# thermodynamics and stability

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.

Questions:

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

EDIT

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.

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