Difference between revisions of "Bromous acid"
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| Section8 = {{Chembox Related | | Section8 = {{Chembox Related | ||
| − | | OtherFunctn = [[Chlorous acid]] | + | | OtherFunctn = [[Chlorous acid]]<br/>[[Iodous acid]] |
| Function = halous acids | | Function = halous acids | ||
| OtherCpds = [[Hydrobromic acid]]<br/>[[Hypobromous acid]]<br/>[[Bromic acid]]<br/>[[Perbromic acid]] | | OtherCpds = [[Hydrobromic acid]]<br/>[[Hypobromous acid]]<br/>[[Bromic acid]]<br/>[[Perbromic acid]] | ||
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Bromous acid is unstable with respect to [[disproportionation]]: at acid pH, the thermodynamic products are [[bromine]] and [[bromate]]. | Bromous acid is unstable with respect to [[disproportionation]]: at acid pH, the thermodynamic products are [[bromine]] and [[bromate]]. | ||
:5 HBrO<sub>2</sub> → Br<sub>2</sub> + 3 BrO{{su|b=3|p=−}} + 3 H<sup>+</sup> + H<sub>2</sub>O | :5 HBrO<sub>2</sub> → Br<sub>2</sub> + 3 BrO{{su|b=3|p=−}} + 3 H<sup>+</sup> + H<sub>2</sub>O | ||
| − | The initial mechanism is simpler than that for [[chlorous acid]], as the production of [[bromine dioxide]] and the oxidation of [[water]] to [[oxygen]] are negligible. Hence, there are just two significant initial steps:<ref name="pKa2"/> | + | The initial mechanism is simpler than that for [[chlorous acid]], as the production of [[bromine dioxide]] and the oxidation of [[water]] to [[oxygen]] are negligible. Hence, there are just two significant initial steps:<ref name="pKa2"/><ref>{{citation | title = The Decomposition of Hypobromite and Bromite Solutions | first1 = P. | last1 = Engel | first2 = A. | last2 = Oplatka | first3 = B. | last3 = Perlmutter-Hayman | journal = J. Am. Chem. Soc. | year = 1954 | volume = 76 | issue = 7 | pages = 2010–15 | doi = 10.1021/ja01636a092}}.</ref><ref>{{citation | first1 = C. L. | last1 = Lee | first2 = M. W. | last2 = Lister | title = The Decomposition of Aqueous Sodium Bromite | journal = Can. J. Chem. | volume = 49 | issue = 17 | pages = 2822–26 | year = 1971 | doi = 10.1139/v71-470}}.</ref> |
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| − | The [[hypobromous acid]] then disproportionates further to give the final products. There is | + | The [[hypobromous acid]] then disproportionates further to give the final products. There is conflicting evidence concerning the [[catalysis]] of the disproportionation of HBrO<sub>2</sub> by [[bromide]] ions (unlike the case of chlorous acid and [[chloride]] ions).<ref name="pKa2"/><ref name="Massagli">{{citation | first1 = A. | last1 = Massagli | first2 = A. | last2 = Indelli | first3 = F. | last3 = Pergola | title = Kinetic investigation of the decomposition of bromite | journal = Inorg. Chim. Acta | volume = 4 | issue = 4 | pages = 593–96 | year = 1970 | doi = 10.1016/S0020-1693(00)93357-7}}.</ref><ref>{{citation | title = The bromide/bromous acid reaction. Switch-controlling step of the Belousov-Zhabotinskii oscillating system | first1 = Freek | last1 = Ariese | first2 = Zsuzsanna | last2 = Nagy | journal = J. Phys. Chem. | year = 1986 | volume = 90 | issue = 8 | pages = 1496–98 | doi = 10.1021/j100399a005}}.</ref> |
==Acidity== | ==Acidity== | ||
| − | + | {| class="wikitable" align=left style="margin:0 1em 0.5em 0;" | |
| − | |||
|- | |- | ||
! colspan=3 | p''K''<sub>a</sub> | ! colspan=3 | p''K''<sub>a</sub> | ||
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| align=center | [[Chlorous acid|HClO<sub>2</sub>]]<br/>1.96 | | align=center | [[Chlorous acid|HClO<sub>2</sub>]]<br/>1.96 | ||
| align=center | [[Bromous acid|HBrO<sub>2</sub>]]<br/>3.43 | | align=center | [[Bromous acid|HBrO<sub>2</sub>]]<br/>3.43 | ||
| − | | align=center | | + | | align=center | [[Iodous acid|HIO<sub>2</sub>]]<br/>— |
