Latent Heat of Vaporization

Latent Heat of Vaporization:

When solid changes to liquid and more heat is added, e.g. water, as the temperature rises, the water will boil at 100°C. This temperature will remain steady as more heat is added until all the water changes to steam at 100°C. This process is known as Latent Heat of Vaporization.

The heat energyEnergy is the ability to do work. Energy exists in several forms such as heat, kinetic or mechanical energy, light, potential energy, and electrical energy. Units of Energy: The SI unit... More supplied to change the water to steam (or vapour) without a change in temperature is called latent heat of vaporization. The heat energy absorbed to change steam (or vapour) to liquid without a change in temperature is also called latent heat of vaporization.

As the water vapor changes its state from gaseous form to liquid form through a process called condensation, the latent heat of vaporization is released unlike when the water changes its form from liquid to gaseous state through a process called evaporation where the latent heat of vaporization is absorbed.

Specific latent heat of vaporisation is the quantity of heat required to convert a unit mass of a liquid to vapour at the boiling point without a change in temperature.

Specific Latent Heat of Vaporization:

The specific latent heat of vaporization of water is 2260000 Jkg^-1 or 2.26 × 10⁶ Jkg^-1. This is the amount or quantity of heat required to change 1 kg of water at boiling point completely to vapour without a change in temperature. It is also the amount of heat energy released when 1 kg of vapour changes into 1 kg of liquid.

This means, when 1 kg of water changes to steam at 100°C, it will absorb 2260000 Jkg^-1. Similarly, when 1 kg vapour melts at 100°C, it will release 2260000 Jkg^-1 heat.

\( \scriptsize Q = ML_v \)

Where:

Q = Quantity of heat
M = Mass of substance
L_v = Specific latent heat of vaporisation

Because gas molecules have the biggest intermolecular space and the force of attraction between them is nearly non-existent, the latent heat of vaporization is larger than the latent heat of fusion. As a result, converting liquid to gas requires more energy.

Example 6.1.1:

Estimate the quantity of heat required to convert 2 kg of ice at 0°C to steam at 100°C.

Specific heat capacity of water = 4200 Jkg^-1k^-1
Specific latent heat of fusion of water = 336000 Jkg^-1k^-1
Specific latent heat of vaporisation of water = 2260000 Jkg^-1k^-1

Solution

Quantity of heat required = Heat to melt the ice at 0°C + Heat required to boil the water from 0° to 100°C + Heat to boil water to steam at 100°C.

Q_total = Q₁ + Q₂ + Q₃

 Q₁ = mL_f

= 2 × 336000

= 672000 J

Q₂ = mC Δθ

= 2 × 4200 × 100

= 840000 J

Q₃ = mL_v

= 2 × 2260000

= 4520000J

Q_total = Q₁ + Q₂ + Q₃ 

= 672000 + 840000 + 4520000

= 6032000 J

= 6.032 × 10⁶ J