Oxides of Sulphur

Introduction to Sulphur Oxides

Sulfur oxide can be defined as the chemical compound with the chemical formula- SO. It is a toxic gas which is  responsible for the smell of burnt matches. It occurs naturally by volcanic activity and is produced as a by-product of copper extraction along with the burning of fossil fuels which are contaminated with sulfur compounds to make further products.

The sulphur oxides are inorganic compounds which are made up entirely of sulphur and oxygen atoms as we all know. If we talk about planets, In the Earth’s lower atmosphere the most commonly found sulphur oxides are sulphur dioxide which is denoted as: SO₂ and sulphur trioxide which is denoted as SO₃. Some other notable classes of sulphur oxides are listed below which we shall consider.

• The lower sulphur oxides, which have the general formula SMON (m>2n).

• Sulphur monoxide which is written as SO and sulphur dioxide denoted as SO₂, which is formed from the dimerization of sulphur monoxide Which is SO.

• Disulfur monoxide  denoted as S₂O

• The higher oxides of sulphur, in which sulphur exhibits its own oxidation state of +6.

Bonding and Structure

If we observe, we find that oxides of sulphur are formed when substances containing sulphur are burnt in air containing plenty of oxygen. It can be found during the roasting of sulfide ores and at the time of burning of fossil fuels, coals, etc also. One of the most common sources of sulphur that we can relate this to is the emission from vehicles. Sulphur dioxide can be formed naturally also due to volcanic activity and also as a byproduct during the metallurgy of copper as well.

Sulphur trioxide which is on the other hand prepared industrially as a precursor to sulphuric acid and is therefore said as sulphuric anhydride. The oxides of sulphur which are lower are formed as intermediates during the combustion of elemental sulphur and are relatively less stable when compared to SO₂ and SO₃.

SO2 is a molecule bent with C2v known as the symmetry point group. A bond theory of valency  approach considering just p and s orbitals would describe the bonding in terms of resonance between two resonance structures as shown below:

[Image will be Uploaded Soon]

The sulfur–oxygen bond which is made has a bond order of 1.5. There is support for this simple approach that does not invoke d orbital participation in the whole process. In terms of electron-counting formalism, the sulfur atom has formal charge of +1 and oxidation state of +4  in the atom.

Occurrence 

[Image will be Uploaded Soon]

The occurrence of sulphur is found on Earth's surface and exists in very little concentrations and in the atmosphere at about 1 ppm.

On other planets,  it can be present in numerous concentrations but the most significant amongst all is the atmosphere of the planet Venus where it is the third-most significant atmospheric gas at 150 ppm. There, it condenses to form clouds and it is a key component of chemical reactions in the atmosphere and contributes to the global warming process. It has been implicated as a key agent in the warming of early Mars along with estimates of concentrations in the lower atmosphere as high as 100 ppm and though it only exists in trace amounts. 

On both Mars and Venus, as on Earth, its primary source is thought to be volcanic. The atmosphere of  a natural satellite of Jupiter is 90% sulfur dioxide and trace amounts are thought to also exist in Jupiter's atmosphere.

Production

[Image will be Uploaded Soon]

Sulfur oxide is primarily produced for the manufacture of sulfuric acid. In the United States in 1979, 23.6 million tonnes that is 26,014,547 US short tons of sulfur dioxide were used in this way compared with 150 thousand tonnes that is 165,347 US short tons used for other purposes. Most sulfur dioxide is produced by the combustion of elements of sulfur. Some sulfur dioxide is also produced by ores in air and roasting pyrite.

Sulfur dioxide is the product of the burning of sulfur or at times of burning materials that contain sulfur:

S + O2 → SO2, and (ΔH = −297) kJ/mol.

To aid the combustion process, liquefied sulfur that is 140–150 °C, 284-302 °F is sprayed through an atomizing nozzle for the purpose of generating fine drops of sulfur with a large surface area. The reaction is exothermic in nature and the combustion produces temperatures of 1000–1600 °C that is 1832–2912 °F. 

The significant amount of heat which is produced is recovered by steam generation which can subsequently be converted to electricity. The combustion of organosulfur and hydrogen sulfide compounds proceeds similarly. The sulphur roasting of the ores such as pyrite, and cinnabar also called mercury sulfide and it also releases SO2

A combination of these reactions which are mentioned is responsible for the largest source of sulfur dioxide and even volcanic eruptions. These events can release millions of tonnes of SO2 at one go itself. Sulfur dioxide can also act as a byproduct in the manufacture of calcium silicate cement and CaSO4 is heated with sand and coke in this process.