Re: energy in the tank

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

John Barrer wrote:
> I think we should be VERY careful about referring to
> energy stored in the gas tank. It reinforces the
> faulty notion on the part of many students that
> chemical bonds store energy. This is not true since
> energy is released as bonds form; energy must be added
> to a molecule to break bonds. The oxidation of
> gasoline takes us to a lower potential well and energy
> is released in the reaction, but it was not "stored"
> in the gasoline.

I partly agree that there is a common misconception
here, but only partly.

It depends on what you take as your zero of energy.
You can choose that arbitrarily, and it is possible
to choose it so that the "energy in the tank" statement
is 100% correct.  (Of course you can choose unwisely
and make incorrect statements, but when somebody
speaks of "energy in the tank" we ought to give him
the benefit of the doubt.)

Here's an analogy:  Consider the gravitational
potential energy of a planet in its orbit around
the sun.  It is conventional to take E=0 at
infinity, whereupon the energy of the planet is
negative.  No problem.

Now consider the energy of a heavy book on a high
shelf in the classroom.  It is conventional and
sensible to think that the book has positive
gravitational potential energy.  The unstated
but perfectly reasonable assumption is that
we are taking the floor (or some such) as the
zero of gravitational potential.  The book has
higher GPE than it would if it were on the floor.

But note that under the other convention (the
astrophysics convention) the book, like the planet,
would have a negative GPE.  The book has much
less GPE than it would if it were way out beyond
Pluto somewhere.

So it is with gasoline in the tank.  Sure, the
hydrogens and carbons are bound together in
the gasoline molecule.  Their energy is negative
relative to infinity.  And the O2 molecules in
the carburetor are bound.  But the atoms in the
reactant molecules are much less tightly bound
than they would be in the product molecules, CO2
and H2O.

So if you analyze the fuel molecules (or book) on
a cosmic scale, yes, they negative energy.  But
if you analyze them on the local, relevant scale,
relative to what's about to happen to them, then
they have positive energy.

> I recently saw a paper which reported the results of
> conceptual testing of Purdue (I believe) undergrad
> science majors. A LARGE majority believed that
> chemical bonds store energy.

If the question was worded the way I think it
was worded, the majority was right.

I can even draw for you the energy level as a
function of the "reaction coordinate":

                   ____
                  /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 shelf.