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Vapour Pressure

Last updated date: 22nd Mar 2024
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Factors Affecting Vapour Pressure

All liquids exhibit a tendency for evaporation. The process of evaporation occurs at the surface of the 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 the liquid escape into space above the surface. The process is called 'evaporation.' If evaporation is carried out in a closed container system, then the vapours of liquid remain in contact with the liquid's surface.

 Like gas molecules, the condensation of molecules also executes the continuous random motion. During these motions, molecules collide with each other and, even with the walls of the container, they lose their energy and return to the liquid state. This process is called 'condensation.'

Evaporation And Condensation

Evaporation and condensation are continuous processes. Hence, after some time, an equilibrium is established at a constant temperature between evaporation and condensation. At the equilibrium number of molecules in the vapour, the state remains stable at a constant temperature.

The liquid's vapour pressure depends on the nature of the fluid and temperature, with an increase of intermolecular force of attraction Vapour pressure of liquid decreases, and with a rising temperature vapour pressure of liquid increases. A mercury manometer may be used to determine the vapour pressure of a liquid.

What is vapour pressure?

Vapour pressure is the tendency of a material to change into a gaseous state. And generally, vapour pressure is the pressure exerted by the vapour of the liquid in Thermodynamic equilibrium with the condensed phases in a closed system. If the temperature of any liquid increases then its vapour pressure also increases. The vapour pressure of a liquid can be measured in various ways. The simplest method to measure vapour pressure is using a manometer in a closed container or flask.

Factors Affecting Vapour Pressure

There are various factors on which vapour pressure depends. They are:

1. Nature of Liquid

The nature of the liquid is explained based on its intermolecular forces. That is to say, as the magnitude of the intermolecular forces rise up, the vapour pressure will dwindle down

2. Effect of Temperature

As the temperature of the liquid increases, the kinetic energy associated with the liquid also increases. And due to this increase in kinetic energy, the escaping tendency of the molecule increases; hence vapour pressure increases. So we can draw the inference that vapour pressure is directly proportional to temperature.

3. Concentration of Solute

The existence of a solute in the liquid will significantly reduce the vapour pressure. And this fall in vapour pressure also differs with respect to the concentration of solute.

4. Vapour Pressure Is Independent of Humidity.

Temperature is the only property that affects the vapour pressure for a certain amount of water vapour in the air. Humidity will act only if all the other variables are constant. So don't get any sort of confusion between the effect of temperature and humidity.

5. Volume

Vapour pressure does not tend to get affected by the volume of the container. As we know, that liquid in the box will be in equilibrium with the vapour Now, suppose the volume of surface S is divided into infinite elementary volumes such that volume is changed, say decreased, then some of the container's vapour turns into a liquid state. And if the volume rises up, some of the liquid is bound to change into its vapour state.

6. Surface Area

Usually, vapour pressure is independent of surface area.

Factors Affecting Vapour Pressure of Liquid

However, the following factors affect the vapour pressure of a liquid at equilibrium.

1. Intermolecular Forces of Attraction

The forces that mediate an interaction between atoms, including powers of attraction or repulsion are called the Intermolecular forces (IMF). For example, the covalent bond, involving sharing electron pairs between atoms, is much stronger than the parties  intermolecular force present between neighbouring molecules.

2. The Volume of the Liquid Present Does Not Affect the Vapour Pressure of a Liquid at Equilibrium.

We can change the volume of a liquid (keeping temperature constant), but the vapour pressure of a fluid at equilibrium will remain the same.

3. The Temperature of the Liquid.

Weaker are the intermolecular forces of attraction, or higher is the temperature of the liquid, higher is the vapour pressure of a fluid at equilibrium.

Unit of Vapour Pressure

The most common unit for vapour pressure is the torr. One torr = 1 mm Hg (one millimetre of mercury).

Most materials have external vapour pressures. For example, water has a vapour pressure of approximately 20 torrs at room temperature (22 °C = 72 °F). Note that the vapour pressure increases with the temperature; water will have a vapour pressure of 760 torr = 1 atm at its boiling point of 100 oC (212 oF).

Boiling Point

The boiling point refers to the temperature of any substance at which the vapour pressure of a liquid becomes equivalent to the pressure encompassing the liquid, and the fluid converts into a vapour.

Relation Between the Standard Boiling Point And The Vapour Pressure of Liquids

The higher the vapour pressure of a liquid at a given temperature, the lower the standard boiling point (i.e., the boiling point at atmospheric pressure) of the liquid.

The vapour pressure chart to the right has graphs of the vapour pressures versus temperatures for a variety of liquids.

As can be seen on the map, the liquids with the highest vapour pressures have the lowest standard boiling points.

For example, at any given temperature, methyl chloride has the highest vapour pressure of any of the liquids in the chart. It also has the lowest standard boiling point (−24.2 °C), which is where the vapour pressure curve of methyl chloride (the blue line) intersects the horizontal pressure line of one atmosphere (atm) of absolute vapour pressure.

FAQs on Vapour Pressure

1. What is the boiling point?

A boiling point is the temperature at which a liquid starts to boil and turns into a vapour or gaseous state. At this temperature, the pressure exerted by surroundings on the liquid is the same as that of pressure exerted by the vapour by the liquid. Vapour pressure will be high if the intermolecular forces of the molecules are low. This means very little energy will be required to separate the molecules and hence the boiling point will also be relatively low.

In Short boiling point decreases as the vapour pressure increases. The higher the vapour pressure of a liquid lower will be its boiling point.

2. Does the vapour pressure depend upon the size of molecules? 

Yes, the vapour pressure depends on the size of the molecules. As bigger the molecule is, the more electrons will be present it and the bigger the London forces are. So bigger molecules will have more molecular weight, and so vapour pressure will also be high. 

pressure at which the rate of sublimation of any solid matches the thermal decomposition of its vapour phase.

3. What are the factors affecting vapour pressure?

Vapour pressure of any liquid is dependent upon some factors which are surface area, type of molecules, and temperature. If the temperature increases, vapour pressure also increases, and conversely if temperature decreases, vapour pressure decreases. With respect to the surface area, vapour pressure has no effect on the surface area of the solid or liquid which is in contact with the gas. And if the intermolecular forces between the molecules are strong then the vapour pressure is relatively low, whereas if the intermolecular forces between the molecules are relatively weak, then vapour pressure is relatively high.

4. What is Raoult’s law?

Raoult's law is a law that states that the freezing and boiling points of an ideal solution are depressed or elevated depending upon the pure solvent. Raoult's law also states that the vapour pressure of the liquid solution is proportional to the mole fraction of the solvent. This law helps to derive the vapour pressure of the liquids. Visit Vedantu to study more about this law.

5. Does vapour pressure depend on the nature of liquids?

Yes, Vapour pressure depends on the nature of the liquid. And this pressure is dependent upon the nature of the liquid in consideration. If the intermolecular forces between the molecules of the liquid are relatively high, then the vapour pressure is low. And if the intermolecular forces between the molecules are relatively low, then the vapour pressure is high. Hence if there is a weak intermolecular force between the molecules, the tendency for the molecules to get evaporated is high. So the vapour pressure will be higher.

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