at near reflux (114°C):
- SnCl4 + 2 H2SO4 → Sn(SO4)2 + 4 HCl
- Pyrolysis of anhydrous tin(IV) sulfate at 150°C - 200°C:
- Sn(SO4)2 → SnO2 + 2 SO3
The advantage of this method over the sodium bisulfate one is that it requires much lower temperatures and can be done using normal borosilicate laboratory glassware without the risk of shattering. A disadvantage is that it generates significant quantities of hydrogen chloride gas which needs to be captured as well.
SO3 may also be prepared by dehydrating sulfuric acid with phosphorus pentoxide.[18]
Applications
Sulfur trioxide is a reagent in
B2O3 stabilized sulfur trioxide was traded by Baker & Adamson under the tradename "Sulfan" in the 20th century.[19]
Safety
Along with being an oxidizing agent, sulfur trioxide is highly corrosive. It reacts violently with water to produce highly corrosive sulfuric acid.
See also
References
- ^ ISBN 978-0-08-037941-8.
- ^ "SULFUR TRIOXIDE CAMEO Chemicals NOAA". Cameochemicals.noaa.gov.
- ^ LCCN 2010038460.
- ^ "Substance:Sulfur trioxide - Learn Chemistry Wiki". Rsc.org.
- ^ "Sulfur trioxide 227692" (PDF). SO3. Archived from the original on 2020-09-01. Retrieved 1 September 2020.
- PMID 10922048.
- ^ Housecroft, Catherine E.; Sharpe, Alan G. (2012). Inorganic Chemistry (4 ed.). Essex, England: Pearson. p. 575.
- .
- ISBN 0-12-352651-5
- .
- ^ a b c Merck Index of Chemicals and Drugs, 9th ed. monograph 8775
- ^ "The Manufacture of Sulfuric Acid and Superphosphate" (PDF). Chemical Processes in New Zealand. Archived from the original (PDF) on 2018-01-27. Retrieved 2016-04-22.
- ISBN 0-471-19957-5
- .
- .
- ^ .
- ^ "How to make sulfur trioxide - YouTube". www.youtube.com. Retrieved 1 September 2020.
- S2CID 93744787. p. 1553:
Sulfur trioxide used was pure, colorless liquid SO3 marketed under the trade name Sulfan by Baker and Adamson