Romans used sulphur dioxide in wine making as they discovered that the empty wine glass smelled fresh on burning sulphur candles inside the glass. The chemical formula of this compound is SO2. It exists in small concentrations in the earth’s atmosphere but it is the third most significant gas in the atmosphere of Venus. It is found in case of volcanic eruptions and in the effluents released from industries.
Structure of Sulphur Dioxide:
This compound is a bent molecule and has a bond order of 1.5. The resonance structure can be described using the valence bond theory using s and p orbitals.
Preparation of Sulphur Dioxide:
In the laboratory, sulphur dioxide is prepared by the reaction of metallic sulphite or a metallic bisulphite with a dilute acid. For example, a reaction between the dilute sulphuric acid and sodium sulphite will result in the formation of SO2.
Na2SO3 + H2SO4 → Na2SO4 + H2O + SO2
Commercially it is obtained as a by-product released from the roasting of sulphide ores. The gas obtained is dried, liquefied and then stored in steel cylinders.
4FeS2 (s) + 11 O2 (g) →2Fe2O3 (s) + 8SO2 (g)
Properties of Sulphur Dioxide:
- It is a colourless gas with a rotten egg odour. It is highly soluble in water.
- It liquefies easily.
- SO2 dissolves in water to form sulphurous acid due to which it possesses an acidic character.
H2O + SO2 → H2SO3
- It does not support combustion nor is it combustible.
- SO2 is a strong oxidizing agent.
2H2S + SO2 → 3S + 2H2O
- It also acts as a reducing agent.
SO2 + 2H2O → H2SO4 + 2H
Uses of Sulphur Dioxide:
- In the food industries, as a food preservative.
- It acts as a bleaching agent to remove the excess chlorine and as a disinfectant.
- In cold storage plant, it acts as a refrigerant.
- It is used as a reagent and a solvent in the laboratory.
Safety measures have to be taken against sulphur dioxide as inhaling this gas causes serious health problems such as premature death and respiratory problems.
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