Skip to content
Related Articles

Related Articles

Improve Article
Save Article
Like Article

Oxides of Sulphur

  • Last Updated : 29 Nov, 2021

Sulphur oxide is a group of sulphur and oxygen compounds, the most important of which are sulphur dioxide (SO2) and sulphur trioxide (SO3), which are both produced in large amounts as intermediary steps in the production of sulfuric acid. The dioxide is sulphurous acid’s acid anhydride (a chemical that reacts with water to generate acid), whereas the trioxide is sulfuric acid’s acid anhydride.

Sulphur oxides are inorganic compounds composed entirely of sulphur and oxygen atoms. The most common sulphur oxides found in the Earth’s lower atmosphere are sulphur dioxide (SO2) and sulphur trioxide (SO3). Other notable classes of sulphur oxides include:

  1. Sulphur monoxide (SO) and disulphur dioxide (S2O2), which is formed when sulphur monoxide dimerizes.
  2. Sulfur monoxide
  3. The higher sulphur oxides have a sulphur oxidation state of +6.

Normally, sulphur oxides are formed when sulphur-containing substances are burned in oxygen-rich air. It is found during the roasting of sulphide ores, the combustion of fossil fuels and coals, and other processes. Vehicle emissions are one of the most common sources of sulphur oxide that we can identify. Sulphur dioxide can be formed naturally as a result of volcanic activity, as well as as a byproduct of copper metallurgy. Sulphur trioxide, on the other hand, is manufactured as a precursor to sulphuric acid and is thus known as sulphuric anhydride. Lower sulphur oxides are formed as intermediates during the combustion of elemental sulphur and are less stable than SO2 and SO3. While there are numerous types of sulphur oxides, the two most important are sulphur dioxide (SO2) and sulphur trioxide (SO3).

Sulphur dioxide

One of the most common sulphur oxides found on Earth and even in space is sulphur dioxide. It is a colourless gas that can be poisonous at times and is soluble in water. Exposure to high concentrations of this gas can be harmful to living things. It has the potential to harm human health.

Structure of SO2: It is angular in shape and has an O-S-O bond edge of 119.50. Sulphur dioxide has two types of pi bonds: p pi-p pi and d pi-p pi. The lengths of the two sulphur-oxygen bonds are similar or equivalent. This means that sulphur dioxide is a hybrid of two canonical structures.

Properties of SO2:

  • Sulphur dioxide is a colourless, acidic gas with a suffocating, pungent odour.
  • It is easily liquefied.
  • It is one of the most simple gases to melt. This is due to the fact that it condenses at room temperature under a pressure of 2 atm.
  • It is highly soluble in water and has an acidic aqueous solution (H2SO3).
  • It’s a type of acidic oxide.
  • It dissolves easily in water.
  • It acts as a strong reducing agent, converting halogens to halogen acids and turning acidified K2Cr2O7 solutions green.


  • It decolorizes the KMnO4 solution and converts ferric to ferrous salts.
  • Because it is acidic, it reacts with NaOH solution to produce sodium sulphite, which then reacts with additional SO2 to produce sodium hydrogen sulphite.

2NaOH + SO2→ Na2SO3+ H2O

Na2SO3+ SO2+H2O→ NaHSO3

  • Carbonates and bicarbonates are also decomposed, releasing CO2 gas.

Na2CO3 + 2SO2+ H2O → NaHSO3 + CO2

NaHCO3 + SO2 → NaHSO3 + CO2

  • It forms milkiness in lime water due to the formation of insoluble calcium sulphite, which disappears after passing SO2 for an extended period of time due to the formation of soluble sodium bisulphite.

CaSO3 + SO2 + H2O → Ca(HSO3)2+ H2O

  • When SO2 reacts with PCl5, it produces thionyl chloride (SOCI), which fumes in moist air and is used in organic chemistry.

PCI5 + SO2 → SOCl2 + POCI3

  • It also functions as an oxidizer and a Lewis base.

2H2S + SO2 → 2H2O +3S

Uses of SO2:

  1. Sulfur dioxide is used in the production of H2SO4.
  2. For fumigation, germicide, and fruit preservation.
  3. Liquid SO2 is used as a refrigerant and a non-aqueous solvent.
  4. It is used in the synthesis of sulphites, sulphuric acid, and hydrogen sulphite.
  5. It is widely used in the sugar industry. It is used in the refining and decolourization of sugar.
  6. It is used to refine lamp oil and other petroleum products.
  7. We use it to colour delicate items.
  8. We use it as an antichlor to remove excess chlorine from substances that have been bleached by chlorine.
  9. It’s a glue thinner.
  10. It is used in refrigerators as a refrigerant.
  11. It is used as a preservative in wines, meat, and dry natural products, among other things.

