Reversible and Irreversible Processes: In the reversible process, the system and surroundings can be restored to the initial state from the final state while in the Irreversible process this cannot be done. Change is the only constant we see around us every day in the world. Ice melts to form water, which evaporates into water vapour, paper burns and becomes smoke and ash, plants and animals grow up, grow old and die etc

In our daily life, we can see these changes and categorize them as reversible and irreversible, for example, water vapour can be condensed to water and frozen back to ice. However, most of these processes such as rusting of iron, paper-burning, growth of plants, etc. Read this article to get more clarity on this.

What are Reversible Processes?

A reversible process is a process in which the system and surroundings can be restored to the initial state from the final state without producing any changes in the thermodynamic properties of the universe (system \(+\) surrounding). Let us consider that the system has undergone a change of state (from \(A\) to \(B\)), as shown in the figure below. Then this process is said to be a reversible process if the system can be restored from state \(B\) to state \(A\), without producing any change in the universe at any moment of time. The reversible process can be reversed completely, and there is no trace left to show that the system had undergone a thermodynamic change.

The important conditions for the reversible process to occur are; first, the process should occur very slowly, and no dissipating forces present (that is, no loss of heat by friction, etc.) and secondly, all of the different states of the system should be in thermodynamic equilibrium with each other.

For example, the above figure shows that a very slow isothermal expansion of an ideal gas is done in a cylinder fitted with a frictionless movable piston. It is a reversible process because the initial and the final states of gas expansion are the same. The reversible process never occurs in actual practice. Thus we can say that it is an ideal or hypothetical process.
Some factors that cause a process to become irreversible are following:

1. Friction
2. Unrestrained expansion and compression
3. Mixing
4. Heat transfer (finite \(\Delta T\))
5. Inelastic deformation
6. Chemical reactions

Types of Reversible Processes

There are two types of reversible process that is given below:

1. Internally Reversible Process: The process is said to be internally reversible if no irreversibilities occur within the boundaries of the system. In these processes, a system undergoes a series of equilibrium states, and when the process reverses, the system passes through the same equilibrium states while returning to its initial state.
2. Externally Reversible Process: In an externally reversible process, no irreversibilities occur outside the system boundaries during the process. For example: If the surface of contact between the system and reservoir is at the same temperature, then heat transfer between a reservoir and a system is an externally reversible process.

Some examples of nearly reversible processes are given following:

1. Frictionless relative motion.
2. Expansion and compression of spring.
3. Polytropic expansion or compression of a gas.
4. Isothermal expansion or compression.
5. Electrolysis.

Quasistatic Process

Quasistatic process means very nearly static process as quasi means almost or near. Let us suppose the weight shown in the above process is made of large numbers of small weights. And one by one, each of these small weights is removed and allowed the system to reach an equilibrium state. Then, we have intermediate equilibrium states, and the path described by these states will not much deviate, as shown in the figure given below. Such a process, the locus of all the intermediate points passed by the system, is known as a quasistatic process. It means this process is almost near to the thermodynamical equilibrium process. The characteristic feature of the quasistatic process is that it is an infinitely slow process.

What are Irreversible Processes?

An irreversible process is a process in which the system and surroundings cannot be restored to their original states once the process is started. It is also called a natural process because this is what happens in nature. Consider an example: when we are driving the car uphill, the car consumes a lot of fuel, and when we are driving down the hill, this fuel can not be returned because the burning of fuel is a combustion process, and it can’t be restored. Hence, this is an irreversible process. Another example of an irreversible process is allowing a certain volume of gas to release into a vacuum and then allowing it to occupy a large space by releasing pressure. During this expansion process, the system and surroundings are not in equilibrium. This phenomenon is called irreversibility.
Since the system will not be in equilibrium at every point during the process, we can not find the states of the system at every point during the process. It is represented by dotted lines.

Irreversibilities are of two types
1. External irreversibilities: In these external irreversibilities, the dissipating effects exist outside the working fluid.
Example: Mechanical friction due to some external source.
2. Internal irreversibilities: In these internal irreversibilities, the dissipating effects exist within the working fluid.

Example: Unrestricted expansion of gas, expansion of a viscous gas, etc
Some more examples of irreversible processes are:

1. Relative motion with friction
2. Combustion
3.  Diffusion
4. expansion
5. Heat transfer
6. Plastic deformation

Summary

A reversible process is a thermodynamic process in which the system and surroundings can be restored to their initial state from the final state without producing any changes in the universe. Examples of reversible processes are boiling of water, electrolysis, expansion, compression of spring, etc.
Quasistatic is an infinitely slow process in which the locus of all the intermediate points passed by the system is in thermodynamic equilibrium. It is a hypothetical concept.
The process is said to be an irreversible process if the system and the surroundings do not return to their original state. Relative motion with friction, combustion, diffusion, etc., are basic examples of irreversible processes.

Reversible and Irreversible Processes– Sample Problems

Q.1. Find the work done for the reversible isothermal process when a container of gas having mass \(m\) changes its states from its initial state \((P_1,\,V_1)\) to the final state \((P_2,\,V_2)\).
Ans: As we know, for an isothermal process, the temperature is fixed, and the ideal equation of gas is given as:
\(PV = mRT =\) constant
i.e., the pressure of a given mass of gas varies inversely as its volume. Suppose an ideal gas goes isothermally (at temperature) from its initial state \((P_1,\,V_1)\) to the final state \((P_2,\,V_2)\).
Now, let consider any intermediate stage with pressure \((P)\) and volume change from \(V\) to \(V + \Delta V\), then we have
\(\Delta W = P \Delta V\)
Taking \((\Delta V \to 0)\) and summing the quantity \(\Delta W\) over the entire process,
\(W = \int\limits_{{V_1}}^{{V_2}} {PdV} \)
\(W = mRT\int\limits_{{V_1}}^{{V_2}} {\frac{{dV}}{V}} = mRT\ln \frac{{{V_2}}}{{{V_1}}}.\)

FAQs on Reversible and Irreversible Processes

The most common doubts on reversible and irreversible processes are addressed here:

Q.1: What is a reversible process, and also give its example? Ans: A thermodynamic process is said to be reversible if the process can be restored. The system and surroundings return to their original states without producing any change in the universe. Isothermal expansion and compression, electrolysis are some examples of reversible processes.

Q.2: What is the difference between reversible and irreversible change? Ans: The changes which can be brought back to their original form are known as reversible changes. For example, melting of ice, expansion, or compression of spring. And irreversible changes are those changes in which the matter cannot be brought back to its original state. For example, burning of fuel, etc.

Q.3: Define the Quasistatic Process? Ans: A process is said to be quasistatic if it will be an infinitely slow process. The locus of all the intermediate points passed by the system is in thermodynamic equilibrium, which means the rate of change is so slow that it seems to be in equilibrium at all times.

Q.4: What do you mean by Expansion? Ans: When the gas escapes into space without doing any work, there is no heat or energy lost in this process. Then, this phenomenon is known as expansion.

Q.5: What is the condition required for a process to be reversible? Ans: A process is reversible if: 1. It is infinitely slow. 2. No dissipating forces occur. 3. Initial and the final states are in the thermodynamic equilibrium with each other.

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