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Sulphur Monoxide and Sulphur Dioxide

  • Last Updated : 25 Feb, 2022

Sulphur is plentiful, multivalent, and nonmetallic. At normal temperature, elemental sulphur is a bright yellow, crystalline solid. Sulfur is the tenth most abundant element in the universe by mass and the fifth most abundant on Earth. Sulfur on Earth is typically found in the form of sulphide and sulphate minerals, while it is occasionally discovered in pure, native form. Almost all elemental sulphur today is created as a byproduct of the removal of sulfur-containing impurities from natural gas and petroleum. The element’s most important commercial application is the manufacturing of sulfuric acid for sulphate and phosphate fertilizers, as well as other chemical processes.

Sulphur is a chemical element with the symbol S and atomic number 16. 

Many sulphur compounds are odoriferous, and organosulfur compounds are responsible for the odours of odorized natural gas, skunk odour, grapefruit, and garlic. The odour of rotting eggs and other biological processes is caused by hydrogen sulphide. Sulfur is required for all life, however, it is nearly always found in the form of organosulfur compounds or metal sulphides. Organosulfur compounds include three amino acids (cysteine, cystine, and methionine) and two vitamins (biotin and thiamine). Sulfur is an essential macronutrient for all living creatures and one of the key chemical components required for metabolic functioning. Let us study about two oxides of sulphur.

Sulphur Monoxide

Sulfur monoxide (SO) is an inorganic chemical. It only exists as a dilute gas phase. It transforms to S2O2 when concentrated or condensed (disulphur dioxide). It has been discovered in space, but it is rarely seen intact on Earth.

Formation of SO

An electric discharge produces an unstable colourless gas in a low-pressure combination of sulphur dioxide and sulphur vapour; upon cooling, it condenses to an orange-red solid that slowly decomposes to sulphur and sulphur dioxide.

Structure and bonding of SO

The SO molecule has a triplet ground state, which means that each molecule has two unpaired electrons. The length of the SO bond in gaseous S2O, SO2, and SO3 is 148.1 pm, which is similar to the length of the SO bond in lower sulphur oxides but longer than the length of the SO bond in gaseous S2O, SO2, and SO3. The molecule is stimulated to the singlet state by near-infrared radiation (with no unpaired electrons). The singlet state is thought to be more reactive than the ground triplet state, much as singlet oxygen is thought to be more reactive than triplet oxygen.

Sulphur monoxide structure

Production and reactions of SO

  • The use of molecules that extrude SO as a reagent in organic synthesis has focused on the production of SO as a reagent. Decomposition of the relatively simple molecule ethylene episulfoxide, as well as more complex ones, such as a trisulfide oxide, C10H6S3O, are examples.

C2H4SO → C2H4 + SO

  • The SO molecule is thermodynamically unstable, first changing to S2O2. SO inserts into alkenes, alkynes, and dienes to form thiiranes, molecules having three-membered sulphur rings.

Safety measures for SO

It is difficult to adequately estimate its risks due to its rarity in our atmosphere and low stability. However, when it is condensed and compacted, it produces disulfur dioxide, which is both deadly and corrosive. This molecule is also very combustible (similar to methane) and releases sulphur dioxide, a deadly gas when burned.

Sulphur dioxide

The chemical compound with the formula SO2 is sulphur dioxide. It is a poisonous gas that causes the odour of burnt matches. It is naturally emitted by volcanic activity and is created as a byproduct of copper extraction and the combustion of sulfur-containing fossil fuels. Sulfur dioxide

a strong odour similar to nitric acid.

Structure and bonding of SO2 

SO2 is a bent molecule. A valence bond theory method that only considers s and p orbitals would explain the bonding as a resonance between two resonance structures. The bond order of the sulfur–oxygen bond is 1.5. There is support for this straightforward technique that does not require orbital cooperation. The sulphur atom has an oxidation state of +4 and a formal charge of +1 according to electron-counting formalism.

Sulphur dioxide structure

Occurrence of SO2 

Sulfur dioxide is present on Earth in extremely low concentrations and in the atmosphere at approximately 1 ppm. It can be found in varying concentrations on other planets, the most notable being the atmosphere of Venus, where it is the third-most abundant atmospheric gas at 150 ppm. It combines with water to generate sulfuric acid clouds, which is a crucial component of the planet’s global atmospheric sulphur cycle and contributes to global warming. It is thought to occur in abundance as ice on the Galilean moons—as subliming ice or frost on Io’s trailing hemisphere, and in the crust and mantle of Europa, Ganymede, and Callisto, potentially even in liquid form and rapidly interacting with water.

