Question

Give the structure of pyrosulphuric acid. How would you prepare it? What would you observe, when colorless $HI$is added to sulphuric acid?

Answer 1

Pyrosulphuric acid is a main constituent of 'fuming' sulfuric acid (oleum). It is also known as disulfuric acid. Which means it has two sulfurs and it has acidic nature.

_Complete step by step solution: _
Pyrosulfuric acid is a strong acid. The chemical formula of pyrosulfuric acid is${{H}_{2}}{{O}_{7}}{{S}_{2}}$. Pyrosulfuric acid can be considered as an anhydride of sulfuric acid (${{H}_{2}}S{{O}_{4}}$).
Pure pyrosulfuric acid is a colorless crystalline solid. It is more commonly found in a less pure form as a fuming, dense, oily liquid which can be colorless to dark brown depending on its purity level.
Structure:

Formation of pyrosulfuric acid:
It is prepared by dissolving sulfur trioxide ($S{{O}_{3}}$) gas in concentrated sulfuric acid. Here is the reaction:
${{H}_{2}}S{{O}_{4}}+S{{O}_{3}}\to {{H}_{2}}{{O}_{7}}{{S}_{2}}$
Pyrosulfuric acid is also obtained as an intermediate during the preparation of sulfuric acid, through the contact process.
When colorless HI is added to pyrosulphuric acid, and its color becomes violet because pyrosulphuric acid oxidises ${{I}^{-1}}$to ${{I}_{2}}$. And below given is the reaction of pyrosulphuric acid with HI:
${{H}_{2}}S{{O}_{4}}+2HI\to 2{{H}_{2}}O+S{{O}_{2}}+{{I}_{2}}$

Note: Pyrosulphuric acid’s main uses are in the manufacture of explosives and dyes, also used widely as a sulfating agent and in petroleum refining, used as a convenient intermediate or form of sulfuric acid for storage and transportation, due to its less corrosive nature. It has similar health effects to those of sulfuric acid.