Re: energy in the tank

["John S. Denker" <jsd@MONMOUTH.COM>]

Cliff Parker <cparker@CHARTER.NET> wrote:
> > Something
> > was at a higher energy level before the gasoline was
> > burned.  After the gas
> > was burned something was at a lower energy level and
> > energy was released.

Right.  Exactly.

Note Cliff's wise use of the relative terms
lowER and highER, to the exclusion of absolute
terms like positive and negative.

> > Where was that energy "stored" before it was
> > released?

It is common to say that the energy was stored in
the fuel, and I don't see anything particularly
wrong in this.  You can quibble about how we allocate
the energy to fuel as opposed to oxidizer, and
various other nits and gnats, but the basic idea
is OK.

John Barrer wrote:
> The energy released in the bond formation process is
> indeed stored in the system of all the dissociated
> molecules. While it's true that you make this all "go
> away" by the choice of a "suitable" zero.

OK....

> But is the
> introduction of negative energy necessary or desirable
> in a first year (HS or college) course? I think not.

I don't think that talking about "positive" or "negative"
absolute energy is ever desirable.  It's a violation of
gauge invariance.  It's a violation of one of the most
useful principles of classical physics.

Negative means lower than zero.  Whose zero are we
talking about?  You can choose your zero -- that's your
gauge freedom.  But I can choose my zero -- that's my
gauge freedom, and you can't take it away.

As I said before, the picture is:

                   ____
                  /ions\
        _________/      \
        C8H18 + O2       \
                          \
                           \
                            \
                             \
                              \_________
                               CO2 + H2O

which makes it clear that relative to other
things of interest, the reactants have higher
energy.  Just like a book on a high shelf.

Which of these energy levels is positive in absolute
terms, and which is negative?  Who cares?!!!
The laws of classical physics certainly don't care.

When I speak here of "classical" physics I'm
including everything except general relativity.
GR probably has something to say about absolute
energy levels, but whatever it says is about 42
orders of magnitude too small to be relevant to
chemistry.  Nobody is going to do GR experiments
to find out "where" the chemical energy is stored,
and I don't even trust the theory to be valid
when extrapolated that far. In any case, if
you want to do the job right, you need to account
for the mass of the atoms, which includes the
electric field-energy in the chemical bonds.  This
has pretty much got to be positive.  Certainly
there's no harm in taking it to be positive if
you want.

Bottom line:  If somebody says there is positive
energy in the fuel tank, you'd better think
twice before contradicting him.