Halous acid

A halous acid, also known as a halogenous acid,^[1] is an oxyacid consisting of a halogen atom in the +3 oxidation state single-bonded to a hydroxyl group and double-bonded to an oxygen atom. Examples include chlorous acid, bromous acid, and iodous acid. The conjugate base is a halite.

Synthesis

One method is to add oxygen to the corresponding hypohalous acid. This can be done with another hypohalous acid—or even the same one.

HBrO + HClO → HBrO₂ + HCl^{[citation needed]}

2 HBrO → HBrO₂ + HBr^{[citation needed]}

The latter is a disproportionation reaction.

Another method of oxidizing hypobromous acid can be used:

HBrO + H₂O - 2e⁻ → HBrO₂ + 2H⁺^{[citation needed]}

The oxidized bromine-containing acid need not contain oxygen originally, as in this comproportionation reaction:

2 HBrO₃ + HBr → 3 HBrO₂^{[citation needed]}

Bromous acid was originally prepared using another method of adding oxygen to hypobromous acid, where the free oxygen was freed from water when the free hydrogen was taken by NO₃ (to form nitric acid) that was freed from silver nitrate when the silver was taken by elemental bromine to form silver bromide.

2 AgNO₃ + HBrO + Br₂ + H₂O → HBrO₂ + 2 AgBr^[2]

Chlorous acid may be generated from salts of the conjugate base, chlorite, which is more stable than chlorous acid.^[3]

Ba(ClO₂)₂ + H₂SO₄ → BaSO₄ + 2 HClO₂^[4]

Stability

Chlorous acid, bromous acid, and iodous acid are all unstable. Chlorous acid is the only isolatable halous acid,^[5] and while it has stable salts, they tend to decompose rapidly, some even explosively, upon heating.^[3] Neither bromous acid nor iodous acid has ever been isolated, and while a few salts of bromous acid have been isolated, the same is not true for iodous acid.^[5]

One method of decomposition of halous acids is disproportionating to the corresponding hypohalous acid and halic acid:

2 HClO₂ → HClO + HClO₃

2 HBrO₂ → HBrO₃ + HOBr

In acidic solutions, chlorous acid tends to decompose via another disproportionation reaction:

5 HClO₂ → 4 ClO₂ + HCl + 2 H₂O^[4]

References

^ Holleman, A. F.; Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Web: Academic Press. p. 439. ISBN 9780123526519. Retrieved 21 March 2019.
^ "Journal of the Society of Chemical Industry. v.25 1906". HathiTrust. Retrieved 2017-04-28.
^ ^a ^b Holleman, A. F.; Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Web: Academic Press. p. 445. ISBN 9780123526519. Retrieved 21 March 2019.
^ ^a ^b Holleman, A. F.; Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Web: Academic Press. p. 444. ISBN 9780123526519. Retrieved 21 March 2019.
^ ^a ^b Wiberg, Egon; Holleman, Arnold Frederick (2001). Inorganic Chemistry. Elsevier. ISBN 0-12-352651-5.^{[page needed]}

[1] Holleman, A. F.; Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Web: Academic Press. p. 439. ISBN 9780123526519. Retrieved 21 March 2019.

[:3-2] "Journal of the Society of Chemical Industry. v.25 1906". HathiTrust. Retrieved 2017-04-28.

[IC445-3] Holleman, A. F.; Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Web: Academic Press. p. 445. ISBN 9780123526519. Retrieved 21 March 2019.

[IC444-4] Holleman, A. F.; Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Web: Academic Press. p. 444. ISBN 9780123526519. Retrieved 21 March 2019.

[Wiberg&Holleman-5] Wiberg, Egon; Holleman, Arnold Frederick (2001). Inorganic Chemistry. Elsevier. ISBN 0-12-352651-5.^{[page needed]}

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