Problem 14 - A Complete Resource Book in Chemistry for JEE Main

Question

The equilibrium between water and its vapour, in an open vessel (a) can be achieved (b) depends upon pressure (c) cannot be achieved (d) depends upon temperature

Step-by-Step Solution

Answer

Equilibrium cannot be achieved in an open vessel as vapour escapes into the atmosphere.

Step 1: Understanding the Problem

The problem is about the equilibrium between water and its vapour in an open vessel. In an open vessel, the system is open to its surroundings, and factors like pressure and temperature might impact the equilibrium state.

Step 1: Identifying an Open System

Recognize that an open vessel allows water vapour to escape into the atmosphere. In such a system, the pressure of the surroundings is constant and matches atmospheric pressure. This makes it difficult for water and vapour to reach an equilibrium since vapour is continuously lost.

Step 2: Analyzing the Conditions for Equilibrium

Equilibrium between water and its vapour occurs when the rate of evaporation equals the rate of condensation. In an open vessel, water molecules can escape into the atmosphere, reducing the possibility of condensation, thus disturbing equilibrium.

Step 3: Evaluating the Options

Options (a) 'can be achieved' and (d) 'depends upon temperature' do not accurately address the inability to contain vapour in an open vessel. Options (b) 'depends upon pressure' is irrelevant as pressure is constant (atmospheric). The correct statement is (c) 'cannot be achieved' due to the open nature allowing vapour to escape.

Key Concepts

Open System

When we talk about an "open system," we mean a system that can exchange both matter and energy with its surroundings. In our scenario with water and its vapor, the open vessel acts as an open system. This means that water vapor can freely escape into the atmosphere.

• The vessel has no lid, so air and moisture can flow in and out.
• This freedom to exchange substances means conditions can change quickly.
• Equilibrium is difficult to maintain because the system is not isolated.

Understanding open systems is crucial in chemistry and physics, as they behave quite differently from closed or isolated systems.

Evaporation and Condensation

Evaporation and condensation are two opposing processes critical to understanding chemical equilibrium in an open system. Evaporation is when water molecules at the surface gain enough energy to change into a gas and enter the atmosphere. Condensation is the reverse process, where water vapor turns back into liquid water.

• Evaporation: This happens when molecules escape from the liquid to the gas phase.
• Condensation: This occurs when gas molecules change into the liquid phase.

In an open vessel, the evaporation process can proceed unhindered because vapor can leave the vessel into the surrounding air. This makes condensation less likely, as there’s less vapor to turn back into liquid, thus preventing equilibrium from being achieved in such a setup.

Vapor Pressure

Vapor pressure refers to the pressure exerted by a vapor in equilibrium with its liquid phase at a particular temperature. It's a crucial concept, especially in the context of an open system.

• Definition: It's the measure of a liquid's tendency to evaporate and become a gas.
• Influence of Temperature: Higher temperatures usually increase vapor pressure because molecules gain more energy to escape the liquid phase.
• In an Open Vessel: Vapor pressure doesn't build up because the vapor leaves the system, preventing the typical balancing act seen in closed systems.

In an open vessel, the vapor pressure doesn't reach the levels necessary to equalize with the liquid phase, since the vapor constantly escapes, affecting the establishment of equilibrium.

Atmospheric Pressure

Atmospheric pressure is the pressure exerted by the weight of air in the Earth's atmosphere. This constant force impacts systems on Earth's surface. In our scenario, atmospheric pressure plays a key role in an open system.

• Definition: It's the force exerted by the weight of air above a given point.
• Role in Open Systems: In an open vessel, the pressure inside is equal to external atmospheric pressure.
• Effect on Equilibrium: Since atmospheric pressure remains constant, it doesn’t influence equilibrium directly in an open system; however, it allows vapor to escape.

Despite being constant, atmospheric pressure permits the exchange of gases and vapors, making it possible for vapor to leave the open system, thus hindering the establishment of equilibrium.

Thermodynamics

Thermodynamics is the branch of physics concerned with heat and temperature and their relation to energy and work. It provides the foundational principles explaining why equilibrium cannot be achieved in an open vessel.

• Energy Transfer: In an open system, energy can enter and exit along with matter, disrupting the balance needed for equilibrium.
• Second Law of Thermodynamics: States that systems tend toward increased entropy, and open systems are constantly moving toward this state.
• Impact on Equilibrium: The continuous energy and matter exchange prevent the balance between evaporation and condensation, as seen in our open vessel.

Understanding these principles helps us see why an open system, where matter and energy flow freely, struggles to reach equilibrium. This forms the core of why the equilibrium between water and its vapor, in an open vessel, indeed "cannot be achieved."