Question

(a) What is meant by calling a process irreversible? (b) After an irreversible process the system is restored to its original state. What can be said about the condition of the surroundings after the system is restored to its original state? (c) Under what conditions will the condensation of a liquid be an irreversible process?

Short answer

(a) An irreversible process is a process that cannot spontaneously return to its initial state, requiring external intervention or work to restore the system. (b) After an irreversible process is restored, the surroundings will be in a different state than before, as it is impossible to return them to their original state without additional intervention or work. (c) Condensation can be irreversible if the process occurs at a non-infinitely slow rate (quasi-static), heat is released and not recoverable, or dissipative forces like friction or viscosity are present, leading to energy dissipation that cannot be recovered by simply reversing the process.

Step-by-step solution

(a) Definition of an irreversible process.

An irreversible process refers to a process that cannot spontaneously return to its initial state. In order for the system to go back to its original state after an irreversible process, an external intervention or work must be performed, making it impossible for the process to go back to its initial state by merely reversing the direction.

(b) Surroundings after an irreversible process is restored to its original state.

If a system has been restored to its original state after an irreversible process, it means that some work or energy must have been transferred between the system and its surroundings. Since the process was irreversible, it is impossible to return the surroundings to their original state without additional intervention or work. Therefore, the surroundings will be in a different state than before the irreversible process began.

(c) Conditions for irreversible condensation.

The condensation of a liquid can be considered as an irreversible process under certain conditions, such as: 1. The process occurs at a rate that is not infinitely slow (quasi-static). In case of a fast condensation, the process will not have time to achieve equilibrium at every stage, making the process irreversible. 2. If during condensation, heat is released into the surroundings, and there is no method to completely recover that heat and return it back to the system. In such a scenario, the process would be considered irreversible as the system and surroundings cannot be restored to their original states without additional intervention. 3. The presence of dissipative forces, such as friction or viscosity, can also cause a process like condensation to be irreversible, as they lead to the dissipation of energy that cannot be recovered by simply reversing the process.

Key concepts

Thermodynamics

Thermodynamics is a branch of physics that studies the relationships between heat, work, temperature, and energy. It delves into the principles governing energy transfer and transformation. The core idea of thermodynamics is to understand how energy moves within a system and between the system and its surroundings.

• There are two types of thermodynamic processes: reversible and irreversible.
• Reversible processes are idealized scenarios where the system can return to its original state without any external influence, like frictionless sliding.
• In contrast, in irreversible processes, once energy has been used or transformed, extra work or energy sources are needed to revert the system to its initial state. This concept is crucial for understanding how energy is oftentimes lost to the surroundings in real-world scenarios.

Thermodynamics provides the framework to analyze such situations and predict the behavior of systems when they undergo changes.

System and Surroundings

In thermodynamics, understanding the concepts of the system and surroundings is essential. A 'system' refers to the portion of the universe that is under observation. Everything outside this specified system is known as the 'surroundings'. Together, the system and its surroundings make up the entire universe for thermodynamic analysis.

• Energy transfer takes place between the system and its surroundings through processes such as heat transfer and work done.
• Reversible and irreversible processes alter the way energy is interchanged between these two definitions.
• When an irreversible process occurs, the total change in the energy state may not be completely regained in both the system and surroundings.

This concept is fundamental in thermodynamics because it helps to analyze the flow of energy and predict changes in both the system and the environment around it during thermodynamic processes.

Equilibrium

Equilibrium in thermodynamics is a state where there are no unbalanced potentials or driving forces within a system. This means the system and its surroundings have reached a state of balance without further energy transfer occurring. Achieving equilibrium can take various forms in different systems.

• For a process to be in equilibrium, there should be no net change in the distribution of energy or matter within the system or with its surroundings.
• In a reversible process, the system moves through a series of equilibrium states, allowing it to return to its original condition with ease.
• An irreversible process, on the other hand, occurs too quickly for equilibrium to be maintained through the steps of the process, meaning energy is often lost, not allowing the process to spontaneously reverse.

Understanding equilibrium is crucial when analyzing processes in thermodynamics as it allows for determining if a process can naturally revert back to its initial state or if intervention is required.

Energy Dissipation

Energy dissipation refers to the process by which energy is transformed from one form to another, often into a less useful form like heat, which becomes challenging to recover and utilize for work. In the context of irreversible processes, energy dissipation plays a critical role:

• Dissipative forces like friction, viscosity, and turbulence contribute significantly to energy loss, making the recovery of energy impossible or inefficient.
• For instance, during condensation, if heat is released to the surroundings and cannot be fully recovered, the process becomes irreversible.
• This energy, once dissipated as heat, integrates into the surroundings, altering its state and thus affecting subsequent processes.

Recognizing how energy dissipation occurs helps to understand why some processes cannot be reversed without additional energy input, highlighting the natural tendency towards increased entropy as predicted by the second law of thermodynamics.