From: Guido de Silvestri (guidods$##$etb.net.co)
Date: Thu Apr 20 2006 - 16:39:43 EDT
Dear Jmarjorie...
When a mixture has a vapor pressure greater than the calculated, it is used
to say that it shows a positive deviation from the Raoult´s Law. The must of
the mixtures are rated to this category. On these cases, the partial
pressure of each of the components is greater than its ideal pressure.
It has to be observed (in a concentration .vs. pressure graph) that as the
concentration of the component becomes to 1, in molar fraction, the partial
pressure for this substance tends tangentially to the ideal... In other
words, the Raoult´s Law is almost applicable to the present substance in
larges concentrations. These are the cases of all the substances, but there
is an exception case, which is, when the components exhibit strong physical
or chemical interaction, such an association inside the vapor phase or an
electrolytic dissociation inside the liquids.
In the cases when a significant deviation from the ideal exists, and the
vapor pressures of the components in both are not so far between them, the
total pressure in a constant temperature curves, could increase to a maximum
(or a minimum) in a given concentration. Those mixtures present an azeotrope
and are known as constant boiling point mixture.
Particularly, to your case, In a first sight, the two boiling points are
very different, but there are two subjects that you have to investigate to
be sure: Firstly, you have to watch out in the literature for the total
pressure-composition system (in the International critical Tables, for
example) related to your compounds and, starting with the data, built the
concentration .vs. pressure graph, and secondly, read about azeotropes in
"The molecular thermodynamics of fluid phase equlibria" written by
Prausnitz, and edited by Prentice Hall - Englewood Cliffs. On this work, Mr.
Prausnitz had shown an extended technique for non ideal solutions trough the
activities coefficients.
Best Regards,
Guido de Silvestri
Process Engineer
-----Mensaje original-----
De: everybody-bounces$##$orglist.net [mailto:everybody-bounces$##$orglist.net] En
nombre de fine chem Enviado el: Martes, 18 de Abril de 2006 03:56 a.m.
Para: everybody$##$orglist.net
Asunto: ORGLIST: Fractional Distillation - seperation of mixtures
Dear All,
This goes out to those expert in fractional distillation : A mixture
contains 2 organic liquids (completely miscible with each other), one with a
boiling point of 80 deg C and the other with a boiling point of 35 deg C.
Can the two liquids be completely seperated by fractional distillation using
a very long column or will they behave as a binary azeotropic mixture at
constant pressure (constant boiling point mixture) and the distillate still
contain proportions of both mixtures ? I'am asking as I have not fully
understood the concept; some say the mixtures are not seperable while others
say the mixtures are completely seperable; am confused. jmarjorie
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