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# Re: [Phys-L] Energy & Bonds

• From: John Denker <jsd@av8n.com>
• Date: Wed, 13 Nov 2013 08:02:00 -0700

On 11/13/2013 07:36 AM, Paul Lulai wrote:

I still need to figure out what the deal is with endothermic
reactions.

http://www.emsb.qc.ca/laurenhill/science/exo_files/image007.jpg)

That's a good question. Fortunately, a good answer is available:
--> Energy does not determine which way the reaction will
proceed. Entropy does.

This idea applies even to the simplest physics situation. The
simple idea that balls roll downhill is not entirely correct.
In a non-disspative system, the ball rolls down the hill,
keeps going, and rolls up the hill on the other side.

If you want the ball to roll down /and stay down/ you need
some sort of friction, some sort of dissipation, some way of
creating entropy. Entropy is the boss.

We can reconcile this with prior intuition as follows: There
are /some/ situations where minimizing the energy is a useful
proxy for maximizing the entropy. This works sometimes but
not always. In other situations, the appropriate proxy is
not the energy E but rather the Helmholtz free energy F or
the Gibbs free enthaply G.

In the example cited above, you would probably find that G
was going down even as E was going up. And remember that
G is only a proxy for the entropy. Entropy is the boss.

For the next level of detail on this, see
http://www.av8n.com/physics/thermo/fundamental.html#sec-fundamental-spontaneous
and then
http://www.av8n.com/physics/thermo/spontaneous.html