Re: thermal energy +- enthalpy

[John Denker <jsd@MONMOUTH.COM>]

At 06:02 PM 3/2/00 -0700, Jim Green wrote:
> > >   entropy is a chemist's portion of the internal energy


At 11:14 PM 3/2/00 -0700, Jim Green wrote:

> I mis-typed -- Didn't someone want to bring up enthalpy in this thread?

1) Do you mean to assert that enthalpy is a chemist's portion of the
internal entropy?????  I don't buy that either.

2) I don't see that bringing in enthalpy was helpful to the
discussion.  But since the issue has been raised.....

The usual definition of enthalpy is
        H = U + pV

a) I don't see what that has to do with chemistry.

b) I don't see why H should be considered a "part" of U.

c) Given the symmetry of the expressions
        H(S,V)! = U(S,V) - (partial U / partial V at constant S) V
and
        U(S,p)! = H(S,p) - (partial H / partial p at constant S) p
I don't see why either H or U should be considered less fundamental than
the other or "part" of the other.

As a warning, I attached exclamation marks to quantities that are not
written in terms of their traditional independent variables.  When people
write H it usually implies that they have chosen S and p as the independent
variables, just as U usually implies that they have chosen S and V as the
independent variables.

Pedagogically speaking, there are lots of reasons why you might want to
switch independent variables from/to (S,V) to/from (S,p).  When you do
that, fudging the potential by +-pV allows you to avoid introducing new
thermodynamic derivatives.  For example if we switch from (S,V) to (S,p)
the quantity
        (partial U / partial p at constant S)
is perfectly well defined and well behaved;  it just doesn't have a
name.  In contrast
        (partial H / partial p at constant S)
is our friend V in its new status as a dependent variable.