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Re: [Phys-l] entropy +- heat



Bernard Cleyet wrote:

Well?

bc, stressed.

I wasn't trying to stress anybody ... but in retrospect I suppose the
fact that some people didn't immediately see how to solve the puzzle
underlines the fact that nontrivial issues are involved.

Here's the deal:

* 1) In chemistry class we learned Hess's law. We also had tables giving
* the heat of reaction to high accuracy, to parts in 10^4 or better. The
* existence of such tables served as pretty strong evidence in support of
* the law: even if the law weren't exact, it must have been very nearly
* exact, or it would have been both impossible and pointless for anybody
* to construct such tables.

OK so far?

==========================================================================

2) No, it's not OK. In school, we had a saying:
"Chemistry class, where we go to learn stuff that cannot possibly be true."

Just because they tell you that the reaction necessarily produces heat
doesn't mean the reaction necessarily produces heat. Just because Hess's
law tells you heat is conserved doesn't mean heat is conserved.

The big fancy book is probably OK as a tabulation of the /enthalpy/ of
reaction. But enthalpy is not the same as heat.

It may be that the last 100 times you carried out a chemical reaction,
the conditions were such that the enthalpy of reaction was immediately
thermalized ... but this is little more than a statement about the
conditions that you chose, and is nowhere near being a law of nature.

Practically anything you do with electrochemistry (plating, batteries,
fuel cells, etc.) serves as a counterexample to the "heat" of reaction
notion.

If you persist in thinking about the "heat" of reaction, you are at
risk for unending stress. That is why I so emphatically recommended:

*** Don't quantify the heat; quantify the energy and entropy. ***

There are tables of the standard entropy as well as the standard
enthalpy of various compounds.

The hydrogen/oxygen fuel cell is not a perpetual motion machine for the
simple reason that it is not a heat engine. Its efficiency is vastly
greater than what you would get if you thermalized the energy (by simply
burning the hydrogen and oxygen, creating vast amounts of new entropy)
and then used the thermal energy in a heat engine.

Fully understanding this requires a slightly sophisticated notion of
thermal equilibrium, namely the idea that /most/ of the modes of a
system can be in equilibrium, while a few special modes are verrrry
far from equilibrium. Any energy-storage device -- or more generally
any metastable system -- illustrates this point. Examples and more
discussion can be found at
http://www.av8n.com/physics/thermo-laws.htm#sec-metastable-t