## Reversible and Irreversible Processes – Class 11 | Chapter – 12 | Physics Short Notes Series PDF for NEET & JEE

Reversible and Irreversible Processes: In thermodynamics, a process is a change in the state of a system from an initial state to a final state. Processes can be classified as reversible or irreversible based on whether they can be reversed by an infinitesimal change in the system’s parameters.

What are Reversible Processes?

Reversible processes are idealized thermodynamic processes that can be reversed by an infinitesimal change in the system or the environment, without any net production of entropy. In other words, a reversible process is one that can be undone by a tiny change in the system or the environment, with no change in the total entropy of the universe.

Properties of Reversible Processes

• Quasistatic: The process proceeds very slowly, so that the system is always close to equilibrium, and the system can be assumed to be in quasi-equilibrium at each instant.
• Infinitesimal changes: The changes in the system are infinitesimally small, so that they can be approximated by a series of infinitely small steps.
• No net production of entropy: The process is carried out without any net production of entropy, which means that the entropy of the system and its surroundings remains constant.

Examples of reversible processes include the Carnot cycle, isothermal expansion of an ideal gas, and adiabatic reversible expansion of an ideal gas. Reversible processes are idealizations and can never be achieved in practice, but they provide useful benchmarks for understanding the limits of thermodynamic efficiency and for designing practical systems that approach the ideal.

What are Irreversible Processes?

Irreversible processes are thermodynamic processes that cannot be reversed by any infinitesimal change in the system or the environment, and which result in a net production of entropy. In other words, an irreversible process is one that proceeds in a single direction, and cannot be undone by a small change in the system or the environment, and which always increases the total entropy of the universe.

Properties of Irreversible Processes

• Non-quasistatic: The process proceeds rapidly or in a non-equilibrium way, so that the system deviates from equilibrium at each instant of the process.
• Finite changes: The changes in the system are finite, and cannot be approximated by a series of infinitesimal steps.
• Net production of entropy:The process is accompanied by a net production of entropy, which means that the entropy of the system and its surroundings increases.

Examples of irreversible processes include combustion, diffusion, heat transfer through a finite temperature difference, and most natural processes. Irreversible processes are prevalent in the real world and are responsible for many phenomena that we observe, such as the degradation of energy quality, the diffusion of solutes, and the flow of fluids through porous media.

Factors Affecting Reversibility

The reversibility of a thermodynamic process is affected by several factors, including:

• Friction:Friction causes energy losses and dissipates heat, resulting in irreversibility. The degree of friction in a system determines the amount of energy that is lost irreversibly.
• Heat transfer: The rate of heat transfer can affect the reversibility of a process. Heat transfer across a finite temperature difference leads to irreversibility, whereas heat transfer across a negligible temperature difference results in reversibility.
• Pressure gradients:Irreversible processes occur when there are non-uniform pressure gradients in the system. Reversible processes occur when the pressure is uniform throughout the system.
• Chemical reactions:Chemical reactions can be irreversible if they proceed to completion, or they can be reversible if they reach an equilibrium state.
• Time constraints: Rapid processes are usually irreversible, while slow processes are more likely to be reversible.
• Finite changes: Irreversible processes result from finite changes in the system, whereas reversible processes result from infinitesimal changes.

In general, the reversibility of a process depends on the level of detail in which the system is described and the level of control that can be exerted over the system. The idealized Carnot cycle is reversible because it is performed under controlled conditions and assumes no friction or heat transfer across finite temperature differences. In reality, all processes are irreversible to some degree, and irreversibility’s result in energy losses and reduced efficiency.

## Examples of Reversible and Irreversible Processes

Examples of reversible processes include:

• A gas expanding or compressing slowly and reversibly, so that the pressure and volume of the gas are always in equilibrium with the surroundings.
• A reversible chemical reaction, in which the products can react to regenerate the reactants with no net change in energy.
• A reversible adiabatic process, in which no heat is exchanged between the system and the surroundings.

Examples of irreversible processes include:

• A gas expanding rapidly and irreversibly, so that its pressure and volume are not in equilibrium with the surroundings.
• A chemical reaction that proceeds irreversibly, with the products unable to react to regenerate the reactants.
• A process in which heat is transferred irreversibly from a hot object to a cold object, resulting in an increase in the entropy of the system.

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