All the concepts in classical thermodynamics are defined for systems in thermodynamic equilibrium, i.e., for systems where there are no changes. If the rate of change is very small, however, the concepts of classical thermodynamics can still be applied. A process that changes very slowly is referred to as reversible, as its direction is allowed to reverse itself at any moment.
Because the change from initial state to final state is so slow, the process can be assumed to be proceeding trough a series of closely spaced quasi-equilibrium states. The entire time evolution of each of the state variables can be obtained from classical thermodynamics.
The vast majority of processes in nature are irreversible and many systems are in states that have not reached equilibrium yet. With suitable care, it is sometimes possible to apply classical thermodynamics to such processes. Irreversible thermodynamics (also known as non-equilibrium thermodynamics) attempts exactly that by suggesting principles which are relaxed toward equilibrium or steady state. Needless to say, a general theory of non-equilibrium thermodynamics is not existing.
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