Question

(a) What is special about a reversible process? (b) Suppose a reversible process is reversed, restoring the system to its original state. What can be said about the surroundings after the process is reversed? (c) Under what circumstances will the vaporization of water to steam be a reversible process? (d) Are any of the processes that occur in the world around us reversible in nature? Explain.

Short answer

A reversible process is an idealized thermodynamic process that occurs infinitesimally slowly, allowing the system and surroundings to remain in equilibrium and having no dissipative effects. When a reversible process is reversed, both the system and the surroundings return to their initial states. The vaporization of water to steam can be considered reversible if it occurs infinitesimally slowly, the pressure exerted by the steam exactly matches the vapor pressure of water, and there is no heat loss to the surroundings. However, most real-world processes are irreversible due to dissipative effects and entropy increase.

Step-by-step solution

(a) Define a reversible process

Reversible processes are idealized thermodynamic processes that occur infinitesimally slowly, allowing the system and surroundings to remain in a state of equilibrium throughout the entire process. These processes are characterized by having no dissipative effects (such as friction or heat loss), and, if reversed, both the system and the surroundings will return to their initial states.

(b) Characteristics of the surroundings after reversing a reversible process

When a reversible process is reversed, it means that all the changes that occurred in the system are undone, and the system returns to its original state. In this case, it can be said that the surroundings also return to their initial state. This is because, in a reversible process, the system and the surroundings have been in equilibrium throughout the entire process, and any changes in the system have had an equal and opposite effect on the surroundings.

(c) Conditions for reversible vaporization of water to steam

For the vaporization of water to steam to be considered a reversible process, it must occur under specific conditions. These conditions are as follows: 1. The process should be carried out infinitesimally slowly, allowing the system (water and steam) and its surroundings to remain in equilibrium throughout the entire process. 2. The pressure exerted by the steam should exactly match the vapor pressure of water at the given temperature. 3. There should be no heat loss to the surroundings, and the entire process should occur under adiabatic conditions. In reality, it is nearly impossible to fulfill these conditions, making the vaporization of water to steam an irreversible process in practice.

(d) Real-world processes and reversibility

In the real world, most processes are irreversible in nature. This is primarily because any process occurring in a finite amount of time will involve dissipative effects such as friction, heat transfer, and mass transfer, which result in an increase in entropy. In a truly reversible process, the entropy of the system must remain constant, and it cannot involve any dissipative effects. Therefore, while the concept of reversible processes is useful for understanding and simplifying various thermodynamic analyses, it is generally not observed in practice.

Key concepts

Thermodynamics

Thermodynamics is fundamentally the study of energy, work, and heat—and how they interconvert and interact with matter in a system. Specifically, it deals with the principles that govern the energy exchanges and transformations within a system and between the system and its environment. Through thermodynamics, we understand that energy cannot be created or destroyed, only transferred or converted (the first law of thermodynamics), and that there's a natural direction in which energy transformations and transfers tend to occur, seeking disorder or randomness (the second law of thermodynamics).

The concept of a reversible process is rooted in thermodynamics and such a process represents the epitome of efficiency—a hypothetical scenario where no energy is wasted as dissipative effects, and complete equilibrium between system and surroundings is maintained. These ideal processes help us to define the boundaries of what is theoretically possible and provide a stark contrast to the irreversible processes, which align with the realities of the physical world.

Equilibrium

Equilibrium in thermodynamics refers to a balanced state where a system experiences no net change over time. This means that, at equilibrium, all opposing processes occur at equal rates and macroscopic properties, such as temperature and pressure, remain constant. There are different types of equilibrium, such as thermal, mechanical, and chemical equilibrium, each highlighting a specific balance of conditions.

In the context of a reversible process, a system achieving thermodynamic equilibrium throughout allows the process to occur in a manner that is perfectly synchronized with its surroundings. As a system approaches this balance, it does so progressively and in infinitesimal steps, facilitating the absolutely reversible nature of the transformation—if and only if such ideal conditions could be met.

Entropy

Entropy is a measure of the level of disorder or randomness in a system. Think of it as the currency of energy dispersal throughout a system and its surroundings. According to the second law of thermodynamics, the total entropy of an isolated system can never decrease over time; it can only increase or remain constant. In real-world scenarios, it's this law that drives processes to be irreversible because natural processes tend to move towards higher entropy.

Connection with Reversible Processes

In a truly reversible process, the entropy of a system doesn't increase, because the process occurs in a manner that allows the system to remain in equilibrium, and entropy changes are exchanged perfectly between the system and surroundings. However, such conditions are idealized and not typically found in nature. Irreversibility is fueled by the natural drive towards entropy increase, making real-life processes deviate from the reversible ideal.

Vaporization of Water

The vaporization of water is the process by which water is converted from its liquid form into vapor or steam. This phase change is an endothermic process, meaning that it absorbs heat from the surroundings. From the standpoint of thermodynamics, when water vaporizes at a temperature and pressure where the liquid and vapor phases are in equilibrium, it does so with an increase in entropy, because the gaseous state is more disordered than the liquid state.

Can Vaporization be Reversible?

For vaporization to be reversible, several stringent conditions must be fulfilled, including the process being adiabatic (no heat exchange with the surroundings), isothermal (constant temperature), and carried out so slowly that the system remains in equilibrium at all times. In practice, achieving these conditions is extremely challenging, hence vaporization, like many other natural processes, is typically irreversible. This helps us appreciate the gap between the idealized models used to teach principles of thermodynamics and the complex, often non-reversible processes witnessed in real-life scenarios.