Q:

Enthalpy change in irreversible adiabatic isobaric process

In an irreversible adiabatic isobaric process why is the enthalpy change given as $\Delta H<0$. Any help is appreciated.

A:

In what people call an adiabatic "isobaric" irreversible process, the pressure of the gas is suddenly dropped to a lower value at time zero, and then held at that value for the remainder of the process.  But this is not strictly isobaric because of the sudden pressure change at time zero.  So Q does not have to equal $\Delta H$.  The temperature decreases and, for an ideal gas, this means that its enthalpy decreases.  

