We know that ΔH=qp=ΔU+w where w=PextΔV so that means that the enthalpy change is only for constant external pressure processes. So how can we say that in a adiabatic reversible expansion, ΔH=CpΔT? Since the temperature case for that system won't be for a constant pressure system? And is the work in the equation always based on the external pressure, PextΔV? For example if the internal pressure is 10Pa and the external 20Pa would the work we calculate be 20Pa?