Vapour Pressure – Composition

Introduction

Vapour pressure is defined as a pressure exerted by the vapours of a liquid at the equilibrium state is called the vapour pressure of the liquid at the given temperature. It is also termed the equilibrium vapour pressure.

When a liquid is heated, its molecules overcome the forces holding them in the liquid form and escape into a gaseous phase. Due to this, they create more molecules in the gaseous phase, and this amount forms pressure right above the surface of the liquid, which leads to the vapour pressure.

Vapour Pressure :

• Liquid molecules are heated at a constant temperature. They gained kinetic energy that is distributed within the molecules of the liquid. As for gas, increasing the temperature increases both the average kinetic energy of the gaseous molecules in the liquid and the range of kinetic energy of each molecule.
• If we assume that there is a minimum amount of energy that is required to break the intermolecular force between the liquid particles, which holds them together, then there are some molecules in the liquid that always have the kinetic energy that is greater than the minimum amount of energy.
• The molecules that have energy higher than this minimum amount of energy are the ones that readily escape from the surface of the liquid, and it has to be on the surface of the liquid for the process of evaporation of vaporisation to take place and convert into a gas molecule.

• When the number of gas molecules above the surface of the liquid increases it creates a pressure and this pressure is referred to as the vapour pressure. 

• As soon as some of the vapour molecules have formed, a fraction of them will collide with the liquid’s surface, converting it to a liquid. and this process is called condensation.

• Therefore the process of evaporation and condensation plays an important role in where the vapour pressure can be measured and observed.
• As the number of molecules in the vapour phase grows, so does the number of collisions between them, and as the surface grows, so does the number of molecules leaving the liquid surface, until a steady-state is reached in which exactly as many molecules per unit time leave the liquid surface and collide back.

At this point, the pressure above the surface of the liquid stops increasing and remains constant at a given temperature.

Saturated Vapour Pressure

When two opposing processes occur at the same rate, therefore, there is no net change in the system. This constitutes the dynamic equilibrium. When a liquid is confined in a chamber, the molecule evaporates and condenses constantly, but the volume of liquid molecules or vapour molecules does not vary. As a result, the equilibrium vapour pressure of the liquid, also known as saturated vapour pressure, is the pressure exerted by the vapour in dynamic equilibrium with the liquid surface.

When a liquid is placed in an open container, the majority of the vapour molecules escape into the air and do not collide with the liquid surface, resulting in no equilibrium. The liquid will continue to evaporate until there is no more liquid in the container under these conditions. The pace at which this occurs is determined by the liquid’s vapour pressure and the heating temperature.

• Volatile liquids have high vapour pressure and tend to evaporate readily.

Example : Gasoline, acetone

• Non-volatile liquids have low vapour pressure and evaporate slowly.

Example : Glycerin

Divide volatile liquid and non-volatile liquids don’t have a clear conclusion. We mostly compare the liquid with the vapour pressure of water. According to the vapour pressure of water, they are divided into volatile liquid and non-volatile liquid.

• Liquids that possess greater vapour pressure than water are considered volatile.
• Liquids that possess lower vapour pressure than water are considered non-volatile.

The equilibrium vapour pressure of any substance at a specific temperature is a characteristic that can be defined as molecular mass, melting point, freezing point, and boiling point.

Factors affecting Vapour Pressure :

• Vapour pressure is directly proportional to the temperature. That is, if there is a rise in temperature, the vapour pressure also increases.
• Vapour pressure is inversely proportional to the nature of liquid and magnitude of intermolecular forces. If there is less intermolecular force, then the vapour pressure increases.

Relation Between Boiling Point and Vapour Pressure :

Vapour pressure plays an important role in the determination of boiling point. As we increase the temperature of a liquid, the vapour pressure of the liquid increases till it reaches the point at which the vapour pressure of the liquid is equal to the atmospheric pressure. 

Therefore the boiling point is defined as the point at which the liquid boils exactly at one atmospheric pressure.

Boiling points differ when the atmospheric pressure changes. For example, at altitude, the water boils faster than at lower altitudes because of the difference in atmospheric pressure due to elevation.

Raoult’s Law:

It states that “ For a solution having two volatile liquids their vapour pressure is directly proportional to their respective mole fractions” 

Component 1: P1 ∝ x1  

                         P1 = P1° x1

Component 2: P2 ∝ x2  

                         P2 = P2° x2

P total = P1 + P2 

            = (P1° x1) + (P2° x2)

It can also be written as ,

     P total = P1°(1-x2) + P2°x2

     Ptotal = P1° + (P2°-P1°)x2.

The following conclusions can be drawn from Raoult’s law equation:

1. The total vapour pressure of a solution can be related to the mole fraction of any component. 
2. The total vapour pressure of the solution varies linearly with the mole fraction of component 2 in the solution.
3. Considering the vapour pressure of the pure components 1 into the total vapour pressure of the solution could be increased or decreased by increasing the mole fraction of component 1.

Conclusion 

Vapour pressure relating to the boiling point and heat of vaporisation is one of the most important characteristics applied in food preparation.

Another important application of vapour pressure is a pressure cooker where the total pressure of the liquid is utilised to cook the food it is seen in our everyday life.

By knowing the vapour pressure of each liquid, we know how we should store them in the Laboratories and how they should be used appropriately.

The above mentioned a few applications of vapour pressure where we use them in our daily life and the laboratory.

This article highlights the definition of vapour pressure, saturated vapour pressure, the relationship between Vapour pressure and boiling point, and Raoult’s law.