1. The problem statement, all variables and given/known data I am trying to understand the implications of the principle that the maximum non-expansion work, dwadd is equal to the Gibbs energy change for a reversible process. 2. Relevant equations For a reversible process, dwadd = dG. This is provided that the process takes place at constant pressure and temperature. 3. The attempt at a solution The source of my confusion is to do with the fact that if the Gibbs energy change of a system, dG, is zero at constant temperature and pressure, then that system is at equilibrium. When a system undergoes a reversible change, it passes through a series of equilibrium states before attaining its final state. Hence, for a reversible process, dG = 0 But, dG = dw add So that seems to imply that for any reversible process, the non-expansion work available is always zero, which doesn't seem to be true. Would greatly appreciate any clarifications. Thanks in advance.