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Re: [Phys-L] Energy & Bonds



-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of Paul Lulai
Sent: Wednesday, November 13, 2013 8:37 AM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Energy & Bonds

SNIP

Springs... I think part of my problem is that I mentally construct an image of
springs holding atom-to-atom to form molecules. It works as a mental model
for holding stuff together. However, it causes great problems when thinking
about a reaction. It is easy and natural (for me at least) to think about energy
stored in the springs holding atoms together. It is NOT easy nor natural (for
me at least) to think that when the ions are roaming free and the fictional
springs are not connected... there is actually more potential energy in the
system of ions.
If I completely abandon the idea of springs, it is easier to understand a more
accurate (i think) model of electrical potential energy for bonding.

Yeah, a spring analogy can be troubling. Consider, however, that if you are separating items that are tied together, so you must do positive work (and increase the potential energy) to move them apart. Electrical forces have infinite range, so the ions are never really roaming free, there is just more potential energy in the system because you did work to increase the separation. If they get close to other "attractive" particles, they lose potential energy by attaching to those new particles (with new springs?). The energy idea works, and if you want to keep the spring idea, you have to realize that new springs replace old springs, and enough work can break springs.

I wonder about the E=mcc thought. At what grade-level are we doing a
disservice to students talking about conservation of mass as opposed to
conservation of mass-energy? Kids have seen that equation and have little
idea of what it means. They do seem to appreciate learning what it means.


YES, we do a disservice to them . Conservation of mass false in chemical reactions; conservation of energy is true. Teach them early about mcc and delta mcc. It helps make bonding explainable without using springs. An O=O molecule has less mass than two widely separated O atoms. If you do work on the molecule slightly greater than the mass difference (x c^2), you "break the bonds" and get two atoms. The strange implication is that the O=O molecule does NOT contain two O atoms, but it is formed from two O atoms (with the release of energy as mcc decreases), and we can extract (with a little work) two O atoms. How do we know that it doesn't have two O atoms? Because an O=O molecule will not react with other atoms in the same way that two separate O atoms will.

Another example: a deuteron is formed from the combination of a proton and a neutron, but the deuteron IS NOT does not contain a proton and a neutron. How do we know? Because 1) it doesn't have enough mass and 2) a neutron will decay into a proton, electron, and anti-neutrino but the deuteron is stable. Yeah, we have these integers Z, N, and A, but those reflect the numbers of constituents which can be used to form the nuclear lump. So we colloquially say that the O-16 nucleus has 8 protons and 8 neutrons, but it really doesn't, because there is mass missing. There are no distinct particles in the nucleus.

That is a bit of a brain-dump, but I think it summarizes things to this point.

I still need to figure out what the deal is with endothermic reactions. I am
reading some old chem books, but they don't mention or allude to my
questions so far.
http://www.emsb.qc.ca/laurenhill/science/exo_files/image007.jpg)

Thanks again to everyone for their help and insight.

Paul.

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