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Re: energy in the tank



OK. Let's try this. I'd like to try to describe the process from
photosynthesis to fossil fuel to internal combustion engine with a
chemically accurate account of the energetic changes involved in the
relevant reactions. Please amend the following if you think its not 100%
right.

Here goes:

During photosynthesis, a series of reactions occurs during which bonds are
broken in molecules of water, CO2, and a 5-carbon sugar-bisphosphate. New
bonds are then formed, rearranging the atoms involved as two molecules of a
3-carbon sugar-phosphate and molecular oxygen. This is an endergonic
process. The energy required to break the bonds in the reactants comes from
sunlight. The energy released when the bonds in the products are formed is
less than what was required to break the bonds in the reactants, so the
increased energy of the products (delta G, if you wish) comes from the
sunlight that drives the whole process.

Some of the reduced carbon compounds formed by plants become fossil fuels. I
won't go into the details of that process here. (someone else can add them
if they wish.) When refined fossil hydrocarbons such as gasoline are
oxidized in a car's engine, C-C and C-H bonds are broken, and O-O bonds in
molecular oxygen are also broken. This requires a net input of energy, but
more energy is released when the bonds in the products form (ideally mostly
CO2 and water, but unfortunately, also a lot of partially oxidized crud).
Hence, the combustion is exergonic. The activation energy required to break
the bonds in the reactants comes initially from the spark of the car's
ignition system, and once the combustion gets going it becomes
self-propagating, with the energy released as products are formed activating
additional reactant molecules. Much of the energy released drives the PV
work of moving the pistons.

OK. How's that?

Jon Greenberg

----------
From: John S. Denker
Reply To: phys-l@lists.nau.edu: Forum for Physics Educators
Sent: Wednesday, February 6, 2002 8:57 AM
To: PHYS-L@lists.nau.edu
Subject: Re: energy in the tank

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.