Table of Contents
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-
| 2SO2 + O2 ————> 2SO3 |
(b) By distillation of H2SO4 in presence of phosphorus pentoxide.
| H2SO4 + P2O5 ————> 2HPO3 + SO3 |
(c) By oxidation of sulfur trioxide with ozone. Example-
| 3SO2 + O3 ————> 3SO3 |
(d) By decomposition of ferric sulfate below 600°C.
| Fe2(SO4)3 ————> Fe2O3 + 3SO3 |
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.
| H2O + SO3 ————> H2SO4 |
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.
| H2O + SO3 ————> H2SO4 H2SO4 ⇌ 2H+ + SO4– |
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.
| 2NaOH + SO3 ————> Na2SO4 + H2O |
(c) Action of basic oxides- It combines with basic oxides like CaO to give the corresponding salt.
| CaO + SO3 ————> CaSO4 |
(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-
| HCl + SO3 ————> ClHSO3 (Chloro sulphonic acid) |
(b) Action of Iodine-
| I2 + 6SO3 ————> I2(SO3)6 |
(3) Formation of Oleum- It readily dissolves in concentrated H2SO4 to give oleum or fuming sulfuric acid.
| H2SO4 + SO3 ————> H2S2O7 |
(4) Oxidizing Property- It is a powerful oxidizing agent. The oxidizing action is due to the liberation of nascent oxygen.
| SO3 ————> SO2 + (O) |
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.
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