We know that for constant pressure thermodynamic processes which only expansion work is possible, dH=dQp. My question is, is it necessary that both work W and Qv be reversible to arrive this relation? What if the heat transfer is irreversible? Similar question for dU=dQv, does the heat transfer need to be reversible? i.e. Besides the work of the system do we also need to consider the reversibility of δQp and δQv in order to determine whether dU=δQv and dH=δQp ? Another question is, do the above relations have anything to do with whether or not the system is an ideal gas? I have heard from a lecture that du≠dqv in Joule's free expansion for non-ideal gas. Consider reversibility and whether it's ideal gas there are four combinations of situations (ideal gas reversible,ideal gas irreversible, non-ideal gas reversible, non-ideal gas irreversible). I get confused with how these factors affect the thermodynamic relations. Is it true that reversible process can only occur in the form of infinitesimal expansion/compression? In addition, in a process is it possible that the work is irreversible but the heat transfer is reversible? Or the work is reversible but the heat transfer is irreversible?