The key difference between SO2 and SO3 is that SO2 is a colourless gas at room temperature, whereas SO3 is a colourless to white crystalline solid.
SO2 is sulfur dioxide while SO3 is sulfur trioxide. Both are oxides of sulfur.
CONTENTS
1. Overview and Key Difference
2. What is SO2
3. What is SO3
4. Side by Side Comparison – SO2 vs SO3 in Tabular Form
5. Summary
What is SO2?
SO2 is sulfur dioxide. It is a colourless gaseous compound containing sulfur and oxygen atoms. SO2 is the chemical formula of this compound. Therefore, it contains a sulfur atom bonded to two oxygen atoms via 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. The sulfur atom has 6 electrons in its outermost orbital. Therefore after forming two double bonds with the oxygen atoms, there are two more electrons remaining; these electrons exist as a lone electron pair.
Hence, we can determine the geometry of the SO2 molecule; it is angular geometry. SO2 is polar due to its geometry (angular) and the presence of a lone electron pair.
Figure 01: Structure of Sulfur Dioxide
Sulfur dioxide is considered as a toxic gas. Hence, if there is SO2 in the atmosphere, it will be an indication of air pollution. Also, this gas has a very irritating smell. The molecular mass of sulfur dioxide is 64 g/mol. It is a colourless gas at room temperature. The melting point is about -71°C, whereas the boiling point is -10°C.
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 the +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 is sulfur trioxide. It is a solid compound containing one sulfur atom that binds with three oxygen atoms. SO3 is the chemical formula of this compound. Here, each oxygen atom has a double bond with the sulfur atom. The sulfur atom is in the centre of the molecule. A sulfur atom has 6 electrons in its outermost orbital. Hence, after forming three double bonds with the oxygen atoms, there are no more electrons remaining on the sulfur atom as in sulfur dioxide. Thus, this determines the geometry of the SO3 molecule; it has trigonal planar geometry. SO3 is non-polar due to its geometry (trigonal planar) and the absence of a lone electron pair.
Figure 02: Geometry of Sulfur Trioxide
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 45oC. At room temperature and pressure, sulfur trioxide is a white crystalline solid compound which will fume in the air. It has a pungent odour. 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 the industrial scale. It is because sulfur trioxide is the anhydride form of sulfuric acid.
SO3(l) + H2O(l) → H2SO4(l)
The above reaction is very rapid and exothermic. Therefore, control methods should be used when using sulfur trioxide for industrial sulfuric acid production. Besides, sulfur trioxide is an essential reagent in Sulfonation process.
What is the Difference Between SO2 and SO3?
SO2 is sulfur dioxide while SO3 is sulfur trioxide. Both are oxides of sulfur. The key difference between SO2 and SO3 is that SO2 is a colourless gas at room temperature, whereas SO3 is a colourless to white crystalline solid. Furthermore, the oxidation state of sulfur in sulfur dioxide is +4 while in sulfur trioxide it is +6. Due to the presence of a lone electron pair and their geometry, sulfur dioxide is a polar compound, while sulfur trioxide is a nonpolar compound. The following infographic summarizes the difference between SO2 and SO3.
Summary – SO2 vs SO3
SO2 is sulfur dioxide, and SO3 is sulfur trioxide. Both are oxides of sulfur. The key difference between SO2 and SO3 is that SO2 is a colorless gas at room temperature, whereas SO3 is a colourless to white crystalline solid.
Reference:
1. Brandt, Malcolm J., et al. “Chemical Storage, Dosing and Control.” Tworts Water Supply, 2017, pp. 513–552., doi:10.1016/b978-0-08-100025-0.00012-0.
Image Courtesy:
1. “Sulfur-dioxide-2D” By The original uploader was Richtom80 at English Wikipedia. – Transferred from en.wikipedia to Commons (Public Domain) via Commons Wikimedia
2. “Sulfur-trioxide-2D-dimensions” By Jynto – Own work, based on File:Sulfur-trioxide-2D-dimensions.png (Public Domain) via Commons Wikimedia