So recently I decided to go back and attempt to make sense of enthalpy (intuitively) as a concept, something which I'd always used mathematically but never bothered to fully understand, to my great regret. I was hoping someone could double-check my logic and reasoning to make sure I have this all together and to answer some questions.
From what I've gathered, enthalpy is an arbitrary definition which under certain conditions allows us to observe how much heat (q) is either released or absorbed by any particular chemical reaction.
Enthalpy (H) is defined H = U + PV. Because it is composed of state functions, enthalpy must also be a state function, therefore ΔH = ΔU + ΔPV. By employing the definition of change in internal energy and constant pressure conditions, we can reduce this to show that ΔH = qp. Because enthalpy remains a state function, this shows us that under conditions of constant pressure, how much heat is absorbed or released by a chemical reaction will always be the same. Meaning, when pressure is constant, enthalpy is simply heat content.
My issue is this:
1) Does Enthalpy, as a concept, have any value in situations where pressure is not being held constant? Under these conditions heat (q) will not be a state function, meaning that it will be path-dependent for any chemical reaction.
2) Why is work not considered a state function? The difficulty I'm having with this is that when I first saw enthalpy defined as H = U + PV it was explained that because all the variables contained in the definition were state functions that enthalpy would also be a state function. Intuitively, this makes sense; a state function will always have the same values if returned froma final to an initial state, therefore mathematically enthalpy will have the same values between any two states. However, work is composed of both pressure and volume, both state functions, hence shouldn't it also be a state function?
3) Under conditions of constant pressure, is work also considered a state function? Under these conditions internal energy will still be ΔU = qp + w. Because internal energy and heat are both state functions, mathematically the only way to keep this consistent is for work to also be a state function.
Thank you to everyone in advance and sorry for the long post.