My teacher was teaching me that how work done in isothermal reversible expansion is greater than irreversible expansion and also work done in isothermal irreversible compression is greater than that for reversible compression. He then said if someone tries to go from 1st state to the 2nd step ( let it be expansion ) and then come back to the first state ( compression) through isothermal irreversible process for both 1st to 2nd and 2nd to 1st, he will not be able to get back to the first state. The reason he to told that work done in both the cases were different and so would be the final state ( i.e. the first state ). Please explain.