|- | |- | ||
| align=center | [[Chloric acid|HClO<sub>3</sub>]]<br/>< 0 | | align=center | [[Chloric acid|HClO<sub>3</sub>]]<br/>< 0 | ||
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| + | The [[acid dissociation constant]] of bromous acid can be calculated from the observed kinetics of its disproportionation:<ref name="pKa2"/> ''K''<sub>a</sub> = 3.7(9){{e|−4}} (p''K''<sub>a</sub> = 3.43). This is consistent with that which would be expected from [[Periodicity|periodic trends]]. | ||
| + | |||
| + | Hence, bromous acid would be expected to form salts with a wide range of metal ions. Nevertheless, only the [[bromite]]s of the [[Alkali metal|alkali]] and [[alkaline earth metal]]s are well characterized.<ref name="Kirk-Othmer"/> They are not prepared by the neutralization of bromous acid, but by the controlled disproportionation of the corresponding [[hypobromite]] at pH 11–12 and 0 °C<ref name="Massagli"/><ref>{{citation | first1 = R. | last1 = Kircher | first2 = R. | last2 = Periat | title = Preparation du bromite de baryum cristallisé | journal = Ind. Chim. Belge, Suppl. 1 | pages = 877 | year = 1959}}.</ref><ref>{{citation | inventor1-first = René | inventor1-last = Kircher | inventor2-first = Robert | inventor2-last = Periat | assignee = Société d'Etudes Chimiques pour l'Industrie et l'Agriculture | title = Verfahren zur Herstellung von Bariumbromit durch Disproportionierung von Bariumhypobromit | country-code = DE | patent-number = 1076096 | publication-date = 1960-02-25}}.</ref> or by heating the corresponding [[bromate]] in vacuo.<ref>{{citation | first1 = A. J. | last1 = Downs | first2 = C. J. | last2 = Adams | editor1-first = J. C., Jr. | editor1-last = Bailar | editor2-first = H. J. | editor2-last = Emeleus | editor3-first = R. | editor3-last = Nyholm | editor4-first = A. F. | editor4-last = Trotman-Dickenson | title = Comprehensive Inorganic Chemistry | volume = 2 | publisher = Pergamon | location = Oxford | year = 1973 | pages = 1419–20}}.</ref> By comparison with the corresponding [[chlorite]]s, the bromites of less [[Electronegativity|electropositive]] cations might be expected to be explosively unstable.<ref>{{Greenwood&Earnshaw1st|pages=1007–9}}.</ref> | ||
| + | |||
| + | Bromous acid itself may be prepared by acidifying a solution of a bromite.<ref name="pKa2"/> It is also thought to be produced in small amounts by the photolysis of bromates, particularly in the [[Belousov–Zhabotinskii reaction]]. | ||
==Notes and references== | ==Notes and references== | ||
Latest revision as of 16:26, 2 January 2011
| Bromous acid | |
|---|---|
| IUPAC name | Bromous acid[note 1] |
| Other names | Bromic(III) acid Hydroxy-λ3-bromanone Hydroxidooxidobromine |
| Identifiers | |
| InChI | InChI=1/BrHO2/c2-1-3/h(H,2,3) |
| InChIKey | DKSMCEUSSQTGBK-UHFFFAOYAC |
| Standard InChI | InChI=1S/BrHO2/c2-1-3/h(H,2,3) |
| Standard InChIKey | DKSMCEUSSQTGBK-UHFFFAOYSA-N |
| CAS number | [] |
| ChemSpider | |
| Properties[2][3] | |
| Chemical formula | HBrO2 |
| Molar mass | 112.91 g mol−1 |
| Acidity (pKa) | 3.43(11) |
| Related compounds | |
| Other halous acids | Chlorous acid Iodous acid |
| Other compounds | Hydrobromic acid Hypobromous acid Bromic acid Perbromic acid |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) | |
Bromous acid, HBrO2, is an oxoacid of bromine. Its existence as a molecular species has been considered doubtful[4][5] but, although it is unstable with respect to disproportionation, its lifetime in aqueous solution is sufficient for spectroscopic study.[3][6] It is an intermediate in the Belousov–Zhabotinskii reaction.[6][7][8][9][10] Some salts (bromites) are known.[4]
Contents
Disproportionation
Bromous acid is unstable with respect to disproportionation: at acid pH, the thermodynamic products are bromine and bromate.
