From: Guido de Silvestri (guidods$##$etb.net.co)
Date: Thu Apr 20 2006 - 17:00:58 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
New Yahoo! Messenger with Voice. Call regular phones from your PC and
save big.
_______________________________________________
ORGLIST - Organic Chemistry Mailing List
Website / Archive / FAQ: http://www.orglist.net
To post a message (TO EVERYBODY) send to everybody$##$orglist.net
To unsubscribe, send to everybody-request$##$orglist.net the message: unsubscribe your_orglist_password your_address
This archive was generated by hypermail 2.1.4 : Tue Feb 13 2007 - 14:17:01 EST