Vapour Pressure of Liquid Solution

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Vapour pressure is a key idea, particularly when we researching on solutions. Vapor is produced when some of the molecules in a liquid escape into the gas phase when it is placed in a closed container. An equilibrium is eventually reached when the rates of condensation and evaporation are equal. Vapour pressure is the force that vapour molecules exert a force, when they are in equilibrium.
Vapour Pressure of Liquid Solution-Vapour pressure
Vapour pressure

Vapour Pressure of Liquid Solution

The vapour pressure of a liquid (solvent) varies when a solute is introduced. Understanding boiling points, colliding properties, and a variety of industrial uses including chemical separation and distillation all depend on this idea.

Vapour Pressure of a Pure Liquid

Temperature and intermolecular forces determine a liquid’s vapour pressure:
• Because their molecules can readily escape into the gas phase, liquids with weak intermolecular interactions, such as acetone, have greater vapour pressures
• Higher temperatures raise vapour pressure as more molecules have the energy to evaporate; liquids with strong intermolecular interactions, such as water, have lower vapour pressure because their molecules fit tightly together securely
Vapour Pressure of Liquid Solution-Molecules
Molecules

Vapour Pressure of Solutions of Solids in Liquids

  • Vapour pressure of a solution of non-volatile solute is lower than that of pure liquid (solvent).  Ps  <  P0
This is called lowering of vapour pressure. Delta p = p0 – ps
Where, delta p  = Lowering of vapour pressure.
 p0 = Vapour pressure of pure liquid (solvent)
ps = Vapour pressure of a solution.
Cause:- In a pure liquid the entire surface is occupied by the molecules of the liquid. Rate of evaporation will be proportional to the number of molecules present in the upper surface.
When a non-volatile solute is added to a solvent the surface has molecules of solute and solvent both. Thus, the number of molecules of solvent present in upper surface is less.

Vapour Pressure of Liquid Solution

Temperature and intermolecular forces determine a liquid’s vapour pressure:
• Because their molecules can readily escape into the gas phase, liquids with weak intermolecular interactions, such as acetone, have greater vapour pressures.
• Higher temperatures raise vapour pressure as more molecules have the energy to evaporate; liquids with strong intermolecular interactions, such as water, have lower vapour pressure because their molecules grip together securely.
The total vapour pressure of solutions with two volatile components (such as a mixture of water and alcohol) is determined by the vapour pressure of each component and the quantities of each component in the mixture.
Raoult’s Law : Its state that for a solution of volatile liquids, the partial vapour pressure of each component of the solution is directly proportional to its mole fraction present in solution.
French chemist François-Marie Raoult proposed a law to describe how vapour pressure changes in solutions.

Raoult’s Law for Non-Volatile Solutes

For a solution with a non-volatile solute, Raoult’s law states that:
Psolution = P t0solven × Xsolvent  Where:
  • Psolution  = Vapour pressure of the solution
  • P0​ solvent = Vapour pressure of the pure solvent
  • Xsolvent​ = Mole fraction of the solvent
The vapour pressure of a solution is always lower than that of the pure solvent because the solvent’s mole fraction is always less than 1.

Raoult’s Law for Volatile Solutions

If both the solute and solvent are volatile (like a mixture of alcohol and water), the total vapour pressure is given by:
Ptotal = P0AXA + P0BXB​ Where:
  • P0A are the vapour pressures of pure components A and B
  • XA and XB​ are their mole fractions in the solution
Accordingly, the contribution of each component based on its percentage and individual vapour pressure determines the vapour pressure of a mixture.

Types of Solutions Based on Vapour Pressure

  1. Perfect Answers
    Observe Raoult’s law precisely.
  • Intermolecular forces remain unchanged upon mixing.
  • A combination of benzoene and toluene is one example.
2. Imperfect Remedies
  • Strong or weak interactions might cause a deviation from Raoult’s law.
  • Acetone and ethanol exhibit a positive deviation, meaning their vapour pressure is higher than anticipated.
  • Hydrochloric acid and water have a negative deviation, or lower vapour pressure than anticipated.
Applications of Vapour Pressure
  1. Boiling Point Elevation: When solute is added, the vapour pressure decreases, raising the boiling point.
  2. Distillation: This process is used in industries to separate mixtures according to variations in vapour pressure.
3. Climate and Weather: Humidity and evaporation are influenced by vapour pressure.


Note

One important characteristic of liquids and solutions is vapour pressure. The presence of solutes, temperature, and intermolecular forces all affect it.
The boiling of water and the recovery of industrial solvents, can be explained by an understanding of vapour pressure. Raoult’s law is a vital idea in chemistry since it offers a practical means of forecasting variations in vapour pressure in solutions.
Different solutions having same vapour pressure are called isopiestic solutions.
The pressure that a liquid’s vapour exerts at a specific temperature when it is in equilibrium with its liquid phase is known as vapour pressure. Temperature and the strength of the liquid’s intermolecular forces determine this.

Because more molecules acquire sufficient energy to escape into the vapour phase, vapour pressure rises in tandem with temperature.

According to Raoult’s Law, a solution’s vapour pressure is directly correlated with its solvent mole fraction. The solution’s vapour pressure is always lower than the solvent’s when the solute is non-volatile.
By taking up space near the liquid surface, the solute particles lower the vapour pressure by limiting the number of solvent molecules that can escape into the vapour phase.
By taking up space near the liquid surface, the solute particles lower the vapour pressure by limiting the number of solvent molecules that can escape into the vapour phase.
Boiling degrees, distillation procedures, humidity levels, and even the efficacy of volatile chemicals and perfumes are all influenced by vapour pressure.
When the vapour pressure of a liquid reaches the air pressure outside, the liquid boils. Both a greater boiling point and a lower boiling point are associated with lower and higher vapour pressure, respectively.

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