Suppose this three processes, at same T and P, each of them on thermodynamical equilibrium:

atoms -->molecule 1

atoms --> molecule 2

atomsdifferent --> molecule 3

Where atoms are infinitely separated, and may be different. The compositions of molecules 1 and 2 are the same, but molecule 3 has different atoms.

From experiments we can calculate thermodynamic functions. Suppose we have them.


Is there any thermodynamic function that able us to order those compounds in stability? Would you talk about G, about H?


An opinion

Stability is only related to $\Delta G$, if $\Delta G$ all systems are stable. If Δ$\Delta G$ is different from zero, the bigger $\Delta G$, the bigger tendency to move from that conditions (more unstable). $\Delta H$ is not a criterion for stability.

It rests a question: why $\Delta G^0$ is different can't be interpreted in the same sense than $\Delta G$? From experiments we can calculate thermodynamic functions.


The various thermodynamic potentials are appropriate depending on what is held fixed. I wont' go through the derivations because they should be in any thermodynamics book, but here are a few examples:

  • When energy and volume are constant, entropy goes to its maximum.
  • When temperature and volume are constant, the free energy $F = U - TS$ should be minimized.
  • When temperature and pressure are constant, the Gibbs energy $G = U - TS + PV$ should be minimized.

In chemical reactions usually temperature and pressure and held fixed (because that's what happens if you have a reaction open to the environment), so $G$ is the relevant function.

Edit: if you want to compare various systems for stability, simply compute $\Delta G$ for all of them, $\Delta G$ being the difference in Gibbs energy between the two states you're comparing (so a bunch of atoms vs a bunch of molecules). The one with the largest $\Delta G$ is the most stable, because it needs a bigger energy input to go the other way.

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  • $\begingroup$ but what is the criteria for stability? $\endgroup$ – user153036 Sep 2 '17 at 16:06
  • $\begingroup$ I edited to be clearer $\endgroup$ – user153036 Sep 2 '17 at 16:07
  • $\begingroup$ @HernanMiraola like it says in the third bullet point, the system is stable when $G$ is at is minimum. $\endgroup$ – Javier Sep 2 '17 at 16:07
  • $\begingroup$ But I am talking about comparing different and equal systems.. $\endgroup$ – user153036 Sep 2 '17 at 16:08
  • $\begingroup$ If each of those systems are on equilibria all will be zero $\endgroup$ – user153036 Sep 2 '17 at 16:11

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