Question

In the laboratory $S{O}_3$ can be prepared by

• A. Burning Hydrogen Sulphide in the air
• B. Burning Sulphur in the air
• C. Both a and b
• D. By two stage pyrolysis Sodium hydrogen sulphate

Answer 1

The correct option is D By two stage pyrolysis Sodium hydrogen sulphate

In most cases, whenever sulphur is burnt in air, $S{O}_2$ is the product as the activation energy for the formation of $S{O}_3$ is pretty high. Expensive catalysts like Platinated Asbestos cannot be used in the everyday laboratory.
But if you are eager to prepare $S{O}_3$ without burning a hole through your wallet, we can employ the two step pyrolysis of Sodium Hydrogen Sulphate –$NaHS{O}_4$
1. Dehydrating $NaHS{O}_4$ at ${315}^{\circ }C$, We get
$2NaHS{O}_4\to N{a}_2{S}_2{O}_7+H_{2}O$
2. Cracking at ${460}^{\circ }C$
The intermediate is called Sodium Pyrosulphate, which on cracking at ${460}^{\circ }C$ yields $S{O}_3$
$N{a}_2{S}_2{O}_7\to N{a}_{2}S{O}_4+S{O}_3$
What can you comment on the oxidation state of the Sulphur atom(s) throughout this two-step synthesis? Is there a change in Sulphur's Oxidation state?

Can you think of one more way of preparing $S{O}_3$? What will happen if we dehydrate Sulphuric acid using Phosphorus Pentoxide? Yes, we will get $S{O}_3$.