Difference Between SO2 and SO3

Main Difference – SO₂ vs SO₃

SO₂ and SO₃ are inorganic chemical compounds formed by the combination of sulfur atoms and oxygen atoms. SO₂ stands for sulfur dioxide, and SO₃ stands for sulfur trioxide. These are gaseous compounds. They have different chemical and physical properties. These compounds are called oxides of sulfur since they are formed from the reaction between sulfur and O₂ molecules. The main difference between SO₂ and SO₃ is that SO₂ has two oxygen atoms bonded to a sulfur atom whereas SO₃ has three oxygen atoms bonded to a sulfur atom.

Key Areas Covered

1. What is SO2
     – Definition, Chemical Structure and Properties, Oxidation State
2. What is SO3
     – Definition, Chemical Structure and Properties, Sulfuric Acid Production
3. What is the Difference Between SO2 and SO3
    – Comparison of Key Differences

Key Terms: Acid Rain, Lone Electron Pair, Oxygen, Oxidation State, Sulfur, Sulfur Dioxide, Sulfuric Acid, Sulfur Trioxide

What is SO₂

SO₂ stands for sulfur dioxide. Sulfur dioxide is a gaseous compound composed of sulfur and oxygen atoms. The chemical formula of sulfur dioxide is SO₂. Therefore, it is composed of a sulfur atom bonded to two oxygen atoms through covalent bonds. One oxygen atom can form a double bond with the sulfur atom. Hence, the sulfur atom is the central atom of the compound. Since sulfur element has 6 electrons in its outermost orbital after forming two double bonds with the oxygen atoms, there are 2 more electrons remaining; these can act as a lone electron pair. This determines the geometry of the SO₂ molecule as angular geometry. SO₂ is polar due to its geometry (angular) and the presence of a lone electron pair.

Figure 1: Chemical Structure of SO2

Sulfur dioxide is considered as a toxic gas. Hence, if there is SO₂ in the atmosphere, it will be an indication of air pollution. This gas has a very irritating smell. The molecular mass of sulfur dioxide is 64 g/mol. It is a colorless gas at room temperature. The melting point is about -71^oC whereas the boiling point is -10^oC.

The oxidation state of sulfur in sulfur dioxide is +4. Therefore, sulfur dioxide can also be produced by the reduction of compounds composed of sulfur atoms that are in a higher oxidation state. One such example is the reaction between copper and sulfuric acid. Here, sulfur in sulfuric acid is in the oxidation state of +6. Therefore, it can be reduced to +4 oxidation state of sulfur dioxide.

Sulfur dioxide can be used in the production of sulfuric acid which has a number of applications in the industrial scale and the laboratory scale. Sulfur dioxide is also a good reducing agent. Since the oxidation state of sulfur is +4 in sulfur dioxide, it can easily be oxidized to +6 oxidation state, which allows another compound to be reduced.

What is SO₃

SO₃ stands for sulfur trioxide. Sulfur trioxide is a solid compound composed of one sulfur atom bonded to three oxygen atoms. The chemical formula of sulfur dioxide is SO₃. Each oxygen has formed a double bond with the sulfur atom. The sulfur atom is in the center of the molecule. Since sulfur has 6 electrons in its outermost orbital, after forming three double bonds with the oxygen atoms, there are no more electrons remaining on sulfur atom. This determines the geometry of the SO₃ molecule as trigonal planar geometry. SO₃ is non-polar due to its geometry (trigonal planar) and the absence of a lone electron pair.

Figure 2: Chemical Structure of SO3

The molecular mass of sulfur trioxide is 80.057 g/mol. The melting point of SO₃ is about 16.9 °C whereas the boiling point is 45^oC. At room temperature and pressure, sulfur trioxide is a white crystalline solid compound which will fume in the air. It has a pungent odor. The oxidation state of sulfur in sulfur trioxide is +6.

In its gaseous form, sulfur trioxide is an air pollutant and is a major component in acid rains. However, sulfur trioxide is very important in the production of sulfuric acid in industrial scale. This is because sulfur trioxide is the anhydride form of sulfuric acid.

SO_3(l)    +    H₂O_(l)        →      H₂SO_4(l)

The above reaction is very rapid and exothermic. Therefore, control methods should be used when using sulfur trioxide for industrial sulfuric acid production. In addition, sulfur trioxide is an essential reagent in Sulfonation process.

Difference Between SO₂ and SO₃

Definition

SO₂: SO₂ stands for sulfur dioxide.

SO₃: SO₃ stands for sulfur trioxide.

Nature

SO₂: SO₂ is a gaseous compound composed of sulfur and oxygen atoms.

SO₃: SO₃ is a solid compound composed of one sulfur atom bonded to three oxygen atoms.

Molar Mass

SO₂: The molar mass of SO₂ is 64 g/mol.

SO₃: The molar mass of SO₃ is 80.057 g/mol.

Melting Point and Boiling Point

SO₂: The melting point of SO₂ is about -71°C whereas the boiling point is -10°C.

SO₃: The melting point of SO₃ is about 16.9 °C whereas the boiling point is 45°C.

Oxidation State

SO₂: The oxidation state of sulfur in SO₂ is +4.

SO₃: The oxidation state of sulfur in SO₃ is +6.

Oxidation

SO₂: SO₂ can be further oxidized.

SO₃: SO₃ cannot be further oxidized.

Polarity

SO₂: SO₂ is polar due to its geometry (angular) and the presence of a lone electron pair.

SO₃: SO₃ is non-polar due to its geometry (trigonal planar) and the absence of a lone electron pair.

Conclusion

SO₂ and SO₃ are inorganic compounds that are named as sulfur oxides. SO₂ is a gaseous compound at room temperature. SO₃ is a solid (crystalline) compound at room temperature. The main difference between SO₂ and SO₃ is that SO₂ has two oxygen atoms bonded to a sulfur atom whereas SO₃ has three oxygen atoms bonded to a sulfur atom.

Reference: 

1. “Sulfur dioxide.” Wikipedia, Wikimedia Foundation, 3 Jan. 2018, Available here.
2. “SULFUR TRIOXIDE.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine, Available here.
3. “SO3 Molecular Geometry, Lewis Structure, and Polarity Explained.” Geometry of Molecules, 21 July 2017, Available here.

Image Courtesy:

1. “Sulfur-dioxide-ve-B-2D” By Ben Mills – Own work (Public Domain) via Commons Wikimedia
2. “SO3 Sulfur trioxide” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia