[MordorFires]

I'm afraid I don't have a specific homework problem but more of a conceptual question. We just finished going over enthalpy, entropy and gibbs free energy and I'm a bit confused on some of it.

For one, under constant temperature and pressure, dS = dH/T. Also, dS_surr = - dH/T. By this, dS = -dS_surr. And as dS_total = dS + dS_surr, does it follow that at constant temperature and pressure, the entropy of the universe doesn't change as the equation reduces to dS_total = 0?

My other question is similar and refers to the Gibbs Free Energy equation, dG = dH - TdS. As this is under constant pressure and temperature, can't dS be replaced by dH/T, which would effectively reduce the equation to dG = 0?

Naturally, I get the feeling that there's more to it than that. I guess I'm just missing something. I mean, from my perspective, it seems as though entropy and free energy and even enthalpy can be interpreted differently based on these equations...

Oh and last question. Are the elements in their most stable states at standard temperatures simply defined to have a standard gibbs free energy of formation of 0, since that can't be shown by the dG = dH - TdS equation as they have nonzero entropies?

Thank you very much for even reading this. I've found since I started college that chemistry is very interesting when it's taught in a way that's understandable, far different from my high school experience.