What is the change in enthalpy for an adiabatic expansion?

Question

I am just wondering if ∆H can be equal to 2∆U for an adiabatic expansion as q=0 in adiabatic process so ∆U would be equal to work done for changing volume and ∆H=∆U+work done in expansion.

Answer 1

First of all, in adiabatic expansion, $\Delta U=-\ work$ (according to your sign convention). Secondly, $\Delta H=\Delta U+\Delta (PV)$, not $\Delta H=\Delta U+\ work$. But, Work = $\Delta (PV)$ only if P is constant (equal to the starting gas pressure), which can't be the case because you have to drop the pressure on the gas to get to expand.