All About Vapour pressure

The pressure created by the vapour of a liquid (or solid) above the liquid’s surface is known as vapour pressure. This pressure is created in a closed container at a specific temperature in a thermodynamic equilibrium state. The equilibrium vapour pressure determines the rate of evaporation of a liquid. As the temperature rises, so does the vapour pressure. The boiling point of a liquid is the point at which the ambient pressure equals the pressure exerted by the vapour.

Characteristics of Vapour Pressure

Nature of Liquid

Intermolecular forces in liquids are weak. Heating the molecules of a liquid can aid them transition to the vapour phase, increasing the liquid’s vapour pressure. Acetone and benzene, for example, have a higher vapour pressure than water at a given temperature.

Temperature Effect

The kinetic energy associated with the liquid increases as the temperature of the liquid rises. As a result of this increase in kinetic energy, the molecule’s inclination to escape increases, resulting in an increase in vapour pressure. As a result, we can deduce that vapour pressure is proportional to temperature.

Volume

The volume of the container has little effect on the vapour pressure. The liquid in the box will, as we know, be in equilibrium with the vapour. If the volume of surface  is divided into infinite elementary volumes, the volume of the container is altered, say lowered, and some of the vapour is converted to a liquid form. And as the volume increases, some of the liquid will condensate and become vapour.

Concentration of Solute

The presence of a solute in the liquid lowers the vapour pressure substantially. And the drop in vapour pressure varies depending on the solute concentration.

Intermolecular Forces of Attraction 

The Intermolecular forces are the forces that mediate interactions between atoms, such as attraction or repulsion (IMF). The covalent bond, which involves atoms sharing electron pairs, is substantially stronger than the normal intermolecular force that exists between molecules.

Unit and Measurements

The standard units of pressure are used to measure vapour pressure. Pressure is a derived unit with the dimension of force per area recognised by the International System of Units (SI), which uses the pascal (Pa) as its standard unit. One newton per square metre (Nm-2 or kgm-1s-2) equals one pascal.

For common pressures between 1 and 200 kPa, measuring vapour pressure experimentally is a simple method. The most precise findings are obtained at the boiling point of substances, while readings less than 1kPa result in substantial inaccuracies. Purifying the test substance, isolating it in a container, expelling any external gas, and then measuring the equilibrium pressure of the substance’s gaseous phase in the container at various temperatures are common procedures.

When care is made to guarantee that the entire substance and its vapour are at the appropriate temperature, accuracy improves. Submerging the confinement area in a liquid bath, as with the use of an isoteniscope, is a common method.

Boiling Point

The vapour pressure of the liquid increases proportionally as the temperature of the liquid rises. It reaches a point where the liquid’s vapour pressure equals the air pressure. The vapours at the surface begin to escape into the atmosphere at this temperature, and the liquid undergoes a phase shift. This temperature is equal to the liquid’s boiling point.

The liquid’s standard boiling point is stated as:

1 atm equals 102325 Pa, or 1 bar equals 105 Pa.

Heat of Vapourization

When we heat a liquid, its energy increases, which leads to a rise in the overall temperature. The extra heat is used up by the molecules at the boiling point to overcome the intermolecular force of attraction in the liquid and shift to a gaseous form.

The amount of heat provided by this process when 1 mole of liquid is changed into a gaseous state is known as the Heat of Vaporisation.

Conclusion

Vapour pressure, which rises with temperature, is used to quantify a substance’s tendency to change to a gaseous or vapour form. The temperature at which the vapour pressure at a liquid’s surface equals the pressure exerted by its surroundings is known as the boiling point. The vapour pressure of a liquid at any temperature is the pressure exerted by the vapour present above the liquid in equilibrium with the liquid at that temperature.