[SGVLUG] [OT] "Gibbs" and ??? (was: Help spread the word)

[Dustin Laurence]

Good thing we're not wandering off-topic here. :-)

On Sat, Jun 23, 2007 at 02:33:34AM -0700, David Lawyer wrote:
> 
> Upon looking it up G = H - TS where H is enthalpy,

Which is the same as what I said, as the enthalpy is E + pV.

> entropy.  G is of course the Gibbs state variable.  By finding
> delta-G for const. temp. and pressure, we can determine whether or not
> a certain chemical reaction will proceed spontaneously.

No.  You can determine for sure it will *not* go if the free energy
isn't there, but available free energy does not determine it *will* go.
It might be kinetically forbidden by too great a potential barrier
(activation energy is the chemical jargon I believe).  A famous extreme
example is diamond, which is thermodynamically unstable (free energy is
available to transform into, say, graphite) at ordinary temperatures and
pressures (diamonds are *not* "forever") but is kinetically stable
"enough" (the reaction rate is mind-bogglingly slow).

http://www.chm.bris.ac.uk/webprojects2002/mjames/carbon.html

offers some pretty pictures.  This is (one reason) why it takes great
heat and pressure to produce diamond; you have to get onto a part of the
phase diagram where diamond is the ground state and the kinetics are
favorable enough for the reaction to complete in however much time is
available (less in the laboratory than in nature, one assumes).

An even more radical example from nuclear chemistry: if thermodynamics
were the whole story the ordinary matter in the universe would be
entirely nickel-62 or iron-56 (depends on conditions, but nickel-62 is,
I believe, the lowest-energy nucleus under Earth conditions and probably
iron-56 is at higher temperatures and pressures).  Why doesn't this
happen?  Kinetics.  In conditions where the kinetics is more favorable,
this can in fact happen.

The "cartoon" way to understand all this is to imagine a marble in the
"crater" of a volcano-shaped hill.  Energetically, it should roll down
the hill, as there is energy available (the "free energy" here is
essentially the difference in height between the bottom of the central
crater and the "plain" outside and beyond the "volcano").  But
kinetically, it is forbidden as long as the marble doesn't have the
energy to get over the potential barrier (the "activation energy" is the
difference in height between the bottom of the crater and the top of the
crater rim).  The "reaction" won't go unless conditions give the marble
enough kinetic energy to roll over the rim.

Dustin

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