The property exerted by a vapor in equilibrium with the liquid at a given temperature is called the vapor pressure of the liquid at that temperature. At high temperatures, there are greater chances of the formation of vapor from liquid, and the vapor pressure of the liquid is high at high temperatures.
In a pure liquid, there is only solvent-solvent interaction i.e. only solvent molecules are present. When the pure solvent is heated then all the solvent molecules have an equal chance to evaporate and give the high vapor pressure. But when the nonvolatile solute is added to a solvent there is both solute and solvent molecule i.e. in solution there is solvent-solute interaction. From the solution, the solvent molecule is evaporated but not the solute molecule from the surface. So, the vapor pressure of the solution is lower than that of the pure solvent.
Raoult’s law for relative lowering of vapor pressure of the solution
The relative lowering of the vapor pressure of the solution is equal to the mole fraction of the solute in the solution.
Where, P = vapor pressure of the pure solvent
Ps = Vapor pressure of the solution
n = no. of moles of solute
N = No. of moles of solvent
The vapor pressure of the pure solvent is caused by the number of molecules evaporating from its surface. Greater is the number of molecules evaporating from the surface greater is the vapor pressure when a non-volatile solute is dissolved in solution., the fraction of the solute molecules blocks fraction of a surface, and this causes the lowering of vapor pressure. The vapor pressure of the solution is therefore determined by the number of molecules of the solvent present at any time in the surface which is proportional to the mole fraction i.e.
In the case of pure solvent, n = 0 and Ps = P. Therefore, the mole fraction of solvent is 1. Thus, P = K, so equation i can be modified as
A solution is said to be ideal if the escaping tendency of each component is proportional to the mole fraction of that component in the solution. That is the partial vapor pressure of the component is directly proportional to its mole fraction in the solution.
A solution will be ideal if the liquid phase obeys Raoult’s law. Therefore, it is true for the vapor phase of the solution that the Daltons’ law of partial pressure must be obeyed by each component of the vapor phase over the liquid solution.
Elevation of Boiling point is directly proportional to Lowering of vapor pressure
The vapor pressure-temperature curve for the solution lies below the curve of the pure solvent. This is because when non-volatile solutes are added to the solvent it decreases the vapor pressure of the solution.
When the temperature is increased, the rate of evaporation is increased and hence vapor pressure of the solution is also increased. The vapor pressure of the solution does not reach the atmospheric pressure the solution will not boil. In the above diagram the temperatures corresponding to pint C, D & F are Tb, T1, & T2 and these are the boiling points of pure solvent, solution-1, and solution-2 respectively. So boiling point of a solution containing non-volatile solute is always greater than that of the pure solvent. The difference between the boiling point of the solution and the pure solvent is called the elevation of the boiling point.
What is vapor pressure?
The property exerted by a vapor in equilibrium with the liquid at a given temperature is called vapor pressure.
What is the relation between lowering of vapor pressure and elevation of boiling point?
The elevation of boiling point is directly proportional to the lowering of vapour pressure.