The anhydride of perchloric acid (HClO₄) is ?

Perchloric acid (HClO₄) is a strong oxoacid of chlorine, where chlorine is in its highest oxidation state of +7. The acid anhydride of an oxoacid is a compound that can react with water to regenerate the acid. For perchloric acid, the corresponding anhydride is dichlorine heptoxide (Cl₂O₇).

The structure of Cl₂O₇ consists of two chlorine atoms connected through an oxygen bridge, with three oxygen atoms double-bonded to each chlorine. When Cl₂O₇ reacts with water, it forms two molecules of HClO₄:

Cl₂O₇ + H₂O → 2 HClO₄

This reaction shows that Cl₂O₇ is the anhydride of perchloric acid because it can be hydrated to give the parent acid. Dichlorine heptoxide is highly reactive and a strong oxidizing agent due to the high oxidation state of chlorine.

Other oxides of chlorine, such as Cl₂O, ClO₂, and Cl₂O₆, are oxides of chlorine in lower oxidation states and correspond to other acids. For example, Cl₂O is the anhydride of hypochlorous acid (HClO), ClO₂ is related to chlorous acid (HClO₂), and Cl₂O₆ is the anhydride of chloric acid (HClO₃).

Understanding the concept of acid anhydrides is important in inorganic chemistry because it explains how non-metal oxides react with water to form acids. Cl₂O₇ is particularly important in laboratory chemistry and industrial processes as a precursor to perchloric acid and as a strong oxidizing agent in chemical reactions.