Surely in a reaction that occurs purely in aqueous solution, for instance, you can safely neglect P and V but not Q, ΔU or ΔH? This is the approach my textbook takes.
My bad. I was assuming non reactive gaseous systems.
So the equations I wrote above all still work, for any system? I'll recap:
ΔU=ΔH-pΔV (always)
The identity of Q (Q=amount of heat released) changes depending on the conditions. If the pressure is constant, Q per mole = ΔH. If the volume is constant, Q per mole = ΔU = change in internal energy as well.
If neither pressure nor volume change, then from the definition of ΔH, ΔU=ΔH=Q (amount of heat released = change in internal energy = enthalpy change).
Of course ΔU=ΔH-Δn
gasRT is still true in all cases (as the volume of liquid or aqueous solution in your system cannot change if it all remains liquid or in solution, right?).