Write a short note on reversible and irreversible processes.

Explanation

Imagine some process in which thermodynamic system goes from an initial state i to a final state f. During the process the system absorbs heat Q from the surroundings and performs work W on it.
Now if this process reverses and bring both the system and surroundings to their initial state with no other effect anywhere ? For most processes in nature this is not possible.
The spontaneous processes of nature are irreversible.
Examples for this : (1) The base of a vessel on an oven is hotter than its other parts. When the vessel is removed, heat is transferred from the base to the other parts, bringing the vessel to a uniform temperature. So vessel cools to the temperature of surroundings.
The process cannot be reversed, means a part of the vessel will not get cooler spontaneously and warm up the base. It will violate the Second Law of Thermodynamics, if it did.
(2) The combustion reaction of a mixture of petrol and air ignited by a spark cannot be reversed.
(3) Cooking gas leaking from a gas cylinder in the kitchen diffuses to the entire room. The diffusion process will not reverse and bring the gas back to the cylinder.
(4) Rusting of iron
(5) Erosion of rocks
(6) Ageing of all animals
(7) Flow of heat
Irreversibility is a nature rule. It arises mainly from two causes :
(i) Many processes like a free expansion or an explosive chemical reaction take the system to non-equilibrium states.
(ii) In most processes like friction, viscosity and other dissipative effects are present and can be minimized but not fully eliminated, because all these processes are irreversible.
Reversible Processes : A process that can be retraced back in the opposite direction in such a way that the system passes through the same states as in the direct process, and finally the system and surroundings return to their original states, with no other change anywhere else is called a reversible process.
Reversible process is an ideal process, it is the basic principle of thermodynamics. A completely reversible process cannot be realized in practice but under certain conditions somewhat reversible process can be done.
For reversible process following conditions should be necessary :
(1) Process should be quasi-static process. Means the system can be considered to be passing through equilibrium states during every stage. In pressure and in temperature have no much difference.
(2) The process must be very slow.
(3) During this process dissipative effects like friction, viscosity or resistive forces which are responsible for energy loss can be minimized.