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< | '''Trouton’s rule''' states that the [[entropy of vaporization]] is almost the same value, about 85–88 J K<sup>−1</sup> mol<sup>−1</sup>, for various kinds of [[liquid]]s at their boiling points.<ref>Compare {{cite book |url=http://books.google.co.uk/books?id=SpIV3J_mwiMC&pg=PA87 |title=Physical Chemistry |author=David Warren Ball}} (85 J K<sup>−1</sup> mol<sup>−1</sup>) and {{cite book |url=http://books.google.co.uk/books?id=OUIaM1V3ThsC&pg=PA206 |title=Chemistry: Principles and Practice |author= Daniel L. Reger, Scott R. Goode, David W. Ball}} (88 J K<sup>−1</sup> mol<sup>−1</sup>)</ref> The entropy of vaporization is defined as the ratio between the [[enthalpy]] of vaporization and the boiling temperature. It is named after [[Frederick Thomas Trouton]]. | ||
Mathematically, it can be expressed as: | |||
:<math>\Delta \bar S_{vap} = 10.5R </math> | |||
where R is the [[gas constant]] | |||
Trouton’s rule is valid for many liquids; for instance, the entropy of vaporization of [[toluene]] is 87.30 J K<sup>−1</sup> mol<sup>−1</sup>, that of [[benzene]] is 89.45 J K<sup>−1</sup> mol<sup>−1</sup>, and that of [[chloroform]] is 87.92 J K<sup>−1</sup> mol<sup>−1</sup>. Because of its convenience, the rule is used to estimate the enthalpy of vaporization of liquids whose [[boiling point]]s are known. | |||
The rule, however, has some exceptions. For example, the entropies of vaporization of [[Properties of water|water]], [[ethanol]], and [[formic acid]] are far from the predicted values. The entropy of vaporization of XeF<sub>6</sub> at its boiling point has the extraordinarily high value of 136.9 J K<sup>-1</sup> mol<sup>-1</sup>.<ref>{{cite book|title=Encyclopedia of Inorganic Chemistry|year=2005|publisher=Wiley|isbn=978-0-470-86078-6|edition=2nd|editor=R Bruce King}}</ref> The characteristic of those liquids to which Trouton’s rule cannot be applied is their special interaction between molecules such as [[hydrogen bonding]]. The entropy of vaporization of water and ethanol shows positive deviance from the rule; this is because the hydrogen bonding in the liquid phase lessens the entropy of the phase. In contrast, the entropy of vaporization of formic acid has negative deviance. This fact indicates the existence of an orderly structure in the gas phase; it is known that formic acid forms a [[Dimer (chemistry)|dimer]] structure even in the gas phase. Negative deviance can also occur as a result of a small gas phase entropy owing to a low population of excited rotational states in the gas phase, particularly in small molecules such as methane - a small [[moment of inertia]] I giving rise to a large rotational constant B, with correspondingly widely separated rotational energy levels and, via [[Maxwell–Boltzmann distribution]], a small population of excited rotational states and hence a low rotational entropy. | |||
Trouton's rule validity can be increased by considering | |||
:<math>\Delta \bar S_{vap} = 4.5R + R\ln T</math> | |||
Here, if ''T'' = 400 K, we find the original formulation for Trouton's rule. | |||
Another equation which ''T''<sub>boiling</sub> < 2100 K is Δ''H''<sub>boiling</sub> = 87 ''T''<sub>boiling</sub> − 0.4 J/K. | |||
== References == | |||
{{reflist}} | |||
== Further reading == | |||
* {{cite journal | title = On Molecular Latent Heat | author = Trouton, Frederick | journal = Philosophical Magazine | year = 1884 | volume = 18 | pages = 54–57 | url = http://books.google.com/?id=zpAOAAAAIAAJ&pg=PR3&lpg=PR3&dq=trouton+philosophical+magazine+1884}} - Publication of Trouton's rule | |||
* [[Peter Atkins|Atkins, Peter]] (1978). ''Physical Chemistry'' [[Oxford University Press]] ISBN 0-7167-3539-3 | |||
[[Category:Chemistry theories]] | |||
Revision as of 15:11, 26 July 2013
Trouton’s rule states that the entropy of vaporization is almost the same value, about 85–88 J K−1 mol−1, for various kinds of liquids at their boiling points.[1] The entropy of vaporization is defined as the ratio between the enthalpy of vaporization and the boiling temperature. It is named after Frederick Thomas Trouton.
Mathematically, it can be expressed as:
where R is the gas constant
Trouton’s rule is valid for many liquids; for instance, the entropy of vaporization of toluene is 87.30 J K−1 mol−1, that of benzene is 89.45 J K−1 mol−1, and that of chloroform is 87.92 J K−1 mol−1. Because of its convenience, the rule is used to estimate the enthalpy of vaporization of liquids whose boiling points are known.
The rule, however, has some exceptions. For example, the entropies of vaporization of water, ethanol, and formic acid are far from the predicted values. The entropy of vaporization of XeF6 at its boiling point has the extraordinarily high value of 136.9 J K-1 mol-1.[2] The characteristic of those liquids to which Trouton’s rule cannot be applied is their special interaction between molecules such as hydrogen bonding. The entropy of vaporization of water and ethanol shows positive deviance from the rule; this is because the hydrogen bonding in the liquid phase lessens the entropy of the phase. In contrast, the entropy of vaporization of formic acid has negative deviance. This fact indicates the existence of an orderly structure in the gas phase; it is known that formic acid forms a dimer structure even in the gas phase. Negative deviance can also occur as a result of a small gas phase entropy owing to a low population of excited rotational states in the gas phase, particularly in small molecules such as methane - a small moment of inertia I giving rise to a large rotational constant B, with correspondingly widely separated rotational energy levels and, via Maxwell–Boltzmann distribution, a small population of excited rotational states and hence a low rotational entropy. Trouton's rule validity can be increased by considering
Here, if T = 400 K, we find the original formulation for Trouton's rule.
Another equation which Tboiling < 2100 K is ΔHboiling = 87 Tboiling − 0.4 J/K.
References
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Further reading
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There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang - Publication of Trouton's rule - Atkins, Peter (1978). Physical Chemistry Oxford University Press ISBN 0-7167-3539-3
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