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.

Step 1: Understanding Equilibrium

In chemistry, equilibrium refers to a state where the concentration of reactants and products remain constant over time. For the equilibrium between water and its vapour, this occurs when the rate of evaporation equals the rate of condensation.

Step 2: Conditions in an Open Vessel

In an open vessel, the system is open to the surrounding environment, allowing water molecules to escape freely. This prevents the vapour pressure, necessary to establish equilibrium, from building up.

Step 3: Analysing the Options

(a) suggests an equilibrium can be established, but in an open system, vapour continually leaves, preventing equilibrium. (b) suggests dependence on pressure, but pressure cannot build in an open system. (c) suggests equilibrium cannot be achieved, consistent with an inability to build vapour pressure. (d) suggests temperature dependence, which is true in closed systems but irrelevant if equilibrium can't be established.

Step 4: Select the Correct Answer

Considering the open nature of the vessel prevents the build-up of necessary vapour pressure for equilibrium, the most suitable answer is (c) cannot be achieved.

Key Concepts

Vapour Pressure

Vapour pressure is a crucial concept when discussing chemical equilibrium involving liquids and vapours. When liquid molecules evaporate, they exert a pressure in the gaseous phase, known as vapour pressure. This is an indication of a liquid's volatility.
A high vapour pressure means the substance easily transitions from liquid to gas, while a low vapour pressure means the opposite.
In a closed system, vapour pressure plays a critical role in reaching equilibrium. In such a system, molecules evaporate, and as the vapour builds up, it exerts a pressure back onto the liquid, leading eventually to a state where evaporation and condensation occur at equal rates. But in an open system, vapour pressure cannot build up because molecules escape to the surroundings.

Evaporation and Condensation

Evaporation and condensation are processes that govern the interchange of states from liquid to gas and vice versa. - **Evaporation:** This is where molecules gain enough energy to leave the liquid phase and enter the gaseous phase. It is a surface phenomenon and depends on the temperature. - **Condensation:** This process involves gas phase molecules losing energy and returning to the liquid state.
For equilibrium to occur, the rate of evaporation must equal the rate of condensation. This balance ensures that the amount of liquid and vapour remains constant over time.
However, in an open system, evaporated molecules escape, disrupting this balance, thus preventing equilibrium.

Open System

An open system allows for an exchange of matter with its surroundings. In the context of liquid-vapour equilibrium, this means molecules can leave the system entirely, affecting equilibrium conditions.
In an open vessel, water vapour escapes into the environment, preventing the build-up of vapour pressure needed for equilibrium.
Key features of open systems are:

• They cannot maintain a constant vapour pressure due to the escape of molecules.
• Molecules from the liquid continue to evaporate without limit, altering the equilibrium dynamics.
• It highlights the importance of system boundaries in chemical atmospheres.

Understanding the nature of open systems is essential as it explains why equilibrium cannot be achieved here, unlike in closed systems where conditions remain fixed.