Question

In thermodynamics, a process is called reversible when: (a) Surroundings and system change into each other (b) There is no boundary between system and surroundings (c) The surroundings are always in equilibrium with the system (d) The system changes into the surroundings spontaneously

Short answer

The correct option is (c): "The surroundings are always in equilibrium with the system."

Step-by-step solution

Define Reversibility in Thermodynamics

In thermodynamics, a reversible process is defined as a process that can be reversed by an infinitesimal change in a variable without leaving any net change in the system and the surroundings. This typically means that the system remains in thermal, mechanical, and chemical equilibrium with its surroundings throughout the process.

Analyze Option (a)

Option (a) suggests that surroundings and system change into each other. Reversible processes do not involve systems and surroundings changing into one another. Rather, they focus on maintaining equilibrium. Thus, option (a) is incorrect.

Analyze Option (b)

Option (b) suggests no boundary between system and surroundings. In thermodynamics, while a reversible process occurs, clear boundaries often exist. The focus is on equilibrium, not boundary removal. Thus, option (b) is not correct.

Analyze Option (c)

Option (c) indicates that the surroundings are always in equilibrium with the system. This aligns with the definition of reversible processes, as the system and surroundings should be in continual equilibrium throughout the process.

Analyze Option (d)

Option (d) suggests that the system changes into the surroundings spontaneously. Spontaneous changes, where control over the small, incremental reversal of the process is lost, contradict the notion of reversibility. Thus, option (d) is incorrect.

Conclusion and Selection of Correct Option

After analyzing each option and using the definition of a reversible process, we determine that option (c) is correct: "The surroundings are always in equilibrium with the system." This is the fundamental characteristic of reversible processes.

Key concepts

Equilibrium in Thermodynamics

In thermodynamics, equilibrium is the state where all macroscopic flows of properties like matter, energy, and momentum stop exchanging between a system and its surroundings. This balance ensures that the system’s properties remain constant over time, and any change that occurs is microscopic and occurs in a balanced manner.

Imagine having a cup of hot coffee in a room. Initially, the coffee is warmer than the room air, resulting in the coffee losing heat and the room air gaining it. Eventually, both the coffee and the air will reach the same temperature. At this point, they've achieved thermal equilibrium.

Thermodynamic equilibrium involves mechanical, chemical, and thermal equilibrium simultaneously. This means no net exchange occurs in pressure, chemical potential, and temperature, respectively.

• Mechanical Equilibrium: No pressure change within the system and its surroundings.
• Chemical Equilibrium: No chemical reaction occurrence and balanced chemical potential.
• Thermal Equilibrium: Uniform temperature, with no heat transfer.

The concept of equilibrium is pivotal in reversible processes, because these processes navigate small, infinitesimal changes that pass through a sequence of equilibrium states.

Reversible and Irreversible Processes

The distinction between reversible and irreversible processes is central to understanding thermodynamics. A reversible process is idealized and occurs in such a way that the system remains almost in equilibrium with its surroundings throughout the process.

Think of reversible processes like moving a book across a table by applying a feather-light touch. The book doesn’t speed up or slow down dramatically, but instead, moves ever so slightly with each gentle push. This slow-paced change allows you the possibility to reverse the action seamlessly, without any net change occurring in the system and the surroundings.

• Reversible Processes: Operate in infinitesimal steps, always near equilibrium.
• Irreversible Processes: Occur spontaneously, often quickly, and without the ability to return both system and surroundings back without a net change.

In real life, truly reversible processes do not occur due to friction, heat loss, and other inefficiencies. However, they are crucial in theoretical studies where they provide insights into efficiency and energy transformations.

Thermodynamic Systems and Surroundings

Thermodynamic systems and their surroundings form the foundation of studying processes in thermodynamics.

Imagine a system as a defined quantity of matter or a region in space, carefully separated by boundaries from everything else in the universe, known as the surroundings. This division allows us to focus on studying the energy and matter exchanges between the two.

• Open System: Both energy and matter can cross the boundary (e.g., a boiling pot without a lid).
• Closed System: Only energy can cross the boundary, matter is static within the system (e.g., a sealed jar).
• Isolated System: Neither energy nor matter can cross the boundary (e.g., an insulated thermos flask).

Understanding these categories helps in predicting how systems evolve and interact with their environments. In the context of reversible processes, maintaining a keen understanding of these interactions is crucial, as any exchange affects system equilibrium and process reversibility.