Reversible Irreversible Processes - Definition, Example, FAQs

In thermodynamics, processes can be classified into two types: reversible and irreversible. A reversible process is an idealized scenario where a system undergoes changes in such a way that it can return to its original state without leaving any net change in the surroundings. This process happens infinitely slowly, allowing the system to stay in equilibrium at every stage. In contrast, an irreversible process occurs spontaneously, where the system and surroundings cannot revert to the original state, typically due to factors like friction, heat loss, or rapid changes.

In real life, most processes are irreversible, such as burning fuel in a car engine, where energy dissipates as heat, and cannot be fully recovered. However, reversible processes are more theoretical, often used to understand efficiency limits, like in the case of idealized Carnot engines. Understanding the difference between these processes helps in designing more efficient systems in engineering and thermodynamics.

In this section, we will study in detail the two very important thermodynamical processes – Reversible and irreversible processes in thermodynamics, reversible process example, irreversible process example

Anything that can get back to its previous position, that is its original/initial state.

In Hindi reversible means prativartee .

What is a Reversible Process in Thermodynamics?

In thermodynamics a process is called a reversible process if it can be reversed in order to obtain the initial state of a system, that is this thermodynamic process can be reversed. This is the condition of reversibility.

The reversible process is being carried out infinitesimally slowly, this means the reversible process takes infinite time to complete.

Work obtained in this process is maximum because of the negligible amount of heat loss.

It is in an equilibrium position at all stages of the process.

Entropy Change for Reversible Process

The entropy of the universe always increases during spontaneous changes.

During reversible changes, the entropy of the system may change but that of the universe stays constant.

It means that spontaneous changes are always irreversible.

During reversible adiabatic changes, the entropy of the system is zero. These are some features of the reversible process.

Examples of Reversible Processes in Thermodynamics are

1. Melting of the ice
2. Boiling of the water
3. Freezing of the water
4. Electric current flows through zero resistance. And many more.
   Hence, these are some reversible cycles in daily life.

Irreversible Meaning ( in Hindi also )

Anything which cannot get back to its previous position that is its original/initial state.

In Hindi irreversible means achal.

Define Irreversible Process in Thermodynamics.

In thermodynamics, a process is called irreversible if it cannot be reversed in order to obtain the initial state of a system, that is it cannot be reversed.

The irreversible process is being carried out rapidly, which means it takes a finite time for completion. This becomes the cause of irreversibility/causes of irreversibility.

In this process work obtained is not maximum. There is a loss of heat in an irreversible process.

Entropy Change for Irreversible Process

If the process is reversible then the total entropy of an isolated system always increases. The change in the entropy of the universe must be greater than 0 for an irreversible process.

Examples of Irreversible Processes in Thermodynamics are

Some examples of irreversible changes are,

1. Cooking of a food
2. Rusting of iron
3. Tearing of any object
4. Friction movements and many more.
   Hence, these are some irreversible cycles in daily life.

Difference Between Reversible and Irreversible Processes

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Solved Examples Based on Reversible and Irreversible Process

Example 1: Which statement is true for a reversible thermodynamic process

1) The internal structure does not change due to chemical reaction

2) is the P-V diagram of a reversible process

3) No permanent change left in any of the bodies taking part in the process

4) All of the above

Solution:

Reversible Process

A process can be reversed in such a way that all changes occurring in the direct process are exactly repeated in the opposite order and in an inverse sense.

wherein

No permanent change is left in any of the bodies taking part in the process or in the surroundings.

The system should remain in chemical equilibrium

Example 2: Which of the following conditions is true for a process to be reversible

1) complete absence of dissipative force

2) The process should be infinitely slowing

3) The system should remain in thermal equilibrium

4) all of the above

Solution:

Condition of a reversible process

1) Complete absence of dissipative force.

2) The process should be infinitely slow.

3) The temperature of the system must not differ appreciably from the surroundings.

wherein

No process is reversible in the true sense.

e.g. extremely slow contraction of spring.

No dissipative forces should be present

All parts of the system and the surroundings should remain at the same temperature

Hence, the answer is the option (4).

Example 3: Which of the following is an example of an irreversible process

1)the flow of current through a conductor

2) the free expansion of gas

3) decay of organic matter

4) all of the above

Solution:

Irreversible process

Any process that is not reversible is an irreversible process.

e.g. all practical processes.

When a current flows through a conductor, some heat is produced.

Hence, the answer is the option (4).

Example 4: If one mole of an ideal gas at ( P₁, V₁) is allowed to expand reversibly and isothermally (A to B ), its pressure is reduced to one-half of the original pressure (see figure). This is followed by a constant volume cooling till its pressure is reduced to one-fourth of the initial value $(B \rightarrow C)$. Then it is restored to its initial state by a reversible adiabatic compression (C to A). The net work done by the gas is equal to :

1) $-\frac{R T}{2(\gamma-1)}$
2) 0
3) $R T \ln 2$
4) $R T\left(\ln 2-\frac{1}{2(\gamma-1)}\right)$

Solution:

$\mathrm{AB} \rightarrow$ Isothermal process

$
\mathrm{W}_{\mathrm{AB}}=\mathrm{nRT} \ln 2=\mathrm{RT} \ln 2
$

$\mathrm{BC} \rightarrow$ Isochoric process
Work done in the Isochoric process-

$
\Delta W=P \Delta V \quad \text { as } \Delta V=0
$
So $\Delta W=0$

$
\mathrm{W}_{\mathrm{BC}}=0
$

$\mathrm{CA} \rightarrow$ Adiabatic process
Work done in adiabatic process

$
\begin{aligned}
& W=\int_{V_i}^{V_f} P d V=\frac{\left[P_i V_i-P_f V_f\right]}{(\gamma-1)}=\frac{n R\left(T_i-T_f\right)}{(\gamma-1)} \\
& \mathrm{W}_{\mathrm{CA}}=\frac{\mathrm{P}_1 \mathrm{~V}_1-\left(\mathrm{P}_1 / 4\right) \times 2 \mathrm{~V}_1}{(1-\gamma)}=\frac{\mathrm{P}_1 \mathrm{~V}_1}{2(1-\gamma)}=\frac{\mathrm{RT}}{2(1-\gamma)} \\
& \mathrm{W}_{\mathrm{ABCA}}=\mathrm{RT} \ln 2+\frac{\mathrm{RT}}{2(1-\gamma)}=\mathrm{RT}\left[\ln 2-\frac{1}{2(\gamma-1)}\right]
\end{aligned}
$

Hence, the answer is the option (4).

Summary

A reversible process occurs when a system switches state in such a manner that there is a possibility of reversing the switch with no effect on either the system or its environment. Such processes are characterized by systems being at thermodynamic equilibrium all through them and the fact that transitions between states are made at infinitely low speeds to support stability. On the other hand, an irreversible process is where the system and its surroundings go through changes which cannot be completely undone; spontaneous typically such processes that occur with finite rates result in deviations from thermodynamic equilibrium.