enthalpy is defined as h = u + pv so, dh = du + d (pv) if the process is constant pressure, dh = du + p dv (change in internal energy + boundary work ) during constant pressure process, if heat is added to the system, it increases its internal energy as well as it does the expansion boundary work. so the net heat added will increase the enthalpy of the system. if the process is constant volume, dh = du + v dp (what is this extra term v dp??). during constant volume process, heat added will just raise the internal energy. so what happens to enthalpy ?