I'm currently reading section 19-7 in McQuarrie's physical chemistry textbook, but I'm getting a bit hung up on the possibility of reversible processes at constant pressure, since up to this point the only reversible processes treated were isothermal.
I think I understand the whole point of postulating a reversible process. At every point in a reversible process, the system and surroundings are essentially in equilibrium. This implies that the state variables will change smoothly, and so the techniques of integral calculus can be used to calculate the changes in the state functions which result from the reversible process.
Crucially, since state functions are path-independent, the reversible process can act as a bridge between states, and the changes it evokes in a state function will hold, despite reversible processes being non-physical.
In the case of a constant volume process (assuming that the system and surroundings are electrically neutral) there is no work performed, and so the change in internal energy associated with a process will simply be the energy transferred as heat.
Since most processes (e.g. chemical reaction) take place at constant pressure, we also find that the energy transferred as heat will be equal to the change in internal energy minus the work (which reduces to the external pressure times the change in volume for the case of constant pressure).
I'm trying to imagine a small amount of gas expanding and while (quite obviously) it won't affect the pressure of the rest of the universe, it seems possible to imagine that the system's pressure is still infinitesimally greater at every step, expanding the volume of the system while negligibly affecting the external pressure. Now that I've typed this all out, it indeed does seems necessary that this is the case, or else the expansion would never occur, but I'm going to post this in the hopes that any underlying flaws in my thought process will be pointed out.