Subject: CCL:A question about enthalpy

From: [email-masked] (Brad Thompson)
Date: Sat, 14 May 94 22:32:11 CDT

Regarding Anthony Scott's question about the enthalpy difference
between to independent atoms A:  (6/2) RT and a molecule A-A with a
very weak bond: (7/2) RT, which Scott describes as "two systems that
appear to be similar" --

As long as the two atoms are bound, from an enthalpy standpoint it is
of little consequence if the bond vibrational frequency "tends to
zero" (Scott's term) as long as it doesn't actually reach zero.  As
the frequency decreases, there are more vibrational levels thermally
accessable, and the energy of any sizeable assembly of molecules
(such as a mole) stays the same.  Vibrational degrees of freedom can
store potential energy, something translations and rotations can't do.  

Of course, at some point the approximations we use will break down,
but for any "normal" very weak bond, A-A is not all that "similar"
to two A's thermodynamically.

                    | H. Bradford Thompson   [Brad]
[email-masked]       | Scholar in Residence, Chemistry & Physics
                    | Gustavus Adolphus College, St. Peter MN 56082-1498