Short Answer

Latent heat of vaporization is the amount of heat energy required to convert a unit mass of a liquid into gas at its boiling point without changing its temperature. It is an endothermic process because energy is absorbed to overcome intermolecular forces in the liquid.

For example, when water boils at 100°C, it absorbs heat from the surroundings without any rise in temperature. The heat absorbed during this process is called the latent heat of vaporization.

Detailed Explanation

Latent Heat of Vaporization

Latent heat of vaporization is a specific form of latent heat associated with the liquid-to-gas transition. During this phase change, energy is supplied to the liquid so that molecules can escape from the surface and enter the gaseous phase. This energy does not increase the temperature of the liquid but is used to overcome intermolecular attractions, allowing molecules to move freely in the gas state.

This concept is fundamental in chemistry, physics, and engineering because it explains how heat energy is required for boiling and evaporation, and it is essential for many natural and industrial processes.

Characteristics of Latent Heat of Vaporization

Energy Absorption: Heat energy is absorbed by the liquid during boiling or evaporation.
Temperature Constant: The liquid remains at its boiling point during the phase change.
Substance Dependent: Liquids with stronger intermolecular forces require more energy for vaporization.
Measurement: Expressed in Joules per gram (J/g) or Joules per kilogram (J/kg).

For example:

Water: 2260 J/g
This means each gram of water absorbs 2260 J of heat to convert into steam at 100°C.

Process of Vaporization

During vaporization:

Heat is supplied to the liquid.
Molecules absorb energy to overcome attractive forces between them.
Molecules escape from the liquid surface as gas.
Temperature remains constant until all liquid is converted to gas.

This process is endothermic, with heat coming from the surroundings or an external source.

Intermolecular Forces and Latent Heat of Vaporization

The latent heat of vaporization depends on the strength of intermolecular forces:

Strong forces (e.g., hydrogen bonding in water) → High latent heat.
Weak forces (e.g., van der Waals in organic liquids) → Low latent heat.

Example:

Water has strong hydrogen bonding → 2260 J/g latent heat of vaporization.
Ethanol has weaker forces → 840 J/g latent heat of vaporization.

The stronger the forces, the more energy is needed to separate molecules for the gas phase.

Examples in Everyday Life

Boiling Water

Water absorbs heat at 100°C to become steam without a rise in temperature.

Evaporation

Liquid water absorbs heat from surroundings to evaporate at room temperature, cooling the environment (like sweating).

Industrial Applications

Steam generation for power plants and heating systems.
Evaporation in cooling towers.
Distillation processes in chemical industries.

Measurement of Latent Heat of Vaporization

Latent heat of vaporization is measured using calorimetry:

Where:

= heat absorbed (Joules)
= mass of the liquid (kg or g)
= latent heat of vaporization per unit mass

This formula allows calculation of energy needed for boiling or evaporation of a liquid.

Significance of Latent Heat of Vaporization

Cooling Effects: Evaporation absorbs heat, cooling surfaces or bodies.
Weather and Climate: Evaporation and condensation of water drive atmospheric energy transfer.
Energy Calculations: Important in engineering and thermodynamics for heating, boiling, and power generation.

Latent heat of vaporization is critical in understanding energy requirements and phase behaviour of liquids.

Conclusion

Latent heat of vaporization is the heat energy absorbed by a liquid to change into gas at constant temperature. It is essential for boiling, evaporation, and various industrial and natural processes. The value depends on the substance and its intermolecular forces, explaining why water requires a high amount of heat to vaporize compared to other liquids.