Re: [Phys-l] entropy +- heat

[John Denker <jsd@av8n.com>]

Ludwik Kowalski wrote:

> The term "excess heat" is often used by CMNS researchers. But I know 
> that for most of them it is nothing more than a convenient synonym for 
> the "unexplained thermal energy."

Ah, it sure would be nice if the misuse of the term were limited to such
a small group....  but alas, the problem is much wider and deeper.

 -- Chemistry books very commonly speak of "heat of formation" when
  they presumably mean enthalpy of formation.

 -- Hess's law is often expressed in terms of conservation of _heat_.
  Yeccchhhh.

 -- Chemical reactions are generally classified as exothermic or
  endothermic.  Better terminology exists, but is not very commonly
  used.

 -- et cetera.


So far, it might sound like I'm just nitpicking the terminology, but
IMHO the issue is much more serious than that ... as the following story
illustrates.

a) In my larval stage, I learned that any given chemical reaction produced
a given amount of heat.  I even had a table giving the heat of reaction
for various interesting cases.  I was particularly interested in the
H2 + 1/2 O2 --> H2O reaction.

b) I knew that the efficiency of any heat engine was limited by the
ratio of two temperatures:  the peak temperature of the working fluid,
and the temperature of the heat sink.

c) Putting those two ideas together, it was straightforward to calculate
an upper bound on the efficiency with which a fuel cell could produce
electricity from O2 and H2.

d) It was reported that the fuel cells in the Apollo service module were
very efficient.  They violated the limit calculated in item (c) by a wide
margin.

I was deeply confused by this.  It didn't figure it out until years later.
And not for lack of trying.

How would *you* explain this to a student?  I tried asking my HS chemistry
and physics teachers:  Does this represent an exception to the laws of
thermodynamics?  Is the service module a perpetual motion machine?  If
thermo doesn't apply to fuel cells, are there really any situations where
it does reliably apply?  How do you know?  Is there perhaps some locally
ultra-high temperature within the fuel cell, so that the efficiency formula
can be salvaged?  But wouldn't a locally high temperature -- right next to
regions of lower temperature -- cause terrible inefficiency all by itself?

I've often said that ideas are primary and fundamental, whereas terminology
is secondary.  Terminology is important insofar as it helps us understand
and communicate the ideas.  In this case, the contrapositive applies:  when
bad terminology leads directly to wrong ideas, we have a real problem, and
it needs to be fixed.

I've also often said that it is usually not worth discussing misconceptions
one by one.  But I make exceptions for misconceptions that are particularly
common and/or particularly destructive.  This includes ``heat of formation''
and ``heat of reaction''.  IMHO students need to be warned about these nasties.

Some useful rules for repairing misconceptions:
 -- Energy is conserved.  Heat is not conserved.
 -- "Hess's law" ===> conservation of energy.
 -- "heat of reaction" ===> enthalpy of reaction
 -- "heat of formation" ===> enthalpy of formation
 -- In equations, H stands for enthalpy.  H does *not* stand for heat.
 -- "Exothermic" ===> exergonic.
 -- "Endothermic" ===> endergonic.

where "===>" means "should be instantly crossed out and replaced by".