Preparation and Properties of Sulfur Trioxide (SO3)

Preparation and Properties of Sulfur Trioxide:

It is an important oxide of sulfur. It is also called an anhydride of sulfuric acid or sulfuric anhydride.

Preparation of Sulfur Trioxide:

Sulfur trioxide is obtained by the methods as under-

(a) By the catalytic oxidation of SO₂ by atmospheric oxygen in presence of a catalyst like platinized asbestos or V₂O₅ at 450-500°C. This is shown in figure-

(b) By distillation of H₂SO₄ in presence of phosphorus pentoxide.

(c) By oxidation of sulfur trioxide with ozone. Example-

(d) By decomposition of ferric sulfate below 600°C.

At higher temperatures, sulfur dioxide is formed.

Properties of Sulfur Trioxide:

Physical Properties of Sulfur Trioxide:

• Sulfur trioxide, at ordinary temperature, is a colourless liquid.
• It fumes strongly in air.
• It freezes into a white crystalline solid at 17°C.
• It is soluble in water giving rise to the fromation of sulfuric acid.

Chemical Properties of Sulfur Trioxide:

Chemically, sulfur trioxide is a reactive compound.

(1) Nature- Sulfur trioxide is an acidic oxide. When dissolved in water, it gives sulfuric acid.

The aqueous solution has a sour taste. The acidic nature is supported as under-

(a) Action of blue litmus paper- It turns blue litmus paper red.

(b) Action of alkalis- It reacts with alkalis like NaOH to give the corresponding salt.

(c) Action of basic oxides- It combines with basic oxides like CaO to give the corresponding salt.

(2) Addition Reactions- Sulfur trioxide shows unsaturated character and is capable of forming a large number of addition compounds with acids and some non-metals.

(a) Action of HCl-

(b) Action of Iodine-

(3) Formation of Oleum- It readily dissolves in concentrated H₂SO₄ to give oleum or fuming sulfuric acid.

(4) Oxidizing Property- It is a powerful oxidizing agent. The oxidizing action is due to the liberation of nascent oxygen.

Uses of Sulfur Trioxide:

It is used-

• As a dehydrating agent in the manufacture of explosives.
• In the manufacture of fuming sulfuric acid and chlorosulfonic acid.