Vapour Pressure

At average temperatures, a constituent with a high vapour pressure is volatile. 

Volatility is the material property of a substance that describes its readiness to vaporise. Vapour pressure is also referred to as equilibrium vapour pressure. It is the pressure exerted by the vapour when it is in a thermodynamic equilibrium state. The exerted pressure is at a specified temperature with the condensed phases of solid or liquid. This vapour pressure study material will give you deep insights into all related concepts with examples and formulae.

Vapour Pressure

In a constantly heated vessel with some water, the compounds of the liquid can be seen shifting at different speeds in various directions within it. This is because of the various kinetic energies of the liquid’s molecules.

When a fluid is heated, the molecule’s energy present in the liquid rises. This causes it to turn lighter and take up the liquid’s surface. This is referred to as ‘evaporation’. The molecules visible on the surface of the liquid are referred to as ‘vapour’.

The evaporation process repeats steadily, although the liquid’s temperature remains constant. When some compounds of the liquid in the vapour state collide with the wall surfaces of the containers, they may be changed back to the liquid phase. The name given to this process is condensation.

The equilibrium vapour pressure is created to serve as an index of a liquid’s evaporation rate. It is known that the propensity of particulates to break free from a liquid is related. The particulate can also break free from a solid state. Volatile material has high pressure of vapour at normal temperatures. It should be noted that vapour pressure refers to the pressure exerted by a vapour above the surface of the liquid. 

Measurement of Vapour Pressure

Vapour pressure can be measured through standard pressure units. The International System of Units identifies pressure as a sourced unit with force per area, and the pascal (Pa) is assigned as its standard unit. A pascal is equivalent to one newton per square metre.

The executing phase of vapour pressure is a simple process for familiar pressures ranging from 1 to 200 kPa. The most precise findings are achieved close to the boiling points of compounds, and measurements less than 1 kPa generate massive errors. Purifying the test substance, isolating it in a container, evicting any new gas, and measuring the equilibrium pressure of the vapour form phase of the compound in the container at various temperatures seem to be standard techniques. When precautions are taken to ensure that the entire compound and its vapour are at the specified temperature, accuracy improves. 

Boiling Point

Higher elevations have lower atmospheric pressures, and water boils at reduced temperatures. As the temperature of a liquid rises, its vapour pressure rises correspondingly. It gets to a point where the liquid’s vapour pressure is equivalent to the atmospheric pressure. Nearing this temperature, vapours reach near the surface and begin to escape into the surrounding air, and the liquid goes through a phase transformation. This temperature is known as the liquid’s boiling point.

The Heat of Vaporisation

When we increase the temperature of a liquid, its corresponding energy increases, leading to an increase in the air temperature and thermal gradient. The particles take up the excess heat at the boiling point to resolve the interparticle force of attraction. This happens in the liquid state and during the transition to the gaseous state.

The heat supplied during the process when 1 mole of liquid is converted into a gaseous condition is known as the heat of vaporisation.

The liquid’s vapour pressure, which increases with temperature, measures a substance’s tendency to transform into a gaseous or vapour state. A liquid’s boiling point is the lowest temperature at which the vapour pressure at its exterior corresponds to the pressure applied by its environment. The vapour pressure at any temperature is defined as the pressure applied by the vapour, which is available above the fluid in equilibrium with the liquid at a certain temperature. 

Raoult’s Law

Raoult’s law states that the vapour pressure of a component in the solution is equivalent to the vapour pressure of the pure component multiplied by its molar concentration in the solution. 

The equation of Raoult’s law is written mathematically as:

 Psolution = ΧsolventP0solvent

Where, PSolution = the solution’s vapour pressure

XSolvent = the solvent’s mole fraction

P0solvent = the pure solvent’s vapour pressure

We’ll understand more about the law’s underlying principle by looking at the example below. 

Consider a container with a solution of volatile liquids A and B. Because A and B are both volatile, both particles of A and B would be present in the vapour phase. 

As a result, both A and B’s vapour particles impose differential pressure, adding to the pressure gradient just above the solution. 

Conclusion

The pressure applied by vapour above that of a liquid’s surface is referred to as vapour pressure. The kinetic energy of a liquid’s molecules tends to increase as its temperature increases. As the molecule’s kinetic energy tends to increase, so does the number of moles that transfer into such a vapour, elevating the vapour pressure. Any substance’s vapour pressure rises nonlinearly to temperature.