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Re: [Phys-l] Bonds, energy; formerly: More Stupidity in state exams



On 08/10/2011 10:34 AM, chuck britton wrote:
as one with a touch of physics training, I'm quite comfortable with
hearing someone say that 'Energy is Released' when a heavy nucleus
such as U or Pu is 'split'. These nuclei have a 'High Energy' content.

I'm less comfortable when I hear folks say that when the 'High
Energy' ATP molecule is split, the energy is released. This seems to
be a VERY common statement in the Bio world.

Am I being too picky??

Splitting an H2 molecule doesn't release energy.
Letting it recombine with a similarly split O2 molecule DOES release
a bit of energy.

The two scenarios -- nuclear reactions and chemical reactions -- are
closely parallel, as you might imagine.

In both cases, you need to look at the reactants *and* the products.
In particular, if you want to obtain energy by splitting uranium,
it very much depends on how you split it, i.e. it depends on what
products you are going to produce. If you start with 235U and try
to split it into 235 individual nucleons, that does not liberate
energy; in fact it is very strongly disfavored. Similarly if you
start with 235U and try to skive off a single nucleon, resulting in
one product of mass 1 and one product of mass 234, that is also
energetically disfavored.

On the other hand, if you start with 235U (or more likely 236U) and
let it fission into two products of mass in the low hundreds, that
is definitely exergonic. The binding energy /of the products/ is
crucial.

So ... in a nuclear reaction, just as in a chemical reaction, it makes
sense to say that a reactant has high energy /relative to the products/
... and the binding energy of the products is crucial.

We say that 238U is metastable against spontaneous fission, because
although spontaneous fission is allowable, energetically and otherwise,
it is very unlikely. The analogy to chemistry holds here also: there
are plenty of molecules that are metastable against decomposition.
Familiar examples include hydrogen peroxide, TNT, RDX, nitroglycerin,
TATP, et cetera.