Why does the energy difference of a reversible process not equal that of an irreversible process in an adiabatic expansion? [duplicate]

This question already has an answer here:

Suppose that one mole of an ideal gas at $P_1$ and $V_1$ adiabatically expands to $P_2$ and $V_2$, reversibly and irreversibly (two separate processes). Since $\Delta U$ is a state function, why is it wrong to say that $\Delta U_{rev} = \Delta U_{irrev}$? (Since in an adiabatic process, there is no heat transfer, we know that $\Delta U = w$)?

marked as duplicate by Jan, bon, Todd Minehardt, Wildcat, M.A.R.Oct 15 '15 at 19:22

• I can't answer this question but I still want to nitpick (how typical of me). $U$ is the state function, not $\Delta U$. – orthocresol Oct 15 '15 at 18:10
• Both $U$ and $\Delta U$ are state functions. – notorious Oct 15 '15 at 18:11
• A state function is a property of a system. $U$ is a property of a system, and $\Delta U$ is, loosely speaking, a "property" of a process that said system undergoes. – orthocresol Oct 15 '15 at 18:13