leaving
For reversible processes, this restoration is
made without leaving any net change on the
surroundings, whereas for irreversible
processes, the surroundings usually do some
work on the system and therefore does not
return to their original state.
The second law of thermodynamics states
that no heat engine can have an efficiency
of 100 percent. Reversible processes can be
viewed as theoretical limits for the
corresponding irreversible ones.