VAPOUR PRESSURE

 VAPOUR PRESSURE 
(the pressure of a vapour in contact with its liquid or solid form.)

Properties of solution affected by vapour pressure

• Vapour pressure of solution decreases on addition of a non-volatile solute to a volatile solvent.
• Properties of solutions affected by the decrease of vapour pressure includes-

1. Relative lowering of vapour pressure of the solvent
2. Depression of freezing point of the solvent
3. Elevation of boiling point of the solvent and
4. Osmotic pressure of the solution.

• These properties of solution depend on the number of solute particles present in the solution regardless of their nature relative to the total number of particles present in the solution. These properties are termed as colligative properties derived from a Latin word with co meaning together ligare meaning to bind

Definition

Vapour pressure of a liquid/solution is the pressure exerted by the vapours in equilibrium with the liquid/solution at a particular temperature. 

Vapour pressure ∝ escaping tendency

 Vapour pressure of liquid solutions and Raoult’s Law :

(Raoult’s law for volatile solutes)

 Raoult’s law states that for a solution of volatile liquids, the partial vapour pressure of each component in the solution is ∝ to its mole fraction.

Consider a solution containing two volatile components 1and 2 with mole fractions x₁ and x₂ respectively. 

Suppose SOLUTIONS & COLLIGATIVE PROPERTIES at a particular temperature,

their partial vapour pressures are p₁ and p₂ and the vapour pressure in pure state are  p⁰₁ and  p⁰₂.

Thus, according to Raoult’s Law, for component 1 

1. The plot of vapour pressure and mole fraction of an ideal solution at constant temperature. 
2. The dashed line I and II represent the partial pressure of the components.
3.  It can be seen from the plot that p1 and p2 are directly proportional to x1 and x2 , respectively.
4.  The total vapour pressure is given by line marked III in the figure.
5.  Mole fraction in vapour phase If y 1 and y 2 are the mole fractions of the components 1 and 2 respectively in the vapour phase then, using Dalton’s law of partial pressures: 
6. p1 = y 1 ptotal p2 = y 2 ptotal 
7. In general pi = y i ptotal

All liquids exhibit tendency for evaporation.

Evaporation takes place at the surface of liquid. If the kinetic energy of liquid molecules overcomes the intermolecular force of attraction in the liquid state then the molecules from the surface of liquid escape into space above surface. The process is called 'evaporation'. If evaporation is carried out in a closed container system then the vapours of liquid remains in contact with surface of liquid. Like gas molecules vapour of molecules also execute continuous random motion. During this motions, molecules collide with each other and also with the walls of the container, losses their energy and returns back to liquid state. This process is called as 'condensation'.
Evaporation and condensation are continues processes. Hence, after some time an equilibrium is established, at constant temperature between evaporation and condensation. At equilibrium number of molecules in vapour state remains constant at constant temperature.
"The pressure exerted by vapours of liquid on the surface of liquid when equilibrium is established between liquid and it's vapour is called VAPOUR PRESSURE of liquid."
    The vapour pressure of the liquid depends on the nature of the liquid and temperature. With increase of intermolecular force of attraction vapour pressure of liquid decrease and with rise of temperature vapour pressure of liquid increases.
  Mercury manometer may be used to determine vapour pressure of liquid.

Vapour pressure is a liquid property related to evaporation. In the liquid (or any substance) the molecules have a distribution of kinetic energies related to the temperature of the system. Because this is a distribution there will always be a few molecules that have enough kinetic energy to over come the attractive potential energy of the other molecules (the intermolecular force), and escape the liquid into the gas phase. In an open container, these molecules will wander off (diffuse) into the room and out into the atmosphere. Eventually all the liquid will evaporate.

For example-

Ice melts to form water, and water evaporates to form water vapor.The pressureexerted by the water vapor is the vapor pressure. In more general terms, vapor pressure is the pressure exerted by a gas in equilibrium with the same material in liquid or solid form.

Vapor pressure or equilibrium vapor pressure is defined as thepressureexerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's evaporation rate.

Vapour pressure is the pressure applied by the vapours on liquids and it has different values for different liquids, even in different conditions.