Re: free electrons vs. "fully ionized"

Posted by Natalie Smith on Nov 21, 2002 at 09:44
(168.216.133.213)

Re: free electrons vs. "fully ionized" (Pasti)

Both HF and HCl are gases at STP, boiling points at 19.5 and -85.1, respectively. What you bought from Aldrich was undoubtedly a water solution. By ultra-pure, they must have meant "without contamination", viewing water, of course, as a constituent rather than a contaminant.
Both of them are azeotropes at about 38% (the similar number probably being coincidental, since this is by weight percent), indicating they are pretty stable (at least as to phase separation). As to how this can be, well, the classical explanation would be that the intermolecular forces possible between the water molecules and HX molecules are stronger than those between the HX molecules themselves. As to explaining THAT, well, I suppose you could argue that the water molecule has more sites available for hydrogen bonding or perhaps the vector sum of the dipoles of the two O-H bonds in the water molecule creates a stronger overall polarity than that of the HX? I don't have access to anything to verify that; perhaps you can.

"What I am trying to say is that what you call polarization is not an indication of lack of degeneracy.Degeneracy can still exist, even for a "polarized" molecule or atom."
Yes, of course, depending on how the different orbital symmetries relate to overall molecular symmetry. But, it will, necessarily be reduced over that of some single, isolated atom or ion.

"what you mean exactly by sigma and sigma star orbitals"
Sorry, it translates from twisted-lingoese into short-hand names for wave-function sums, overlaps, what-have-you that maximise in-line with the internuclear axis, constructively to make the sigma and destructively to make the sigma star. This is to differentiate from pi and pi star interactions that maximize above and below the internuclear axis. p-p interactions can be oriented to produce either sigma or pi overlap. s-p interactions produce only sigma. I'm sure you know all this; those are the LCAO terms.

As to the rest, I still stand by the statement that there is no such thing as a 100% absolute ionic bond. Complete and total electron transfer is a mental construct to describe a process that some systems come very, very close to being, but not 100.000...%. The perfectly ionic and perfectly covalent bonds are ideas at the ends of a spectrum along which all real-world bonds lie. I will readily admit that some lie so very, very close to one or the other of the ends that the purely ionic or covalent bond description can be virtually all that is needed to understand/explain the behavior of the system. But, let's go back to the original bond being discussed - - the Re-O bond. People were blithely talking about +4 and +7 charges as if this were some sort of neatly ionic bond. It is not. That discussion was very misleading. The bonding between Re and O is polar covalent at most. The charge on the Rhenium is nowhere near +4, much less +7, in any compound. People were using simplistic definitions to argue invalid points.
So, you may define ionic how you choose. But, the COMPLETE electron transfer ionic bond described in basic chem texts, does not strictly happen. And it doesn't begin to describe the Re-O bond.

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