Introduction
Some Basic Features of Sulphur Dioxide
Formula of Sulphur dioxide: SO2
Vapour Density: 32
Relative Molecular Mass: 64
Discovery of Sulphur Dioxide
Sulphur dioxide has been known to people for ages. Sulphur used to be burnt to produce sulphur dioxide, which was then used as a fungicide and insecticide. Priestley prepared sulphur dioxide in the year 1774 by heating concentrated sulphuric acid with mercury and called it "Vitriolic acid air". But it was Lavoisier, who proved that it was an oxide of sulphur.
Occurrence of Sulphur Dioxide
Sulphur dioxide is present in volcanic gases; in hot aqueous springs surrounding volcanic regions; in areas where sulphur is extracted by melting it, and in places where coal containing sulphur or sulphur compounds are burnt.
Preparation of Sulphur Dioxide
By direct burning of sulphur in air or oxygen
Sulphur + Oxygen 
Sulphur dioxide
Laboratory Preparation of Sulphur Dioxide
At the end of the reaction the residue left behind is copper sulphate, which is normally blue in color. In the presence of excess concentrated sulphuric acid, it may get dehydrated into grayish white color.
Physical Properties of Sulphur Dioxide
Vapor density : Its vapor density is 32, while the vapor density of air is 14.4. Hence sulphur dioxide gas is 2.2 times denser than air.
Chemical Properties of Sulphur Dioxide - Combustibility and Addition Reactions
Combustibility
Sulphur dioxide gas is not combustible and ordinarily does not support combustion. The only exceptions are burning potassium or burning magnesium, which continue to burn in sulphur dioxide.
Chemical Properties of Sulphur Dioxide - Reducing reactions
In the presence of moisture, sulphur dioxide acts as a powerful reducing agent. On reaction with water it forms sulphurous acid.
Bleaching Action of Sulphur Dioxide
Dry sulphur dioxide does not have any bleaching action. However, if the gas is moist, or if the object to be bleached is moist, then bleaching takes place. Usually only delicate articles such as coloured flowers, silk, wool etc., are bleached by sulphur dioxide.
Tests for Sulphur Dioxide
It forms a white precipitate of barium sulphate when passed through a solution of barium chloride and chlorine water.
Uses of Sulphur Dioxide
The various uses of sulphur dioxide are:
1) In the manufacturing of sulphuric acid, sulphites, and hydrogen sulphite.
2) In the sugar industry for refining and decolorizing sugar.
3) For refining kerosene, and other petroleum products.
Preparation of Sulphur Trioxide
Some Basic Features of Sulphur Trioxide
Formula of Sulphur Trioxide: SO3
Relative Molecular Mass: 80
Physical Properties of Sulphur Trioxide
* Sulphur trioxide is a silken white transparent crystalline solid.
* Sulphur trioxide sublimes on heating.
Chemical Properties of Sulphur Trioxide
Action of Heat
When heated to about 500oC, sulphur trioxide decomposes to give sulphur dioxide and oxygen.
Uses of Sulphur Trioxide
* In the manufacturing of sulphuric acid.
* In the production of oleum.
* As a drying agent for certain gases.
Preparation of Hydrogen Sulphide
Hydrogen sulphide is prepared by the action of sulphuric acid on metal sulphides.
Physical Properties of Hydrogen Sulphide
Vapor Density : Hydrogen sulphide has a vapor density of 17. The vapor density of air =14.4, i.e., it is 1.2 times denser than air.
Chemical Properties of Hydrogen Sulphide
Combustibility
It is a combustible gas, and burns with a blue flame. In a limited supply of oxygen or air, the products are water and sulphur. In excess of air or oxygen, the products are water and sulphur dioxide.
Uses of Hydrogen Sulphide
In the laboratory, hydrogen sulphide acts as a reagent to detect the presence of certain anions and cations present in a given salt.
Discovery of Sulphuric Acid
Of all the acids, sulphuric acid is the most important one. In almost all industries, sulphuric acid is used directly or indirectly. Hence it is called the "King of chemicals". It is said that the material prosperity of a country may be determined by the amount of sulphuric acid consumed by that country.
Occurrence of Sulphuric Acid
In the combined state, it is found in the form of metal sulphates, such as barium sulphate, calcium sulphate, magnesium sulphate, alum etc.
Preparation of Sulphuric Acid
By Dissolving Sulphur Trioxide in Water
Industrial Preparation of Sulphuric Acid by Contact Process
Industrially, the main method of manufacture of sulphuric acid is the Contact Process. In this process, sulphur dioxide is first catalytically oxidized to sulphur trioxide. The trioxide is then absorbed by concentrated sulphuric acid. The product is then diluted to the desired concentration of the acid required.
Physical Properties of Sulphuric Acid
Boiling Point
Pure sulphuric acid on boiling decomposes to give water and sulphur trioxide. During boiling more sulphur trioxide is lost than water. As a result, the concentration becomes 98.33%. The acid of this concentration forms a constant boiling mixture with the remaining small quantity of water at 338oC. Hence sulphuric acid cannot be concentrated beyond 98.33% by boiling. However 100% sulphuric acid can be obtained by dissolving more sulphur trioxide in it.
Chemical Properties of Sulphuric Acid - As Dilute Acid
Sulphuric acid almost completely ionizes in water to form hydronium ions, sulphate ions, and hydrogen sulphate ions.
The acidic properties of sulphuric acid are due to the presence of the hydronium ions.
Chemical Properties of Sulphuric Acid - As a Strong Acid
Sulphuric acid ionizes almost completely in water. Hence it acts as a strong acid. It also behaves as a strong electrolyte. Pure sulphuric acid is a weak electrolyte as it hardly has any water to form hydronium ions. When diluted with water, it ionizes readily. As a result, on dilution, it becomes a stronger electrolyte.
Chemical Properties of Sulphuric Acid - As Concentrated Acid
As a Non-volatile Acid : When compared to hydrochloric acid and nitric acid, sulphuric acid is non-volatile. It also has a high boiling point. So when treated with salts of more volatile acids, and heated, concentrated sulphuric acid displaces the more volatile acids.
Tests for Sulphuric Acid
1) Sulphuric acid turns blue litmus paper red.
2) It gives a white precipitate of barium sulphate, when added to barium chloride solution.
Uses of Sulphuric Acid
In manufacture of fertilizers
Sulphuric acid is used in the manufacturing process of fertilizers such as ammonium sulphate, super phosphate of lime, etc.
Summary
The presence of sulphur dioxide can be tested by reacting with acidified orange colored potassium dichromate solution when it gives green colored chromic salt.
