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what is Trouton’s rule please explain deeply

what is Trouton’s rule
please explain deeply


2 Answers

askIITians Faculty 1246 Points
6 years ago
Trouton's RuleThe enthalpy of vaporisation of many liquids may be estimated using Trouton's rule. If we assume that the liquid and vapour consist of randomly oriented disordered distributions of molecules then the entropy change on vaporisation (vapSmay be calculated using purely statistical theories and should be the same for all substances. Trouton demonstrated experimentally that the standard entropy change of vaporisation (vapSʅ is 88 J K-1mol-1for many substances. At the phase transition where the liquid and vapour are at equilibrium


so that

(G=Gvap-Gliq= (vapH-T(vapS= 0

Rearranging, and noting that the temperature at the phase transition isTbgives an expression for the enthalpy of vaporisation

(vapH=Tb(vapS= 88 J K-1mol-1×Tb

Trouton's rule only holds good for substances in which there is little order in the liquid phase. Substances that form strong hydrogen bonds tend to be ordered, even in the liquid phase, so that the entropy change upon vaporisation is rather bigger than predicted using Trouton's rule. For example, the application of Trouton's rule to predict the enthalpy of vaporisation of water suggests

(vapHʅwater= 88 J K-1mol-1×Tb= 88°× 373 = 32.8 J K-1mol-1

The observed enthalpy change is, however,

(vapHʅwater= 40.7 J K-1mol-1

In fact, the entropy of vaporisation of water is (vapSʅ = 109.1 J K-1mol-1.
2061 Points
6 years ago
The explanation of Trouton’s rule is that a comparable change in volume occurs
(with an accompanying change in the number of accessible microstates) when any
liquid evaporates and becomes a gas. Hence, all liquids can be expected to have
similar standard entropies of vaporization.

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