- 5 HBrO2 → Br2 + 3 BrO−3 + 3 H+ + H2O
The initial mechanism is simpler than that for chlorous acid, as the production of bromine dioxide and the oxidation of water to oxygen are negligible. Hence, there are just two significant initial steps:[3][11][12]
HBrO2 + HBrO2 → HBrO + BrO−3 + H+ k = 800(100) dm3 mol−1 s−1 HBrO2 + BrO−2 → HBrO + BrO−3 k = 39.1(26) dm3 mol−1 s−1
The hypobromous acid then disproportionates further to give the final products. There is conflicting evidence concerning the catalysis of the disproportionation of HBrO2 by bromide ions (unlike the case of chlorous acid and chloride ions).[3][13][14]
Acidity
| pKa | ||
|---|---|---|
| HClO 7.47 |
HBrO 8.80 |
HIO 10.70 |
| HClO2 1.96 |
HBrO2 3.43 |
HIO2 — |
| HClO3 < 0 |
HBrO3 < 0 |
HIO3 0.80 |
The acid dissociation constant of bromous acid can be calculated from the observed kinetics of its disproportionation:[3] Ka = 3.7(9) × 10−4 (pKa = 3.43). This is consistent with that which would be expected from periodic trends.
Hence, bromous acid would be expected to form salts with a wide range of metal ions. Nevertheless, only the bromites of the alkali and alkaline earth metals are well characterized.[4] They are not prepared by the neutralization of bromous acid, but by the controlled disproportionation of the corresponding hypobromite at pH 11–12 and 0 °C[13][15][16] or by heating the corresponding bromate in vacuo.[17] By comparison with the corresponding chlorites, the bromites of less electropositive cations might be expected to be explosively unstable.[18]
Bromous acid itself may be prepared by acidifying a solution of a bromite.[3] It is also thought to be produced in small amounts by the photolysis of bromates, particularly in the Belousov–Zhabotinskii reaction.
Notes and references
Notes
- ↑ Bromous acid is a retained name in IUPAC nomenclature.[1]
References
- ↑ Nomenclature of Inorganic Chemistry; IUPAC Recommendations 2005; Royal Society of Chemistry: Cambridge, 2005; p 287. ISBN 0-85404-438-8, <http://www.iupac.org/publications/books/rbook/Red_Book_2005.pdf>.
- ↑ Faria, Roberto de Barros; Epstein, Irving R.; Kustin, Kenneth Systematic design of chemical oscillators. Part 84. Determination of the pKa of bromous acid. J. Phys. Chem. 1992, 96 (17), 6861–63. DOI: 10.1021/j100196a003.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Faria, R. B.; Epstein, Irving R.; Kustin, Kenneth Kinetics of Disproportionation and pKa of Bromous Acid. J. Phys. Chem. 1994, 98 (4), 1363–67. DOI: 10.1021/j100055a051.
- ↑ 4.0 4.1 4.2 Ukeles, S. D.; Freiberg, M. Bromine, Inorganic Compounds. In Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley: New York, 2002. DOI: 10.1002/0471238961.021815131001031.
- ↑ Cotton, F. Albert; Wilkinson, Geoffrey Advanced Inorganic Chemistry, 5th ed.; Wiley-Interscience: New York, 1988; pp 566–67. ISBN 0-471-84997-9.
- ↑ 6.0 6.1 Ariese, Freek; Ungvarai-Nagy, Zsuzsanna The disproportionation of bromous acid (HBrO2), key species of the Belousov-Zhabotinskii oscillating reaction. J. Phys. Chem. 1986, 90 (1), 1–4. DOI: 10.1021/j100273a001.
