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-

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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