Reversibility, Irreversibility and Carnot cycle

The second law of thermodynamics distinguishes between reversible and irreversible processes.

• If a process can proceed in either direction without violating the second law of thermodynamics, it is reversible process. A reversible process is carried out infinitely slowly with an infinitesimal gradient, so that every state passed through by the system is an equilibrium state. So, a reversible process is a quasi-static process which can proceed in either direction.
  
  
• Given a process, if the attempt to reverse its direction leads to a violation of the second law of thermodynamics, then the given process is irreversible.

Any natural process carried out with a finite gradient is an irreversible process . A reversible process which consists of a succession of equilibrium states, is an idealized hypothetical process, approached only as a limit. It is said to be an asymptote to reality, All spontaneous processes are irreversible.