Re: [Phys-L] Energy & Bonds

[John Denker <jsd@av8n.com>]

On 11/13/2013 09:50 AM, Ron Mcdermott wrote:

>  if the delta-G is negative, a reaction will occur
> spontaneously;

That's true *PROVIDED* we restrict attention to conditions 
of constant pressure P and constant positive temperature T.
  http://www.av8n.com/physics/thermo/spontaneous.html#sec-p-t

Biologists get a lot of mileage out of G, because biological
processes are often well approximated as isothermal and
isobaric.  OTOH elevating the ΔG formula to the status of a
fundamental "definition" is quite wrong in principle.  It is
not good enough for chemistry and nowhere near good enough
for physics.
 -- More than a few chemical reactions are carried out in a 
  closed cell (constant E not constant T, and constant V not
  constant P).
 -- Consider for example the physics of ordinary sound.
  Plot the thermodynamic state of the air parcel on the
  usual P versus V diagram.  The trajectory is neither
  constant-P nor constant-V;  it moves along a diagonal,
  along a curve of constant S, a curve of constant PV^γ.
  FWIW this is not constant-T, either.
 -- For that matter, it's hard even to explain why the
  adiabatic exponent γ is also called the ratio of specific
  heats unless you are willing to consider both Cp and Cv,
  i.e. to consider conditions other than constant P, i.e.
  to consider conditions where ΔG does not predict the
  direction of spontaneous reaction.
 -- etc. etc. etc. etc.

> By this definition, entropy is NOT "always the boss". 

Mother nature does not pay attention to your "definitions".

Also note that the entropy that *is* the boss is the overall
entropy Stotal ... which is not the system entropy Ssys that 
appears in equations such as
       Gsys := Hsys - Tsys Ssys
which is often shorthanded as
       G    := H    -   T  S

This point is explained in detail in any reputable book on
thermodynamics, and also here:
   http://www.av8n.com/physics/thermo/spontaneous.html#sec-analysis
as previously mentioned.

> John seems to equate delta-G with 'entropy',

Thou hast got to be kidding.  I never said any such thing.
I said Stotal determined the direction of spontaneous reaction.
Anybody who conflates that with his own utterly erroneous 
notions about ΔG does so at his own risk.

My previous email also stated explicitly that *sometimes* -ΔG 
could be used as a proxy for ΔStotal.

> With respect to John's RDX example; if the formation occurs spontaneously,
> then the products represent the most stable configuration available based
> on the particles available and the conditions. 

That's another leap out of the frying pan into the fire, a 
leap from the mostly-wrong notion that ΔG is the controlling
principle to the completely-wrong notion that "stability" is
the controlling principle.

ΔG=0 is (sometimes) a criterion for thermodynamic /equilibrium/.
Equilibrium is not the same thing as stability.  Anybody who
spent even a femtosecond looking at the references I provided
last time would know that.
  http://www.av8n.com/physics/thermo/spontaneous.html

>  I'll undoubtedly say something ELSE that John will object to,

If you don't like being contradicted, stop saying wrong things.

Other people on this list, when discussing things they don't
understand, manage to post /questions/ rather than posting
emphatic definitions and waving their credentials around.

I'm really not very impressed by credentials in general.
  http://www.av8n.com/physics/authority.htm