IChO 2003 · Theory problem 18

The strongest chlorine oxoacid

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Which of the following acids is the strongest?

a) perchloric acid, \(\ce{HClO4}\) ( )

b) chloric acid, \(\ce{HClO3}\) ( )

c) chlorous acid, \(\ce{HClO2}\) ( )

d) hypochlorous, \(\ce{HClO}\) ( )

(e) All of them are equally strong because they all contain chlorine ( )

Official English text of the problem.

Answers

  • 18: a) Perchloric acid, \(\ce{HClO4}\), SMILES OCl(=O)(=O)=O. Mark only the box beside a).

Solution

18. The four acids have the following structures, written using conventional localized bonds:

Acid SMILES
Perchloric acid OCl(=O)(=O)=O
Chloric acid OCl(=O)=O
Chlorous acid OCl=O
Hypochlorous acid OCl

For oxoacids of the same central element, acidity increases with the number of additional oxygen atoms attached to that element. These oxygen atoms withdraw electron density, stabilizing the conjugate base formed when the acidic \(\ce{O-H}\) group loses its proton. Delocalization of the conjugate base’s negative charge over oxygen atoms also contributes to this stabilization.

Consequently, the acid-strength order is

\[ \boxed{\ce{HClO4 > HClO3 > HClO2 > HClO}}. \]

Perchloric acid produces the particularly stable, weakly basic perchlorate ion, \(\ce{ClO4^-}\), SMILES [O-]Cl(=O)(=O)=O. This SMILES depicts one localized resonance form; the actual ion has equivalent chlorine–oxygen bonds, with its negative charge delocalized over all four oxygen atoms.

Thus a) perchloric acid is the strongest. Merely containing the same central element does not make these acids equally strong.

The answers were checked against the official answers published by the organisers.