Disagreement on irreversible adiabatic entropy determination often arises from misunderstandings of the Clausius inequality and the nature of entropy changes in irreversible processes. An irreversible adiabatic expansion indeed increases the total entropy of an isolated system or the universe, as stated by the second law of thermodynamics. While it is possible for the entropy change of a system to be negative during an irreversible process, this is offset by a greater positive change in the entropy of the environment, ensuring that the total entropy still increases. The confusion may stem from classroom discussions that label certain processes as non-spontaneous, which can mislead interpretations of entropy changes. Understanding that the total entropy must account for both system and environment clarifies these discrepancies.