Sulphur trioxide

When sulphur dioxide is oxidised, sulphur trioxide is frequently formed. This chemical compound can be found in a variety of forms in a white crystalline solid. When it is liquid, it is colourless. It is a very reactive substance that reacts violently with water. 

When sulphur trioxide is in the form of a vapour, it is considered a major pollutant and one of the components of acid rain. It also produces a lot of fumes in the atmosphere, and its vapour is extremely corrosive. This substance should be handled with extreme care.

Structure of SO3: It is a D3h symmetry trigonal planar molecule in its gaseous state, as predicted by VSEPR theory. As a result, it is said to be a member of the D3h point group. According to the electron-counting formalism, the sulphur atom has an oxidation state of +6 and a formal charge of 0. The Lewis structure contains one S=O double bond and two S–O dative bonds but does not use d-orbitals. The electrical dipole moment of gaseous sulphur trioxide is given as zero. This is due to the 120° angle formed by the S-O bonds.

Properties of SO3:

  • It is an acidic oxide that reacts with water to form H2SO4. As a result, it reacts with CaO to produce CaSO4 and decomposes carbonates to produce CO.
  • When it reacts with HCl, it produces chloro-sulphonic acid (HOSO2Cl).
  • It is a powerful oxidizing agent.
  • Sulfur trioxide reacts with the base sodium hydroxide to form sodium hydrogen phosphate.

Uses of SO3:

  1. Sulphuric acid and other chemicals are made with it.
  2. It’s a crucial reagent in sulfonation reactions.
  3. It can be used as a bleaching agent to remove residual hydrogen peroxide or as a digesting agent to separate pulp from lignin.
  4. Sulfur trioxide is produced as a byproduct of the oxidation of sulphur dioxide.
  5. It is a strong inorganic acid mist containing sulfuric acid that is used in industry or commercial product production.
  6. As a necessary reagent in the sulfonation reaction.
  7. It is used in the production of photovoltaic cells and solar energy devices.
  8. Sulfur trioxide is a reagent that is required in sulfonation reactions. Dyeing, pharmaceuticals, and detergents are produced as a result of these processes. Sulfur trioxide can be produced in situ from sulfuric acid or used as a solution in the acid.

Safety from SO3: Sulfur trioxide, in addition to being a strong oxidizing agent, is highly hygroscopic and corrosive, resulting in severe burns upon ingestion and inhalation. Because its compounds react violently with water and produce highly corrosive sulfuric acid, they should be handled with extreme caution. It should also be kept away from organic materials due to its strong dehydrating nature and ability to react violently with such materials.

Sample Questions

Question 1: Is sulfur dioxide a compound?


Sulfur dioxide is an inorganic substance that is toxic, thick, and colourless. It is produced in massive quantities during the intermediate steps of producing sulfuric acid. Sulfur dioxide has a pungent, unpleasant odour that is similar to the scent of a freshly struck match.

Question 2: Is sulphur dioxide an acid?


Sulfur dioxide is an acid gas, which can be easily demonstrated by filling a gas container with water and a few drops of universal indicator. Sulphurous acid (H2SO3) is formed, which is a weakly dibasic acid.

Question 3: What is the pH of sulfur dioxide?


Sulfur dioxide is used in juices with a high acidity as a function of pH, the percentage of sulfite, bisulfite, and molecular sulphur dioxide in aqueous solution.

Question 4: How the sulphur trioxide and water react?


It takes a lot of heat to form sulfuric acid quickly. This is the reaction that can be used to produce sulfuric acid by converting metal sulphides or sulphur to SO2, catalytically converting it to SO3, and then reacting it with water. However, we combine SO3 gas with fairly concentrated sulfuric acid to produce even more concentrated acid. As a result, we are reacting to a low concentration of water in H2SO4, and the reaction is proportionately less violent.

Question 5: Why is sulphur trioxide electrophilic in nature?


Sulphur can be bonded to three different oxygens twice. The sulphur atom in the middle technically has a formal charge of zero, whereas the oxygens to which it is bonded are extremely electronegative (known as electron hogs), so the sulphur atom in the middle has a partial plus charge on it. As a result, it will accept the electrons in an attempt to compensate.

My Personal Notes arrow_drop_up
Recommended Articles
Page :

Start Your Coding Journey Now!