Re: [Phys-l] internal/external conservative/nonconservative forces!?!?

["John Clement" <clement@hal-pc.org>]

You get energy by forming bonds, and it takes energy to stretch out bonds,
so if you get energy by forming a bond the bond does not "contain" energy.
If you associate energy with the system, then students are less likely to
think that bonds "store energy".  They are less likely to think that
breaking bonds gives you energy.

And the forces are all electromagnetic in chemistry, but how do you transfer
the energy to the E_k of the individual molecules??  That is something I
have never thought about with respect to forces, but using energy
considerations you know it happens.

Getting energy out of the system is a multistep process where you first
break bonds, and then make new bonds.  So there is an energy barrier to
overcome.

The system concept is valuable because it promotes better thinking about
bonds.  It simply does not make sense to say something is in a container
ready to be released when in reality you are taking it (energy) out when
making the container (bond).  Just try asking students whether energy is
released or absorbed when a bond is broken, and I think you may be surprised
at the answer.

John M. Clement
Houston, TX


> If this energy shows up as KE of the byproducts, then I think a student
> could legitimately ask "What force accelerated these byproducts to these
> high speeds?" I don't see how associating energy with the system versus
> the bonds helps the understanding of the chemistry. Simply reciting a
> scenario where PE decreases and KE increases does not produce
> understanding at a basic level - it's simply substituting a model for the
> dynamics. (This is why I became frustrated with Chemistry as an undergrad
> - too much scenario in the introductory courses.)
>
> Bob at PC
>
> > -----Original Message-----
> > From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> > bounces@carnot.physics.buffalo.edu] On Behalf Of John Clement
> > Sent: Wednesday, December 15, 2010 2:27 PM
> > To: 'Forum for Physics Educators'
> > Subject: Re: [Phys-l] internal/external conservative/nonconservative
> > forces!?!?
> >
> >
> > My example of bonds is very important to understanding chemistry.
> > Because
> > students will tell you there is energy in bonds and when you break them
> > you
> > get energy.  So if you dissociate gasoline the student will tell you
> > that
> > releases energy.  The student will tell you the same thing about the
> > Krebs
> > cycle.  This misconception blocks understanding of chemical and
> > physical
> > reactions.
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