hypohalous acid
{{Short description|Class of chemical compounds}}
A hypohalous acid is an oxyacid consisting of a hydroxyl group single-bonded to any halogen. Examples include hypofluorous acid, hypochlorous acid, hypobromous acid, and hypoiodous acid. The conjugate base is a hypohalite. They can be formed by reacting the corresponding diatomic halogen molecule ({{chem2|auto=1|F2}}, {{chem2|auto=1|Cl2}}, {{chem2|auto=1|Br2}}, {{chem2|auto=1|I2}}) with water in the reaction:
:{{chem2|X2 + H2O <-> HXO + HX}}
This also results in the corresponding hydrogen halide, which is also acidic.
Stability
Hypohalous acids tend to be unstable. Only hypofluorous acid has been isolated as a solid, and even it is explosive at room temperature.{{cite journal |author1=W. Poll |author2=G. Pawelke |author3=D. Mootz |author4=E. H. Appelman | title = The Crystal Structure of Hypofluorous Acid : Chain Formation by O−H···O Hydrogen Bonds | journal = Angew. Chem. Int. Ed. Engl. | year = 1988 | volume = 27 | issue = 3 | pages = 392–3 | doi = 10.1002/anie.198803921}} Hypochlorous acid cannot be prepared in anhydrous form.Inorganic chemistry, Egon Wiberg, Nils Wiberg, Arnold Frederick Holleman, "Hypochlorous acid" p.442, section 4.3.1 Hypobromous acid, hypoiodous acid, and their conjugate bases (hypobromite and hypoiodite) are also unstable, undergoing disproportionation reactions like
:{{chem2|3 BrO-(aq) -> 2 Br-(aq) + BrO3−(aq)}}
and
:{{chem2|3 HIO -> 2 HI + HIO3}}
that result in the corresponding hydrogen halides/halide ions and halic acids/halates.{{cite book |last1=Holleman |first1=A. F. |last2=Wiberg |first2=Egon |last3=Wiberg |first3=Nils |title=Inorganic Chemistry |date=2001 |publisher=Academic Press |isbn=9780123526519 |page=451 |url=https://books.google.com/books?id=Mtth5g59dEIC&dq=0123526515&pg=PA451 |accessdate=24 September 2018 |language=en}}
Uses
Hypochlorous acid and hypobromous acid are each dissolved in water to sanitize it, hypochlorous acid in swimming pools and hypobromous acid in hot tubs and spas.{{cite book |last1=Gonick |first1=Larry |last2=Criddle |first2=Craig |title=The cartoon guide to chemistry |url=https://archive.org/details/cartoonguidetoch00gonirich |url-access=registration |publisher=HarperResource |isbn=9780060936778 |page=[https://archive.org/details/cartoonguidetoch00gonirich/page/189 189] |edition=1st |language=English |chapter=Chapter 9 Acid Basics |quote=Similarly, we add HOCl to swimming pools to kill bacteria.|date=2005-05-03 }}
Acidity
Hypohalous acids tend to be weak acids, and they typically get weaker as the halogen progresses further down the periodic table. Hypochlorous acid has a pKa of 7.53.{{Cite book |last = Harris|first = Daniel C.|year = 2009|title = Exploring Chemical Analysis |url = https://archive.org/details/exploringchemica00harr_726|url-access = limited|edition=Fourth |page = [https://archive.org/details/exploringchemica00harr_726/page/n553 538]}} The pKa values of hypobromous acid is higher (meaning that it is an even weaker acid),{{cite book |last1=Holleman |first1=A. F. |last2=Wiberg |first2=Egon |last3=Wiberg |first3=Nils |title=Inorganic Chemistry |date=2001 |publisher=Academic Press |isbn=9780123526519 |page=449 |url=https://books.google.com/books?id=Mtth5g59dEIC&pg=PA449 |accessdate=7 October 2018 |language=en}} at 8.65. The pKa of hypoiodous acid is even higher, at 10.6.{{cite web |title=Oxoacids {{!}} Introduction to Chemistry |url=https://courses.lumenlearning.com/introchem/chapter/oxoacids/ |website=courses.lumenlearning.com |accessdate=26 February 2019 |quote=HOCl pKa = 7.5 < HOBr pKa = 8.6 < HOI pKa = 10.6}}
References
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