From: Carl Feynman (carlf@atg.com)
Date: Thu Aug 28 1997 - 10:08:35 MDT
At 12:44 PM 8/27/97 -0700, The Low Golden Willow wrote:
>On Aug 27, 11:16am, Carl Feynman wrote:
>
>} Organic stuff is mostly carbon, bonded to other carbons, and to hydrogens.
>} Diamond is carbon bonded to carbon. The carbon atoms in the two cases have
>} the same energy, but the hydrogen that you throw away in going to diamond
>} can be reacted with air to release water and enormous amounts of energy.
>
>First point: there's energy in more than the bonds. Diamond is higher
>energy than graphite on the surface; it's kinetics that prevent diamond
>from decaying into lower forms of carbon. I don't know if these
>magnitudes are comparable to bond energies, but I don't think diamond
>and charcoal can be treated the same.
Rearranging graphite to diamond is a very low-energy operation, 0.69
kcal/g-mole.
>Second point: I don't know if "energy" is entirely the right
>concept. As I remember, reactions are thermodynamically favored if the
>free energy of the reaction is negative:
>dG= dH - T*dS
>G being free energy, H being enthalpy, T being temperature, and S being
>entropy.
T dS is usually about 10% as large as dH at room temp. However, if it would
make you feel better, we can calculate it (with the help of the Handbook of
Chemistry and Physics).
You're forgetting all the water vapor that gets thrown off. The entropy of
gases is immensely higher than that of solids. The reaction
C6H12 + 3 O2 -> 6 C + 6 H2O
has a net entropy *gain* of 49 e.u. because it absorbs three moles of gas
and releases six moles of gas. (I will treat living things as if they were
made out of a typical liquid hydrocarbon, C6H12.) This makes the reaction
that much faster than if no entropy gain were involved.
>I think I can safely say that diamond has a lower entropy than
>coal, meaning dG would be more positive, and less likely to happen.
The difference between diamond and graphite is a smidgenly 0.8 e.u. I don't
think coal can be much higher entropy than graphite.
--CarlF
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