[GollumsLunch]

I'm interested in the spontaneity for the following process:

The isothermal expansion of an ideal gas against a constant pressure.

What is the predictor of spontaneity in this case.  The Gibbs Free energy is the predictor of spontaneity under constant temperature and pressure.  In this case we are expanding against a constant pressure, but the pressure of the system is changing.  Is it the pressure of the system, or Pext that must be constant for deltaG to be the predictor of spontaneity?

thank you for any help

[Yggdrasil]

For any process, the total change in entropy, ΔS_tot = ΔS_sys + ΔS_sur, determines the spontaneity of a process.

If ΔS_tot > 0 the process occurs spontaneously.  If ΔS_tot = 0 the process occurs reversibly.  And, if ΔS_tot < 0, the process will not occur spontaneously.

Under the condition of constant pressure, these arguments reduce to the standard rules about ΔG.

(To answer your second question, for ΔG to be a predictor of spontaneity, I think the pressure of the system must be constant).