Vapor Pressure, Characteristics, Raoult՚s Law, Boiling Point, the Heat of Vaporization (For CBSE, ICSE, IAS, NET, NRA 2022)

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What is Vapor Pressure?

  • When a liquid is placed in a vessel that gets continually heated, the molecules of the liquid are seen to be moving at varying speeds in different directions. This happens due to the different kinetic energies possessed by the molecules of the liquid.
  • When the liquid is heated, the energy of the molecules rises; it becomes lighter and occupies the surface of the liquid. This process is known as ‘evaporation’ . The molecules which can be seen on the liquid surface are called ‘vapor’ .
  • The evaporation continues at a constant rate the temperature of the liquid is kept constant.
  • When some molecules of the liquid in the vapor phase, strikes the walls of the containers or the surface of the liquid, it may get converted back to the liquid phase. This process is called condensation.
  • Vapor pressure changes with the temperature of the surroundings and the nature of the liquid.

Characteristics of Vapor Pressure

  • However, as time passes, the number of molecules in the vapor phase increases while the rate of condensation also increases. It reaches a stage where the rate of evaporation is equal to the rate of condensation. This phase is called the stage of equilibrium.
  • As represented by the manometer, at this point the pressure exerted by the molecules is called the vapor pressure of the liquid. Vapor pressure is defined as the pressure exerted by the vapor present above the liquid.
  • A pure liquid experience a greater amount of vapor pressure as against a liquid՚s solution.
  • It is inversely proportional to the forces of attraction existing between the molecules of a liquid.
  • It increases with a rise in the temperature. This is because the molecules gain kinetic energy and thus, vaporize briskly.
Vapour Pressure

Vapour Pressure

The process of evaporation depends on different factors:

Nature of the Liquid

Liquids have weak intermolecular forces. Heating the molecules of the liquid can help change them to the vapor phase and thus increase the vapor pressure of the liquid. For example, Acetone and benzene have higher vapor pressure than water at a particular temperature.

Effect of Temperature

The vapor pressure of the liquid increases with an increase in its temperature. The molecules of the liquid have higher energy at higher temperatures.

Raoult՚s Law

Where is the vapour pressure of component 1 in a pure state?

Similarly, for component 2:

From Dalton՚s Law of partial pressures, we know that,

Using the values of and from equation (1) and (2) respectively we have:

We can draw the following inferences from the above equation:

  • The above equation is a straight line between and whose slope is given by and the y-intercept is equal to .
  • The total vapour pressure above a solution varies linearly with the mole fraction of component 2.
  • The total vapour pressure above a solution depends on the vapour pressure of Component 1 and 2 in their pure state and the mole fraction of component 2 in the solution.

What is a Boiling Point?

  • As we go on increasing the temperature of the liquid, its vapor pressure increases proportionately. It reaches a stage where the vapor pressure of the liquid becomes equal to the atmospheric pressure. At this temperature, the vapors near the surface start escaping to the atmosphere and the liquid undergoes a phase transition. This temperature is defined as the boiling point of the liquid.
  • The standard boiling point of the liquid is given at:

Pressure of or

Boiling Point

What is the Heat of Vaporization?

  • As we provide heat to a liquid, its energy increases, which results in an increase in the overall temperature. At the boiling point, the additional heat is used up by the molecules to overcome the intermolecular force of attraction in the liquid and change to the gaseous state.
  • When 1 mole of liquid is transformed into a gaseous state, the amount of heat provided by this process is known as the Heat of Vaporization.

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