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

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 $\Delta U = w$)?

• – user7951
Oct 15 '15 at 18:09
• I can't answer this question but I still want to nitpick (how typical of me). $U$ is the state function, not $\Delta U$. Oct 15 '15 at 18:10
• Both $U$ and $\Delta U$ are state functions. 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. Oct 15 '15 at 18:13
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– Jan
Oct 15 '15 at 18:15