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Enthalpy of Vaporisation is defined as the amount of heat required to be absorbed to vaporise a certain quantity of liquid at a constant temperature. The total amount of heat required to change a particular amount of liquid into its vapour form, without any rise in the temperature of the liquid, is called Enthalpy of Vaporisation.
It is also called Latent Heat of Vaporisation.
What is Enthalpy?
Enthalpy is a thermodynamic property of a system. It is defined as the total amount of heat present in the system. It is the measure of energy content in a thermodynamic system. Enthalpy is defined as the sum of internal energy and the energy which is formed due to its pressure and volume. Enthalpy is a State Function because the value of enthalpy depends only on the initial and final values of heat in a reaction and not on the path of the reaction.
Enthalpy is an extensive property as its value depends on the amount of substance present in the system.
It is represented by “H”.
Enthalpy is the sum of internal energy and the product of the pressure and volume of a thermodynamic system.
H = U + pV
Where U = internal energy of the system
p = pressure of the system
V = volume of the system
Enthalpy is of several types. Some of them are:
- Enthalpy of Formation: The amount of heat required or generated when 1 mole of a compound is formed is called enthalpy of formation.
- Enthalpy of Combustion: The amount of heat generated when 1 mole of a substance is oxidised completely is called the enthalpy of combustion.
- Enthalpy of Neutralisation: The amount of heat generated when 1 kg of acid completely neutralises 1 kg of base is called enthalpy of neutralisation.
- Enthalpy of fusion: The amount of heat required to transform 1 mole of a substance from solid to liquid state at its melting point is called enthalpy of fusion.
- Enthalpy of Vaporisation: The total amount of heat required to change a particular amount of liquid into its vapour form, without any rise in the temperature of the liquid, is called enthalpy of vaporisation.
What is Vaporisation?
The process by which a substance is converted from its liquid phase into the vapour (gaseous) phase is called Vaporisation. When heat is supplied to a liquid, then the molecules of the liquid gain kinetic energy. As a result of this, the force of attraction between the molecules decreases. Due to the decrease in the force of attraction, the molecules escape into the surrounding area in the form of vapours. In the process of vaporisation, heat is absorbed by the system.
Vaporisation is of two types:
- Evaporation
- Boiling
Evaporation is a surface phenomenon whereas boiling is a bulk phenomenon.
The reverse process of vaporisation is condensation.
Enthalpy of Vaporisation
When a substance undergoes phase change from solid to liquid or liquid to gas, then a particular amount of energy is involved in this transformation of phase. In case of liquid to gas phase change, this amount of energy is called Enthalpy of Vaporisation.
The enthalpy change that occurs due to vaporisation of 1 mole of gas without changing its temperature at constant pressure is called Enthalpy of Vaporisation.
The units of Enthalpy of Vaporisation are J/mol, kJ/mol, kJ/kg, J/g, etc.
The process of vaporisation occurs at a constant temperature. As a consequence of this, the kinetic energy of the molecules in the liquid and gas phase remains the same. The heat energy that is supplied for the process of vaporisation is responsible for only breaking the potential energies of the intermolecular forces in the liquid. Enthalpy of Vaporisation is a function of the pressure at which the transformation from one state to another occurs. The Enthalpy of Vaporisation decreases with an increase in the pressure of the system. It becomes almost negligible at a certain pressure point. This point is known as the critical point. Above the critical point, it is impossible to identify liquid and vapour phases. The substance in this state is known as supercritical fluid.
The Enthalpy of Vaporisation is equal to the sum of change (increase) in the internal energy of the system when a liquid undergoes a phase transition to vapour phase and work is done by the system against the pressure of the surrounding.
The internal energy of the system increases during vaporisation to overcome the intermolecular forces in the liquid. The Greater the Van der Waals forces between the atoms of a molecule, the greater will be the requirement of heat and hence, the greater will be the Enthalpy of Vaporisation.
Example: The Enthalpy of Vaporisation of helium is very low when compared with the Enthalpy of Vaporisation of water. This is because the Van der Waals forces between the atoms of helium are very weak. But, in the case of water, molecules are held together by strong hydrogen bonds.
Enthalpy of Vaporisation Example (Water)
When we put a beaker full of water (at room temperature) on a burner, it undergoes an increase in temperature due to the heat being provided by the flames. We observe that the temperature of water increases to 100 degree Celsius. At this point, the water starts to boil. At this point, heat energy is continuously being used, but the temperature of the water remains at 100 degrees Celsius. This is because the heat is absorbed is used in breaking the intermolecular force of attraction between the water molecules. As a result of this, the water molecules become free and they escape into the surroundings as vapour. When all the molecules of water get vaporised, then the temperature again starts to rise.
Conclusion
Matter exists in three forms – Solid, Liquid and Gas. Substances are interconvertible in these three states of matter. Solid is transformed into liquid by absorbing heat, the process is called melting. The heat involved in the process is called Enthalpy of Fusion. The liquid is transformed into vapour. The heat involved is known as Enthalpy of Vaporisation and the process is called vaporisation. The process of vaporisation occurs at a constant temperature. During the process of vaporisation, the temperature of the system does not change.
