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Sulphur dioxide also known as sulfurous anhydride or Sulfur(IV) oxide is a chemical compound. Its chemical formula is SO2. Sulphur dioxide is a colorless gas with a pungent suffocating smell. It has a boiling point of -10 degree Celsius. It is corrosive in nature and acutely toxic by inhalation. It may cause irritation in your eyes. It can convert in liquid when placed under pressure and easily dissolves in water. Burning of coal and oil are the human activities that release sulphur dioxide in air, while it is naturally released from volcanic eruptions. Sulphur dioxide can be used as a food bleaching agent, in manufacturing chemicals, in paper pulping and as a refrigerant.
Read More: Electronic Configuration of Group 16 Elements
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Key Terms: Oxygen, Sulphur dioxide, atoms, chemical compound, Boiling Point
Read More: Manufacturing of Sulfuric acid through contact proces
Structure of Sulphur Dioxide
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The structure of Sulphur dioxide consists of two oxygen atoms and one Sulphur atom. When combined together they make an angular shape of 119.5 degree. The length of both the bonds are equal and have p pi - p pi and p pi - d pi bond. That is why it can be said that Sulphur dioxide shows a resonance structure with two canonical forms.
Structure of Sulphur Dioxide
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Preparation of Sulphur Dioxide
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Sulphur dioxide can be prepared in various ways, some the most common practices to prepare sulphur dioxide are mentioned here:
- By reacting sodium sulphite with sulphuric acid (laboratory preparation).
Na2SO3 + H2SO4 → Na2SO4 + H2O + SO2↑
- By roasting of sulphide ores Sulphur dioxide is obtained as a by-product for commercial use.
4FeS2 (s) + 11 O2 (g) →2Fe2O3 (s) + 8SO2 (g)
Preparation of Sulphur Dioxide
- Burning of sulphur in the presence of Oxygen.
Δ
S + O2 → SO2
- Heating sulphur in sulphuric acid solution.
- Heating copper with sulphuric acid solution.
- Heating silver with sulphuric acid solution.
Read More: Manufacturing of Sulphuric Acid
Chemical Properties of Sulphur Dioxide
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It is converted into sulfurous acid when dissolved in water.
SO2 + H2O → H2SO3
When it reacts with sodium hydroxide gives sodium sulphate, excess of Sulphur dioxide in this solution gives sodium hydrogen sulphate.
SO2 + 2NaOH → Na2SO3 + H2O
SO2 + Na2SO3 → H2O + 2NaHSO3
Chemical Properties of Sulphur Dioxide
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Sulphur atoms show a property where they can extend the covalency to six and make bonds with elements like oxygen, chlorine. This property is called tetravalent. In the presence of charcoal it reacts with chlorine and gives sulphuryl chloride.
Charcoal
SO2 + Cl2 → SO2Cl2
It gives sulphur trioxide when it reacts with oxygen in the presence of vanadium pentoxide.
V2O5
2SO2 + O2 → SO3
Sulphur dioxide can also act as a reducing agent in the presence of moisture and releases hydrogen. When it comes in contact with ferric salts, the ferric salts are reduced to ferrous salts and similarly it reduces halogens to halogen acids.
2Fe3+ + SO2 + 2H2O → 2Fe2+ + SO4- + 4H+
X2 + SO2 + 2H2O → SO4- + 2X - + 4H+
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Identification of Sulphur Dioxide Gas
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Potassium permanganate solution decolorizes when it comes in contact with Sulphur dioxide.
5SO2 + 2KMnO4 + 2H2O → K2SO4 + 2 MNSO4 + 2H2SO4
Other tests we can do is, it turns K2Cr2O7 (potassium dichromate) solution green.
3SO2 + K2Cr2O7 + H2SO4 → K2SO4 + 2Cr2(SO4)3 + H2O
Sulphur dioxide turns starch iodide paper blue.
5SO2 + 2KIO3 + 4H2O → 2KHSO4 + 3H2SO4 + I2
Read More: Oxides of Nitrogen
Sulphur Dioxide Uses
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Sulphur dioxide is used in various domestic as well as commercial uses, but various safety measures need to be taken care while using it.
Sulphur dioxide is used as:
- Food preservative in the food industries.
- Refrigerant in the cold storage plants.
Prolonged exposure to Sulphur dioxide gas can cause severe genetic, respiratory as well as fertility problems in humans.
Uses of Sulphur Dioxide
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Sample Questions
Ques. How does sulphur dioxide cause acid rain? (1 mark)
Ans. There is sulphur dioxide in air and the excess release of this substance from factories reacts with moisture in air forms sulphuric and nitric acid which then falls on earth with rain.
Ques. What is the pH value of sulphur dioxide? (1 mark)
Ans. In general, the pH value of sulphur dioxide is 3.0 .
Ques. What are the most important things to know about sulphur dioxide? (1 mark)
Ans. It is a colorless gas with a suffocating pungent smell. A long exposure to this gas may cause genetic / fertility problems and may even result in the premature death of an individual.
Ques. What is the major source for sulphur dioxide? (1 mark)
Ans. Fossil fuel combustion is the primary source of sulphur dioxide.
Ques. Why is sulphur dioxide considered as a pollutant? (1 mark)
Ans. Sulphur dioxide is naturally released by volcanic activity. This can lead to the production of secondary pollutants like particulate matter, acid rain and aerosols.
Ques. What is sulphur dioxide used for? (1 mark)
Ans. It is mainly used for the production of sulphuric acid, sulfites. But it can also be used as a reducing agent or as a disinfectant or as a refrigerant or as a bleaching agent. It can also be used as a food preservative.
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