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The commonly used thermodynamic variables are generalized so as to take account of irreversible processes across a thin boundary separating a system from its surroundings. This permits one to relate heat transfer and work performance in irreversible processes to those carried out reversibly. The commonly used functions of state have been generalized to include the contribution of irreversible changes to the entropy of the system. These contributions are specified in terms of experimental observables.
J. M. Honig (2011) studied this question.