Problem 70 - A Complete Resource Book in Chemistry for JEE Main

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 sponta neously

Step-by-Step Solution

Answer

(c) the surroundings are always in equilibrium with the system.

Step 1: Understanding the term 'Reversible Process'

A reversible process in thermodynamics refers to an ideal process where the system and its surroundings are always in near-perfect equilibrium. This means that any changes in the system occur so slowly that the system remains in equilibrium with the surroundings at all times.

Step 2: Analyzing the given options

We need to analyze each option to see which accurately describes a reversible process. - (a) The surroundings and system change into each other: This does not describe a reversible process as it implies a mutual interchange. - (b) There is no boundary between system and surroundings: This neglects the definition of a system in thermodynamics which is separated by a boundary. - (c) The surroundings are always in equilibrium with the system: This aligns with the definition of a reversible process. - (d) The system changes into the surroundings spontaneously: This describes a spontaneous process, not a reversible one.

Step 3: Conclusion based on analysis

Based on our analysis, option (c) is the most accurate. In a reversible process, the system and its surroundings are always in equilibrium, allowing processes to reverse without additional changes in either the system or the surroundings.

Key Concepts

Thermodynamics

Thermodynamics is the branch of science concerned with heat, work, and energy. It explores how energy is transferred and converted between different forms. In thermodynamics, a 'system' refers to a specific part of the universe under study, while everything outside this system is considered the 'surroundings.' Together, they form a boundary where energy and material interactions occur. In reversible processes, these interactions are crucial as they dictate how reversible a system and its surroundings can be. During a reversible process, changes happen extremely slowly, ensuring that both the system and its surroundings remain in a perfect balance or equilibrium.
This allows reversible processes to theoretically return to their initial states with no energy loss, making them ideal models in thermodynamic studies. Understanding such concepts helps in designing more efficient engines and predicting how natural processes will behave.

Equilibrium

The concept of equilibrium is central in reversible processes. A system in equilibrium is one where there are no net changes over time. In the context of a reversible process, the system must be in thermal, chemical, and mechanical equilibrium with its surroundings at all times. This means:

• Thermal equilibrium: Both system and surroundings have the same temperature, so no heat flows between them.
• Chemical equilibrium: No net reaction occurs within the system or between the system and the surroundings, maintaining constant composition.
• Mechanical equilibrium: There are no unbalanced forces, keeping the pressure constant across system boundaries.

In a reversible process, equilibrium allows the system to change states infinitesimally slowly, enabling a seamless transition back to the original state without external influences. Such a perfect state of balance is more of an ideal condition in practice but helps in understanding how systems interact with their surroundings.

System and Surroundings

In thermodynamics, understanding the system and its surroundings is vital to studying energy exchanges. The 'system' is the particular portion of the world that we focus on, and it can be as large as a power plant or as small as a cup of coffee. The 'surroundings' include everything outside the system that interacts with it.
The boundary is a conceptual or physical divide that separates the system from the surroundings. It dictates how energy, work, or matter is allowed to transfer. In a reversible process, the boundary must be such that the system and surroundings can exchange energy or matter without disrupting their equilibrium.
Understanding this division and their interactions is crucial for identifying how reversible changes can occur without adding or losing energy, signifying a model of perfect efficiency. This also helps us define the limits of energy transformations and transport in practical applications.