Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula SO2. This important gas is main combustion product of sulfur compounds and is of environmental concern. SO2 is often described as the "smell of burning sulfur."
SO2 is produced by
volcanoes and in various industrial processes. Since
coal and
petroleum contain various amounts of sulfur compounds, their combustion generates sulfur dioxide. Further oxidation of SO2, usually in the presence of a catalyst such as
NO2 forms
H2SO4, and thus
acid rain.
Encyclopedia
| Sulfur dioxide |
|---|
| |
|
| Other names | Sulfur oxide
Sulfurous anhydride,
'sulphurous anhydride |
| Molecular formula | SO2 |
| Molar mass | 64.054 g mol-1 |
| Appearance | colourless gas |
| CAS number | [7446-09-5] |
| EINECS number | 231-195-2 |
| Properties |
|---|
| Density and phase | 2.551 g/L, gas |
| Solubility in water | 9.4 g/100 mL
|
| Melting point | -72.4 °C |
| Boiling point | -10 °C |
| Critical Point | 157.2°C at 7.87 MPa |
| Acidity | 1.81 |
| Structure |
|---|
| Molecular shape | bent |
| Dipole moment | 1.63 D |
| Thermodynamic data |
|---|
Standard enthalpy
of
formation ?fH°gas | -296.84 kJ mol-1 |
Standard molar entropy
S°gas | 248.21 J K-1 mol-1 |
| Safety data |
|---|
| EU classification | Toxic |
| R-phrases | R23, R34 |
| S-phrases | S1/2, S9, S26
S36/37/39, S45 |
| NFPA 704 | |
| PEL-TWA | 5 ppm |
| IDLH | 100 ppm |
| Flash point | non-flammable |
| RTECS number | WS4550000 |
| Supplementary data page |
|---|
Structure and
properties | n, er, etc. |
Thermodynamic
data | Phase behaviour
Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Related compounds |
|---|
| Other cations | Selenium dioxide
Tellurium dioxide |
| Related compounds | Sulfur trioxide
Sulfuric acid |
Except where noted otherwise, data are given for
materials in their standard state
|
|
Sulfur dioxide is the chemical compound with the formula SO
2. This important gas is main combustion product of sulfur compounds and is of environmental concern. SO
2 is often described as the "smell of burning sulfur."
SO
2 is produced by
volcanoes and in various industrial processes. Since
coal and
petroleum contain various amounts of sulfur compounds, their combustion generates sulfur dioxide. Further oxidation of SO
2, usually in the presence of a catalyst such as
NO2 forms
H2SO4, and thus
acid rain.
Preparation
Sulfur dioxide can be prepared by burning sulfur:
- S + O2 ? SO2
The combustion of
Hydrogen sulfide and organosulfur compounds proceeds similarly.
- 2H2S + 3O2 ? 2H2O + 2SO2
The roasting of sulfide ores such as iron
pyrites and
sphalerite also gives SO
2:
- 4FeS2 + 11O2 ? 2Fe2O3 + 8SO2
- 2ZnS + 3O2 ? 2ZnO + 2SO2
When anhydrous
CaSO4 is heated with coke and sand in the manufacture of
cement, CaSiO
3, sulfur dioxide is a by-product.
- 2CaSO4 + 2SiO2 + C ? 2CaSiO3 + 2SO2 + CO2
Structure and bonding
SO
2 is a bent molecule with C
2v symmetry point group.
n terms of electron-counting formalisms, the sulfur atom has an oxidation state of +4, a formal charge of 0, and is surrounded by 5 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.
Uses
Sulfur dioxide is sometimes used as a preservative in alcoholic drinks, or dried
apricots and other
dried fruits. The preservative is used to maintain the appearance of the fruit rather than prevent rotting. This can give fruit a distinctive chemical taste.
Sulfur dioxide is also a good reductant. In the presence of water, sulfur dioxide is able to decolorize substances that can be reduced by it; thus making it a useful reducing
bleach for
papers and delicate materials such as clothes.
This bleaching effect normally does not last very long.
Oxygen in the atmosphere reoxidizes the reduced dyes, restoring the color. This might explain why older
newspapers turn yellow, because paper used for newspaper is naturally yellow.
Sulfur dioxide is also used to make sulfuric acid, being converted to
sulfur trioxide, and then to oleum, which is made into
sulfuric acid. Sulfur dioxide for this purpose is made when sulfur combines with oxygen. This is called the
contact process.
Prior to the development of
Freons, sulfur dioxide was used as a refrigerant in home refrigerators.
H2SO3 is also called "hydrogen sulfite" or
sulfurous acid.
Emissions
According to the US
EPA , the following amount of thousands of short tons of Sulfur dioxide were released in the
U.S. per year:
| *1999 | 18,867 |
| *1998 | 19,491 |
| *1997 | 19,363 |
| *1996 | 18,859 |
| *1990 | 23,678 |
| *1980 | 25,905 |
| *1970 | 31,161 |
Due largely to the
US EPA’s Acid Rain Program, the U.S. has witnessed a 33 percent decrease in emissions between 1983 and 2002. This improvement resulted from
flue gas desulfurization, a technology that enables SO
2 to be chemically bound in
power plants burning sulfur-containing
coal or
oil. In particular,
calcium oxide reacts with sulfur dioxide to form calcium sulfite:
- CaO + SO2 ? CaSO3
As of 2006,
China is the world's largest Sulfur Dioxide polluter, with 2005 emissions estimated to be 25.49 million tons. This amount represents a 27% increase since 2000, and is roughly comparable with U.S. emissions in 1980.
Aerobic oxidation converts this CaSO
3 into CaSO
4,
gypsum. Most gypsum sold in Europe comes from flue gas desulfurization.
See also
...
- Sulfur-iodine cycle
- National Ambient Air Quality Standards
- Homer City Generating Station: an example of an SO2 polluter.
External links
Appendix: temperature dependence of aqueous solubility
22 g/100ml
15 g/100ml
11 g/100ml
9.4 g/100 ml
8 g/100ml
6.5 g/100ml
5 g/100ml
4 g/100ml
3.5 g/100ml
3.4 g/100ml
3.5 g/100ml
3.7 g/100ml
