Vapour Pressure of Liquid Solutions Chemistry Notes
→ Vapour Pressure : The pressure produced at surface of liquid by molecules of vapour in equilibrium at a certain temperature is called saturated vapour pressure or vapour pressure. The process of evaporation takes place continuously in open containers but in closed containers, the particles in vapour colapse back to liquid surface due to which condensation starts. Evaporation and condensation are two opposite reections. So, equilibrium is established in them after some time.
Evaporation ↔ Condensation
Factors Affecting Vapour pressure :
→ Nature of Liquid : The liquid, in which weak intermolecular forces of attracion are present, has high vapour pressure because more molecules of liquid can come in vapour state after leaving surface of liquid. That’s why, the vapour pressure of diethyl ether is relatively higher than ethyl alcohol.
→ Temperatue : The vapour pressure of liquid increases with increase in temperature because kinetic energy of liquid molecules increases with increase in temperature and more molecules convert in to vapour state after leaving liquid surface. The vapour pressures P, and Pat temperatures T, and T, of a liquid are represented by following relation:
Vapour Pressure of Liquid-Liquid solution :
When solution of a liquid (solute) is formed in a liquid (solvent) then both components in solution are volatile.
→ So, there is contribution of vapour pressure of both components in vapour pressure of solution. Let two volatile components A and B are present in a solution. The partial vapour pressure of these components are PA and PB respectively and vapour presure of solution is P, then
P = PA + PB
Vapour Pressure of Solid-Liquid Solutions :
→ When a non-volatile solute (solid) is mixed in a solvent by heating, then the vapour pressure of solution is lesser than solvent. This is called lowering of vapour pressure. If vapour pressure of pure solvent is p° and vapour pressure of solution is P, then,
Lowering of vapour Pressure = p° – P
→ It is explained as follows – Evaporation is a surface phenomenon in which molecules of solvent change into vapours from the surface of liquid. When a non-volatile solute is added in a solvent then some space at surface taken place by molecules of non-volatile solute due to which the number of particles of solvent at surface decreases. Due to this reason, the rate of evaporation of solution decreases. Hence lowering of vapour pressure takes place.