Vapour Pressure

Vapour Pressure

When a liquid is allowed to evaporate in a closed vessel, part of the liquid evaporates and fills the available space with the vapours. Since the vapours leave the container, these get collected in the vapour state above the surface of the liquid.

Due to vaporisation, liquid changes into vapours and level of liquid decreases. As the evaporation proceeds, the number of gaseous molecules in the vapour phase increases gradually.

These molecules move about at random in a limited space during their random movement, some of these strike the surface of liquid and get condensed. The process of condensation acts in opposite direction to the process of evaporation. Thus, both evaporation and condensation processes go on simultaneously.

A stage is reached when the rate of evaporation becomes equal to rate of condensation and an equilibrium gets established between liquid and vapour phases.

The pressure exerted by the vapours above the liquid surface in equilibrium with the liquid at a given temperature is called vapour pressure.

Vapour Pressure of Liquid Solutions

The vapour pressure of a liquid solution is the pressure exerted by the vapour of the solvent and any volatile solutes in equilibrium with the liquid phase. The vapour pressure of a solution is always lower than the vapour pressure of the pure solvent at the same temperature due to the presence of the solute molecules, which occupy some of the surface area of the liquid and reduce the number of solvent molecules that can escape into the gas phase.

 The vapour pressure of a liquid depends upon-

Nature of liquid.

  • ·         Each liquid has a characteristic vapour pressure because each liquid has different magnitude of intermolecular forces.
  • ·         The liquids, which have weaker intermolecular forces, tend to escape readily into vapour phase and therefore, have greater vapour pressure.
  • ·         For example: dimethyl ether and alcohol have higher vapour pressure than water at a given temperature because of weaker intermolecular forces in them as compared to water.

 Temperature –

The vapour pressure of a liquid increases with increase in temperature. This is due to the fact that with increase in temperature, more molecules will have larger kinetic energies. Therefore, larger number of molecules will escape from the surface of the liquid to the vapour phase resulting in higher vapour pressure.

 Presence of impurities –

 The presence of nonvolatile impurities lowers the vapour pressure, and the presence or addition of more volatile impurities raises the vapour pressure of a liquid.

Raoult’s law

·         The relationship is known as the Raoult’s law which states 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.

·         If Pis the vapour pressure of the solvent over a solution containing non-volatile solute and xA is its mole fraction, then according to Raoult’s law, the vapour pressure of the solvent in the solution,

·         pA =pA° xA

·         p=pA°xA

·         p(solution) = p(pure solvent) × mole fraction of solvent

·         This relationship is known as Raoult’s law.

Raoult’s Law as a special case of Henry’s Law

The vapour pressure of a volatile component in given solution is given by the relation:

pA =PA° xA

where pA° is the vapour pressure of the pure component, pA is the vapour pressure in the solution having mole fraction xA. In the case of solution of a gas in a liquid, the gaseous component is volatile component. Its solubility is governed by Henry law which gives the relation:

p=KH x

where p is the pressure of the gas above the solution and x is its mole fraction ,K is a proportionality constant known as Henry’s constant.

The partial pressure of the volatile component or gas is directly proportional to its mole fraction in solution. The only difference in the two expressions is the proportionality constant PA° (in Raoult’s law) and KH (in Henry’s law). Therefore, Raoult’s law becomes a special case of Henry’s law in which KH becomes equal to vapour pressure of the pure component (pA°).

Vapour Pressure of Solutions of Solids in Liquids

Add a small amount of a non-volatile solute to the solvent (e.g, sugar in water) to form the solution. When evaporation of this solution takes place, the vapour phase again consists of vapours of the solvent (i.e., of water) because the solute is non-volatile.The vapour pressure of the solution is found to be less than that of the pure solvent.

The vapour pressure depends on the escape of solvent molecules from the surface of the liquid. In the case of solution, the non-volatile sugar molecules occupy a certain surface area. As a result lesser number of solvent molecules will escape into vapours. Vapour pressure of the solution will be less than that of the pure solvent or there will be a lowering in vapour pressure. The increase in the concentration of sugar in the solution will further lower the vapour pressure of the solution.