Re: [Phys-L] Energy & Bonds

[Jeffrey Schnick <JSchnick@Anselm.Edu>]

> -----Original Message-----
> From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of Paul Lulai
> Sent: Wednesday, November 13, 2013 9:37 AM
> To: Phys-L@Phys-L.org
> Subject: Re: [Phys-L] Energy & Bonds
>
> First, thanks for all of the help.  I am beginning to improve my understanding.
> George, Jeff, and others have helpfully pointed out that binding energy is a
> misnomer.  I am now thinking of binding energy as being similar to a work
> function value.  I think that is a fair analogy.  I am open to being corrected
> there.
>
> I am still having issues with one (at least) concept.  Endo-thermic graphs (like
> this one: http://bit.ly/184Ow3E (full url below))  show the products with
> more energy than the reactants. This makes me wonder a couple of things.
> First, what 'energy' is being plotted on the y-axis? Second, why is the product
> (which for an endothermic process has a higher 'energy' than the reactants)
> the more likely substance?

To me what makes this diagram
http://bit.ly/184Ow3E
confusing is the position of the horizontal axis of the graph.  It suggests that the 0 of energy is below the curve.  I recognize that the energy in question is only defined up to an additive constant, but I see no point in implying a zero of energy that confuses the issue.  The system is bound both before and after so it makes more sense to depict the energy as being negative for both the reactants and the products.  In fact, I would move the horizontal axis to a position above the entire curve, perhaps tangent to the curve at the top of the curve.  Then the whole thing makes sense.  The reactants are strongly bound.  We add some energy to raise the energy of the particles to about zero and then some energy (less energy than the amount originally added) is given off as they drop down to a weakly bound state.