Preparation of SO2 

  • In the lab, sulphur dioxide is made by reacting metallic sulphite or metallic bisulphite with dilute acid. In a reaction between dilute sulphuric acid and sodium sulphite, for example, it is produced.

Na2SO3 + H2SO4 → Na2SO4 + H2O + SO2

  • It is obtained commercially as a byproduct of the roasting of sulphide ores. The resulting gas is dried, liquefied, and stored in steel cylinders.

4FeS2 (s) + 11O2 (g) →2Fe2O3 (s) + 8SO2 (g)

Properties of SO2 

  1. It is a colourless gas with the odour of rotting eggs. It is extremely soluble in water.
  2. It is easily liquefied.
  3. It dissolves in water to generate sulphurous acid, which gives it an acidic flavour.
  4. It is neither combustible nor conducive to burning.
  5. It is a powerful oxidising agent.
  6. It is also a reducing agent. It also acts as a reducing agent.

Uses of SO2 

  1. As a food preservative in the food industry.
  2. It functions as both a bleaching agent and a disinfectant to eliminate excess chlorine.
  3. In the laboratory, it is employed as a reagent and a solvent.
  4. Although sulphur dioxide is primarily utilised in the production of sulfuric acid, sulphur trioxide, and sulfites.
  5. It is also employed as a disinfectant, refrigerant, reducing agent, bleach, and food preservative, particularly in dried fruits.
  6. Sulfur dioxide is employed as a disinfectant, a refrigerant, a reducing agent, a bleach, and a food preservative, especially in dried fruits, despite its primary usage in the production of sulfuric acid, sulphur trioxide, and sulfites.


ng>Sulphur dioxide in the air

The burning of fossil fuels by power plants and other industrial facilities is the major source in the atmosphere. Smaller sources of emissions include industrial activities such as mineral extraction, natural sources such as volcanoes, and locomotives, ships, and other vehicles and heavy equipment that run on sulfur-rich gasoline.

Health effects of Sulphur dioxide

  • Short-term sulphur dioxide exposure can affect the human respiratory system and make breathing difficult. Asthmatics, particularly youngsters, are particularly vulnerable to the effects of sulphur dioxide.
  • Sulfur dioxide emissions that result in high SO2 concentrations in the air typically result in the creation of other sulphur oxides (SOx). Small particles can arise when SOx reacts with other substances in the environment.
  • Particulate matter (PM) pollution is caused by these particles. Small particles can penetrate deeply into the lungs and, if present in significant quantities, can cause health concerns.
  • Gaseous SOx, in high concentrations, can harm trees and plants by destroying leaves and slowing growth. Acid rain, caused by SO2 and other sulphur oxides, can destroy delicate ecosystems.

Sample Questions

Question 1: What is the nature of sulphur dioxide?


Sulfur dioxide is an acidic gas, which may be easily proved by filling a gas container with water and a few drops of universal indicator. Sulphurous acid is formed, which is a slightly dibasic acid.

Question 2: How does Sulphur dioxide form acid rain?


Sulphur dioxide and nitrogen oxides are released into the atmosphere and carried by wind and air currents, resulting in acid rain. Sulfuric and nitric acids are formed when both oxides react with water, oxygen, and other molecules. They are then mixed with water and other seasonings before being served.

Question 3: What is acid rain?


Rain or any other form of precipitation that is highly acidic, suggesting that it includes significant levels of hydrogen ions, is referred to as acid rain. The pH of most water, including the water we drink, is between 6.5 and 8.

ut acid rain has a pH lower than this.

Question 4: How is sulphur monoxide formed?


An electric discharge produces an unstable colourless gas in a low-pressure combination of sulphur dioxide and sulphur vapour; upon cooling, it condenses to an orange-red solid that slowly decomposes to sulphur and sulphur dioxide.

Question 5: What is the structure of sulphur dioxide?


Sulfur dioxide is a bent molecule. A valence bond theory method that only considers s and p orbitals would explain the bonding as resonance between two resonance structures.

Question 6: How is sulphur dioxide formed?


Sulfur dioxide is produced through the combustion of fossil fuels (coal and oil) and the smelting of sulphur-containing mineral ores (for example, aluminium, copper, zinc, iron and lead). Sulfur dioxide molecules easily dissolve in water to form sulfuric acid.

Question 7: Is sulphur dioxide a compound?


Sulfur dioxide is a hazardous inorganic gas that is thick and colourless. It can be created in large amounts during the intermediary processes of producing sulfuric acid. Sulfur dioxide also has a distinct and unpleasant odour that is akin to the aroma of a freshly struck match.

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