Difference between revisions of "Frost diagram"
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[[File:Manganese Frost pH0.png|thumb|right|254px|The Frost diagram for [[manganese]] at pH 0.]] | [[File:Manganese Frost pH0.png|thumb|right|254px|The Frost diagram for [[manganese]] at pH 0.]] | ||
A '''Frost diagram''' is a graphical representation of the relative stability of different [[oxidation state]]s of an element: specifically a plot of the [[Gibbs free energy of formation]] of different species in or from aqueous solution against [[oxidation number]]. Such diagrams were first introduced by American chemist [[Arthur A. Frost]].<ref>{{citation | title = Oxidation Potential–Free Energy Diagrams| first = Arthur A. | last = Frost | journal = J. Am. Chem. Soc. | year = 1951 | volume = 73 | issue = 6 | pages = 2680–82 | doi = 10.1021/ja01150a074}}.</ref> | A '''Frost diagram''' is a graphical representation of the relative stability of different [[oxidation state]]s of an element: specifically a plot of the [[Gibbs free energy of formation]] of different species in or from aqueous solution against [[oxidation number]]. Such diagrams were first introduced by American chemist [[Arthur A. Frost]].<ref>{{citation | title = Oxidation Potential–Free Energy Diagrams| first = Arthur A. | last = Frost | journal = J. Am. Chem. Soc. | year = 1951 | volume = 73 | issue = 6 | pages = 2680–82 | doi = 10.1021/ja01150a074}}.</ref> | ||
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| + | ==Interpretation== | ||
| + | The vertical axis on a Frost diagram is a measure of [[free energy]]: a chemical system will always tend to towards the state of lowest free energy so, ''other things being equal'', species at the bottom of the diagram will tend to be "more stable" than those higher up. Hence, in the Frost diagram from [[manganese]] at pH 0, the Mn<sup>2+</sup> cation is the "most stable" species at that pH. | ||
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| + | If a species is at a concave point on the curve, such as Mn<sup>2+</sup> and MnO<sub>2</sub>, that species is stable towards [[disproportionation]]. On the other hand, species which are at convex points on the curve, such as Mn<sup>3+</sup>, H<sub>3</sub>MnO<sub>4</sub> and HMnO<sub>4</sub><sup>−</sup>, will tend to disproportionate until they reach the species at concave points. It is important to remember that this "tendency to disproportion" is a thermodynamic description, and says nothing about the kinetics of the reaction: the [[hypochlorite]] ion, for example, is unstable with respect to disproportionation to [[perchlorate]] and [[chloride]], but the reaction is very slow below 70 °C and, even then, gives [[chlorate]] as the oxidized product and not perchlorate.<ref>{{Greenwood&Earnshaw1st|page=1002}}.</ref> | ||
==See also== | ==See also== | ||
Revision as of 12:01, 11 April 2010
A Frost diagram is a graphical representation of the relative stability of different oxidation states of an element: specifically a plot of the Gibbs free energy of formation of different species in or from aqueous solution against oxidation number. Such diagrams were first introduced by American chemist Arthur A. Frost.[1]
Interpretation
The vertical axis on a Frost diagram is a measure of free energy: a chemical system will always tend to towards the state of lowest free energy so, other things being equal, species at the bottom of the diagram will tend to be "more stable" than those higher up. Hence, in the Frost diagram from manganese at pH 0, the Mn2+ cation is the "most stable" species at that pH.
If a species is at a concave point on the curve, such as Mn2+ and MnO2, that species is stable towards disproportionation. On the other hand, species which are at convex points on the curve, such as Mn3+, H3MnO4 and HMnO4−, will tend to disproportionate until they reach the species at concave points. It is important to remember that this "tendency to disproportion" is a thermodynamic description, and says nothing about the kinetics of the reaction: the hypochlorite ion, for example, is unstable with respect to disproportionation to perchlorate and chloride, but the reaction is very slow below 70 °C and, even then, gives chlorate as the oxidized product and not perchlorate.[2]
See also
References
- ↑ Frost, Arthur A. Oxidation Potential–Free Energy Diagrams. J. Am. Chem. Soc. 1951, 73 (6), 2680–82. DOI: 10.1021/ja01150a074.
- ↑ Greenwood, Norman N.; Earnshaw, A. Chemistry of the Elements; Pergamon: Oxford, 1984; p 1002. ISBN 0-08-022057-6.
External links
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See also the corresponding article on Wikipedia. |
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