Introduction:

In chemistry, vapour pressure is a crucial concept, especially in the study of liquid solutions. It refers to the pressure exerted by the vapour of a liquid when it is in dynamic equilibrium with its liquid phase at a given temperature. Understanding the vapour pressure of liquid solutions is essential to grasp concepts like boiling point elevation, freezing point depression, and colligative properties, which are significant for both theoretical and practical purposes in chemistry.

Vapour Pressure of Pure Solvent:

For a pure solvent, the vapour pressure is determined by the temperature and the intermolecular forces within the liquid. At a constant temperature, the vapour pressure of a solvent remains constant. As the temperature increases, the kinetic energy of the molecules increases, causing more molecules to escape into the vapour phase, thus increasing the vapour pressure.

Vapour Pressure of Liquid Solutions:

In a solution, the vapour pressure is different from that of a pure solvent due to the presence of a solute. When a non-volatile solute is added to a solvent, it lowers the vapour pressure of the solvent. This phenomenon can be explained by Raoult’s Law.

Raoult's Law:

Raoult's Law states that the vapour pressure of an ideal solution is directly proportional to the mole fraction of the solvent. Mathematically:

P_solution = P_solvent^0 . X_solvent

Where:

P_solution= Vapour pressure of the solution.

P_solvent^0= Vapour pressure of the pure solvent.

X_solvent = Mole fraction of the solvent.

According to this law:

1. The presence of a non-volatile solute reduces the number of solvent molecules at the surface of the liquid, resulting in lower vapour pressure.

2. The magnitude of vapour pressure lowering is directly proportional to the mole fraction of the solute.

Ideal and Non-Ideal Solutions

1. Ideal Solutions: Solutions that obey Raoult’s Law at all concentrations and temperatures are termed ideal. In such solutions, the interactions between solute-solvent molecules are similar to those between solvent-solvent and solute-solute molecules.

2. Non-Ideal Solutions: Solutions that deviate from Raoult’s Law exhibit positive or negative deviations. This deviation is due to the difference in intermolecular forces between solute and solvent molecules compared to pure solvent molecules.

Positive Deviation: Occurs when the vapour pressure of the solution is higher than predicted by Raoult’s Law. This happens when solute-solvent interactions are weaker than solvent-solvent interactions (e.g., ethanol and acetone).

Negative Deviation: Occurs when the vapour pressure is lower than predicted by Raoult’s Law, implying that solute-solvent interactions are stronger than solvent-solvent interactions (e.g., water and nitric acid).

Colligative Properties and Vapour Pressure:

The lowering of vapour pressure is a key factor in the understanding of colligative properties, which depend on the number of solute particles in a solution rather than the nature of the solute itself. These properties include:

-Relative lowering of vapour pressure

-Boiling point elevation

-Freezing point depression

-Osmotic pressure

Relative Lowering of Vapour Pressure:

The relative lowering of vapour pressure is directly proportional to the mole fraction of the solute. For a solution of a non-volatile solute in a solvent, this can be expressed as:

P_solvent^0 — P_solution ÷ P_solvent^0 = X_solute

Where X_solute is the mole fraction of the solute.

Applications of Vapour Pressure Lowering:

1. Distillation: Vapour pressure lowering is essential in processes like distillation, where components of a solution are separated based on differences in their vapour pressures.

2. Food Preservation: Vapour pressure lowering plays a role in preserving food, as adding salt or sugar reduces the vapour pressure, which helps in reducing the rate of spoilage.

3. Boiling and Freezing Points: Solutions have higher boiling points and lower freezing points than pure solvents due to vapour pressure lowering.

Conclusion:

The concept of vapour pressure in liquid solutions is vital for understanding many chemical processes and real-life applications. From the preparation of solutions to the study of colligative properties, vapour pressure provides insights into how solutes and solvents interact at the molecular level. Mastery of these principles is crucial for students preparing for exams like NEET and board exams.