[ELI5] Why is enthalpy included in Gibbs free energy equation?

Question

G=H-TS

Why is enthalpy included in this equation? Like, what’s so special about enthalpy?
To my understanding enthalpy equates to the “heat content” under atmospheric pressure(constant pressure) condition. And heat content is just a form of energy accodring to the equation U=q+w. So enthlapy itself is literally just a form of energy (energy in the form of heat).

So why has only “enthalpy(H)” been incorporated into the gibbs free equation? Like why only heat and not both hear and work? If we want to incorporate energy that comes from heat(enthalpy) shouldn’t we also have to incorporate energy that comes from doing work into the equation as well? Since q+w=Energy and work is another form of energy? Is it okay to neglect energy that’s coming from work here? If so, why?

Just as a note, this question is coming from general chemistry 2 from my university course. I won’t be able to understand any advanced physics concept 🥲

In: Chemistry

Answer 1

H = U + pV, not heat (or q).

So enthalpy *does* actually include both non- pressure*Volume work and heat, and then has an *additional* term that captures energy from pressure and volume. So we aren’t neglecting w at all, it’s bundled up into H along with q and pV.

Where things get a but confusing is in the details. Enthalpy *is* strictly equal to q in situations where there is no work being done and pressure is constant. As you can imagine, there are convenient experiments one can devise that make this true, which is why you see tables and tables of “heat of formation” values for different reactions labeled as enthalpies. They are values of enthalpy, just under a particular set of conditions. If you ran the reaction under different conditions, you’d get a different number, and it would 1.) no longer be just a “heat of formation” because you’d have to account for the work being done by/on the system and 2.) be significantly less convenient to measure.

So TLDR; enthalpy isn’t *just* heat, except under specific conditions, so we aren’t actually neglecting any energies unless they would be zero anyway.