Vapour Pressure: Raoult’s Law, Evaporation, Boiling Point

Vapour Pressure forms when a liquid that is partially filled in an evacuated container undergoes the process of evaporation to fill the remaining volume of the container with vapour. The vapour formed then exerts pressure on the walls of the container and this pressure is known as the vapour pressure. In this article, we will discuss the characteristics of vapour pressure, Raoult’s law, vapour pressure, boiling point, etc.

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 What is Vapour Pressure?

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Vapour Pressure Phase Transition

At a constant temperature, the pressure exerted by the vapour of a liquid on its surface, when the liquid and its vapour are in equilibrium, is called Vapour Pressure. This process of phase transition of an element or a compound, from its liquid to vapour phase is called the process of Vaporization. When these vapour molecules collide with the surface of the container, they may get converted back to their liquid phase. This process is known as condensation. 

As the liquid present in the container evaporates, the vapours exert pressure on the walls of the container. When equilibrium is established, the pressure exerted by the vapours is known as Vapour Pressure.

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Characteristics of Vapour Pressure

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• Vapour pressure is known as the pressure that is exerted by the vapour on the surface of the liquid.
• Vapour Pressure is affected by the temperature exponentially, which means that the vapour pressure rises with the rising temperature.
• It is not affected by the volume of the liquid in a container. Any two containers having varying volumes but the same temperature will have the same vapour pressure.

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Factors affecting Vapour Pressure

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The vapour pressure is affected by factors such as:

• Temperature: On increasing the temperature, more vapours are produced due to more evaporation. Therefore, the vapour pressure increases.
• Nature of Liquid: Vapour pressure depends on the intermolecular forces among the liquid molecules. The intermolecular forces amongst the liquid molecules are weaker due to which their phases can be changed. Hence, as the intermolecular force increases, the vapour pressure decreases. 

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Raoult’s Law

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Raoult’s Law states that the partial pressure of each component of a solution, for a solution of volatile liquids, is directly proportional to the mole fraction that is present in the solution.

P = X.P^o 

where, P: vapour pressure of the solution

X: mole fraction of solvent

P^o: vapour pressure of pure solvent

Now, let us consider a solution of two volatile liquids, with p₁, x₁ and p₂, x₂ being their respective partial vapour pressures and mole fractions. The total vapour pressure is p(total). 

Now, according to Raoult’s Law, 

p₁ ∝ x₁

and p₁ = p₁^o x₁

where p₁^o: vapour pressure of pure component 1 at the same temperature

Similarly, 

p₂ = p₂^o x₂

According to Dalton’s Partial Pressure Law,

p(total) = p₁ + p₂

p(total) = p₁^ox₁ + p₂^ox₂

= p₁^o (1 - x₂) + p₂^o x₂

= p₁^o + (p₂^o - p₁^o)x₂

Hence, we can conclude that:

• The total vapour pressure can be related to the vapour pressure of both components in the pure phase and the mole fraction of the second component in the solution. 
• The total vapour pressure varies linearly with the mole fraction of component 2. 
• The deduced equation represents a straight line between x₂ and p(total). The slope of the same is given by (p₂^o - p₁^o) and the y-intercept equals p₁^o.
• The total vapour pressure increases or decreases depending upon the vapour pressures of the components. 

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Boiling Point

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As we continue heating the liquid in an open vessel, more and more liquid vapourises from the surface. The vapour pressure of the liquid becomes equal to the external pressure, at a certain temperature. This temperature is called Boiling Temperature at that pressure. 

Vapour Pressure and Boiling Point

• With temperature, vapour pressure increases.
• Water has the greatest intermolecular forces followed by ethyl alcohol and diethyl ether.
• The boiling temperature of a liquid at 1atm pressure is called Normal Boiling Point. 
• If the pressure is 1 bar, the boiling point is called Standard Boiling Point. 

In simpler words, the boiling point of a substance is the temperature at which the vapour pressure of the liquid becomes equal to the external pressure, and the liquid changes into a vapour. 

Processes involving a change in boiling point:

• It is difficult to cook at high altitudes as the atmospheric pressure is low. This low atmospheric pressure means that liquids will boil at a lower temperature, as compared to sea levels. Hence, the pressure cooker is used for cooking food.
• In hospitals, autoclaves are used to sterilize the instruments. These autoclaves increase the boiling point of water by increasing the pressure above the atmospheric pressure. This is done by using a weight to cover the vent.

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Evaporation

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When a liquid kept in a vessel is heated, its molecules gain energy. This gain in energy results in a random motion of the liquid molecules in all directions. Due to this motion, the molecules collide with each other and they transfer energy to each other. When a molecule on the surface of the liquid acquires enough energy, it escapes into the surrounding air. This whole process is termed Evaporation. 

Evaporation and Condensation

Factors affecting evaporation:

• Intermolecular Forces: The stronger is the intermolecular force among the molecules of the liquid, the lower is the rate of evaporation. 
• Temperature: The rate of evaporation is higher with an increase in the temperature of the liquid.
• Surface Area: More area of the liquid results in more surface molecules that can escape into the air. 
• Pressure: Lower exertion on the surface of the liquid results in a higher rate of evaporation. 

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Critical Temperature and Heat of Vaporization

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As the temperature increases, the rate of vaporization increases as well. With more vapours in the air, the density of vapours increases while the density of liquid decreases. At a certain temperature, both have equal densities. 

Vapour Pressure Phase Diagram

• The temperature at which the density of the liquid and the density of the vapour is equal is called Critical Temperature. At this temperature, the clear boundary between liquid and vapours disappears.
• When 1 mole of liquid changes into a gaseous state, the amount of heat that is provided is known as the Heat of Vaporization.

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Things to Remember

• Vapour pressure is known as the pressure that is exerted by the vapour on the surface of the liquid.
• Vapour Pressure is affected by the temperature exponentially, which means that the vapour pressure rises with the rising temperature.
• Raoult’s Law states that the partial pressure of each component of a solution, for a solution of volatile liquids, is directly proportional to the mole fraction that is present in the solution.

          P = xP^o

• The vapour pressure of the liquid becomes equal to the external pressure, at boiling temperature.
• When a molecule on the surface of the liquid acquires enough energy, it escapes into the surrounding air by evaporation.
• The temperature at which the density of the liquid and the density of the vapour is equal is called Critical Temperature.

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