Trouton's rule

Trouton’s rule states that the entropy of vaporization is almost the same value, about 87-88 J K^-1 mol^-1, for various kinds of liquids. 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:

$Delta ar S_{vap} = 10.5R$

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 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

$Delta ar S_{vap} = 4.5R + Rln T$

Here, if "T" = 400 K, we find the original formulation for Trouton's rule

Another equation which "T"_boiling < 2100 K is Δ"H"_boiling = 87 "T"_boiling − 0.4 J/K.

References

# Peter Atkins (1978). "Physical Chemistry" Oxford University Press ISBN 0-7167-3539-X

Wikimedia Foundation. 2010.

Игры ⚽ Нужен реферат?

Look at other dictionaries:

trouton's rule — ˈtrau̇tənz noun Usage: usually capitalized T Etymology: after Frederick T. Trouton died 1922 English physicist : a statement in physical chemistry: the molar heats of vaporization of pure liquids are proportional to the absolute temperatures of… … Useful english dictionary
Frederick Thomas Trouton — FRS (24 November, 1863 ndash; 21 September, 1922) was an Irish physicist known for Trouton s Rule and experiments to detect the Earth s motion through the luminiferous aether.Trouton was born in Dublin in 1863 and attended Trinity College there,… … Wikipedia
1922 in Ireland — Events*January 2 The first edition of the newspaper Poblacht na hÉireann is published. It is established by republican opponents to the Anglo Irish Treaty who declare their fealty to the Irish Republic. *January 6 The terms of the Anglo Irish… … Wikipedia
1863 in Ireland — Events*The Ulster Railway, which began construction in 1839, reaches Clones. *28 November First edition of The Irish People . [cite book | last=Moody, TW Martin, FX (eds)| year=1967 |title=The Course of Irish History | publisher=The Mercier Press … Wikipedia
Luminiferous aether — The luminiferous aether: it was hypothesised that the Earth moves through a medium of aether that carries light In the late 19th century, luminiferous aether or ether, meaning light bearing aether, was the term used to describe a medium for the… … Wikipedia
Melting point — For the physical processes that takes place at the melting point, see Melting, Freezing and Crystallization The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid… … Wikipedia
Phase transition — This diagram shows the nomenclature for the different phase transitions. A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another. A phase of a thermodynamic system and the states of matter… … Wikipedia
Degenerate matter — is matter that has such extraordinarily high density that the dominant contribution to its pressure is attributable to the Pauli exclusion principle.[1] The pressure maintained by a body of degenerate matter is called the degeneracy pressure, and … Wikipedia
Plasma (physics) — For other uses, see Plasma. Plasma lamp, illustrating some of the more complex phenomena of a plasma, including filamentation. The colors are a result of relaxation of electrons in excited states to lower energy states after they have recombined… … Wikipedia
Critical line (thermodynamics) — In thermodynamics, a critical line is the higher dimensional equivalent of a critical point [1]. It is the locus of contiguous critical points in a phase diagram. These lines cannot occur for a single substance due to the phase rule, but they can … Wikipedia

Academic Dictionaries and Encyclopedias

Trouton's rule

Look at other dictionaries:

Share the article and excerpts

Academic Dictionaries and Encyclopedias

Wikipedia

Trouton's rule

Look at other dictionaries:

Share the article and excerpts

Direct link