- ↑ Sullivan, James C.; Thompson, Richard C. Kinetic study of the cerium(IV)-bromous acid reaction in acid sulfate solution. Implications for the Belousov-Zhabotinskii oscillating reaction. Inorg. Chem. 1979, 18 (9), 2375–79. DOI: 10.1021/ic50199a009.
- ↑ Hynne, F.; Graae Soerensen, P.; Neergaard, H. Oscillations of bromous acid, hypobromous acid, bromide, and cerium(4+) concentrations in the Belousov-Zhabotinskii reaction reconstructed from quenching experiments. J. Phys. Chem. 1991, 95 (3), 1315–18. DOI: 10.1021/j100156a052.
- ↑ Ruoff, Peter; Foersterling, Horst Dieter; Gyorgyi, Laszlo; Noyes, Richard M. Bromous acid perturbations in the Belousov-Zhabotinskii reaction: experiments and model calculations of phase response curves. J. Phys. Chem. 1991, 95 (23), 9314–20. DOI: 10.1021/j100176a052. Foersterling, Horst Dieter; Varga, Margit Bromous acid/cerium(4+): reaction and HBrO2 disproportionation measured in sulfuric acid solution at different acidities. J. Phys. Chem. 1993, 97 (30), 7932–38. DOI: 10.1021/j100132a022.
- ↑ Pelle, Krisztina; Wittmann, Maria; Lovrics, Klára; Noszticzius, Zoltán; Turco Liveri, Maria L.; Lombardo, Renato Mechanistic Investigations of the BZ Reaction with Oxalic Acid Substrate. I. The Oscillatory Parameter Region and Rate Constants Measured for the Reactions of HOBr, HBrO2, and Acidic BrO3− with Oxalic Acid. J. Phys. Chem. A 2004, 108 (25), 5377–85. DOI: 10.1021/jp048817s. Onel, Lavinia; Bourceanu, Gelu; Bitter, István; Wittmann, Mária; Noszticzius, Zoltán Uncatalyzed Reactions in the Classical Belousov−Zhabotinsky System. 2. The Malonic Acid−Bromate Reaction in Acidic Media. J. Phys. Chem. A 2006, 110 (3), 990–96. DOI: 10.1021/jp055259o.
- ↑ Engel, P.; Oplatka, A.; Perlmutter-Hayman, B. The Decomposition of Hypobromite and Bromite Solutions. J. Am. Chem. Soc. 1954, 76 (7), 2010–15. DOI: 10.1021/ja01636a092.
- ↑ Lee, C. L.; Lister, M. W. The Decomposition of Aqueous Sodium Bromite. Can. J. Chem. 1971, 49 (17), 2822–26. DOI: 10.1139/v71-470.
- ↑ 13.0 13.1 Massagli, A.; Indelli, A.; Pergola, F. Kinetic investigation of the decomposition of bromite. Inorg. Chim. Acta 1970, 4 (4), 593–96. DOI: 10.1016/S0020-1693(00)93357-7.
- ↑ Ariese, Freek; Nagy, Zsuzsanna The bromide/bromous acid reaction. Switch-controlling step of the Belousov-Zhabotinskii oscillating system. J. Phys. Chem. 1986, 90 (8), 1496–98. DOI: 10.1021/j100399a005.
- ↑ Kircher, R.; Periat, R. Preparation du bromite de baryum cristallisé. Ind. Chim. Belge, Suppl. 1 1959, 877.
- ↑ Kircher, René; Periat, Robert (Société d'Etudes Chimiques pour l'Industrie et l'Agriculture) Verfahren zur Herstellung von Bariumbromit durch Disproportionierung von Bariumhypobromit. DE Patent 1076096, published 25 February 1960.
- ↑ Downs, A. J.; Adams, C. J. In Comprehensive Inorganic Chemistry; Bailar, J. C., Jr.; Emeleus, H. J.; Nyholm, R., et al., Eds.; Pergamon: Oxford, 1973; Vol. 2, pp 1419–20.
- ↑ Greenwood, Norman N.; Earnshaw, A. Chemistry of the Elements; Pergamon: Oxford, 1984; pp 1007–9. ISBN 0-08-022057-6.
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