A process is said to be irreversible if it cannot reach back to its original state without using external work.

Whenever a process occurs and a substance changes its state from A to B, some of the energy is lost in the universe which cannot be regained. As a result, to bring it back to its original state, some external energy is needed.

In reality, all the processes are irreversible

Types and causes:

Irreversibility is classified according their causes:

i) External: The irreversibility caused by external physical factors like friction, resistance, viscosity, surface tension, finite temperature difference, etc.

The energy lost due to friction can never be regained. Hence, the direction of the process cannot be reversed without supplying external work.

ii) Internal: Irreversibility caused by properties of the working fluid in a process like throttling or free expansion.

When a gas expands, it uses its internal energy to do so. It cannot contract on its own and reverse the process.

iii) Chemical: Irreversibility caused by internal chemical properties like structure, bonds, etc.

When a chemical reaction occurs in association with absorption or liberation of heat, it cannot reverse spontaneously.