Re: [Phys-L] Energy & Bonds

[Bill Nettles <bnettles@uu.edu>]

 -----Original Message-----
> From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of Ron
> Mcdermott
> Sent: Tuesday, November 12, 2013 2:19 PM
> To: Phys-L@phys-l.org
> Subject: Re: [Phys-L] Energy & Bonds
>
> This is one of the more confusing areas in chemistry.  A lot of the confusion
> comes from conventions found in biology and some from positive versus
> negative forces.  I see bits and pieces of the picture have been mentioned
> already...

Snip

> This increase in energy is the
> ACTIVATION ENERGY, and is required to break existing bonds in order for
> new bonds to form.  Breaking bonds ALWAYS absorbs/requires ENERGY.
> Forming bonds ALWAYS releases energy.

Activation energy is an important idea in the nuclear fission process, too. Some energy input is required for most fissioning nuclides (although some will fission spontaneously, c.f. Cf-252). The activation energy is required to separate the proto-fragments enough for the fission process to continue, then the "collapse" into 2 separate fragments which are more tightly bound with less mc^2 and more 1/2mv^2.  For U-235, all you need is the mass-energy difference of (U-235 + neutron) - U-236.  If you have too much energy (an energetic neutron), the proto-fragments fail to separate and the U-236 gives off a photon.  The cross section for this radiative capture increases with increasing neutron KE. Sometimes, more than enough causes process failure.  Pu-239, strangely enough, fissions better with more energetic neutrons.

Instructors should keep in mind that bonded O_2 has less mass than 2 oxygen atoms.  Maybe if biology and chemistry introduced this concept it would be simpler to explain forming and breaking of bonds??  Instructors should spend time up front explaining the MAKING of bonds (decrease of mc^2). Then to break a bond you have to add enough energy to have the separate masses, followed by a rebonding into a different form with less mc^2 and more KE.