From: Jacob Zabicky (zabicky$##$bgumail.bgu.ac.il)
Date: Sun Nov 05 2000 - 06:31:28 EST
Hello Ashutosh,
Let me only make a brief remark on the phrase "this boiling point is 20C
higher than expected". IMHO this way of approaching physical properties is
wrong. One should start by stating a carefully determined physical
property, say, the normal boiling point of methane as an incontrovertible
fact of life, and such-and-such model fails to predict it by 20 full
kelvin, and there's no inkling about why is that so.
First, I don't know which model was used by Pauling for the evaluation. If
it is a model of quite general application, then it is frequently found
that the simplest cases, say, the first couple of members of a homologous
series, show exceptional behavior. All models are based on certain
simplifying assumptions that avoid unsurmountable difficulties in the
calculations. Some simplifications may not be granted for methane.
Second, many models are explicitly or tacitly based on certain parameters
that are properties of the compund. Most popular for VLE (vapor-liquid
equilibrium) calculations are the critical temperature and pressure, and
the acentric factor; the dipole moment may also enter certain models.
Models for predicting normal boiling points lack, IMO, a scientific basis,
because there is nothing special about a pressure of 1 atm regarding the
tendency of a substance to be in two equilibrated phases. In this respect,
the acentric factor is a vapor-pressure dependent property determined at
the reduced temperature of 0.7, with the critical temperature (of each
compound) acting as a unit in a unified scale for all compounds. Reduced
propereties may be of better scientific value for evaluating models. But
then, two or three parameters may not suffice for correctly predicting VLE
for all compounds.
Is Pauling's complain about methane still on the table?
All the best,
Jacob
At 22:59 2/11/0, Ashutosh wrote:
>Hello everyone
>Methane,due to lack of H bonding,shows a low boiling point as expected(-161.5C)
>However,Pauling in the 1960 edition of his 'The Nature of the Chemical
>Bond' says that even this boiling point is 20C higher than expected,and
>he says that nobody knows the explanation why.
>Has an explanation been found after 1960?
>Thanks
>Ashutosh S. Jogalekar
>Department of Chemistry
>University of Pune
>India
>
>
>
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><DIV><FONT face=Arial size=2>Hello everyone</FONT></DIV>
><DIV><FONT face=Arial size=2>Methane,due to lack of H bonding,shows a low
>boiling point as expected(-161.5C)</FONT></DIV>
><DIV><FONT face=Arial size=2>However,Pauling in the 1960 edition of his 'The
>Nature of the Chemical Bond' says that even this boiling point is 20C
>higher than expected,and he says that nobody knows the explanation
>why.</FONT></DIV>
><DIV><FONT face=Arial size=2>Has an explanation been found after
>1960?</FONT></DIV>
><DIV><FONT face=Arial size=2>Thanks</FONT></DIV>
><DIV><FONT face=Arial size=2>Ashutosh S. Jogalekar</FONT></DIV>
><DIV><FONT face=Arial size=2>Department of Chemistry</FONT></DIV>
><DIV><FONT face=Arial size=2>University of Pune</FONT></DIV>
><DIV><FONT face=Arial size=2>India</FONT></DIV>
><DIV> </DIV>
><DIV> </DIV></BODY></HTML>
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