According to first law of thermodynamics ∆Q=∆U+∆W An isolated system is adiabatic. This means ∆Q=0. The first law in this case yields 0=∆U+∆W⇒∆W=−∆U . . . (i) For expansion, ∆W is positive and hence ∆U is negative. This means Tf is less than Ti in both the cases. For the same expansion of volume, the work done in irreversible process is greater than that in reversible one because the system has to work against friction etc. Thus ∆Wirreversible >∆Wreversible ⇒−∆Uirreversible >−∆Ureversible [from equation (i)] ⇒∆Uirreversible <∆Ureversible ⇒∆Tirreversible <∆Treversible ⇒Tf irreversible >Tf reversible