Latent heat of vaporization and fusion

Introduction

In this article, we will be focusing on the concept of latent heat. We will discuss latent heat of fusion and latent heat of vapourization with the help of solved examples and much more. The form of energy transferred between two materials having different temperatures is called heat.

Latent heat

When a substance undergoes a change of state, it emits and transfers a certain amount of heat energy to its surroundings.
Therefore, the latent heat of a substance refers to the amount of heat transferred per unit mass during the change of phase transformation.
For example, when heat is added to a certain quantity of ice, the ice temperature will continue to increase until it reaches 0°C, which is the melting point of ice. At this temperature, even if more heat is added to the ice, the temperature of the ice will not increase. However, the heat will cause the ice to change its state as it will start to melt.
Similarly, during the change of state of liquid to gas, adding more heat to the boiling water will not increase the temperature of the water itself but will lead to vaporization.
The heat required to change the state of a substance depends upon the mass of the substance that is undergoing the change and the heat of transformation.
The quantity of heat required for a substance of mass m to transform one state to another, can be determined by the following equation Q = mL or L = Q/m.

In this equation, the letter L denotes the characteristic of the substance and is known as the latent heat. The value of latent heat also depends upon the pressure. And its value is calculated at standard atmospheric pressure.

The SI unit of latent heat is J kg-1.
Temperature will remain constant even if heat is added or removed during the change of state.
The latent heat of fusion for water is LF = 3.33 × 105 J kg-1, and the latent heat of vaporization for water is LF = 22.6 × 105 J kg-1.
Therefore, to melt 1 kg of ice at 0°C, you need 3.33 × 105 J kg-1 of heat and 22.6 × 105 J kg-1 of heat is required to turn water into steam at 100°C. This also means that at 100°C, water carries 22.6 × 105 J kg-1 less heat than steam at 100°C. Therefore, burns caused due to exposure to steam are more serious than burns caused by boiling water.

Let’s take a look at an example of how to calculate latent heat.

For example, the heat of fusion of ice will be when 0.15 kg of ice at 0 ° C is mixed with 0.30 kg of water at 50 ° C in a container. And the resulting temperature of the mixture is 6.7 ° C. (S water = 4186 J kg-1 K-1)

Heat lost by water = msw (θf –θi) w

= (0.30 kg) (4186 J kg–1 K–1) (50.0 ° C – 6.7 °C)

= 54376.14 J

Heat required to melt ice = m2Lf = (0.15 kg) Lf

Heat required to raise temperature of ice water to final temperature = mI sw (θf –θi) I

= (0.15 kg) (4186 J kg–1 K –1) (6.7 °C – 0 °C)

= 4206.93 J

Heat lost = heat gained

54376.14 J = (0.15 kg) Lf + 4206.93 J

Lf = 3.34 ×105 J kg–1.

Latent heat of fusion

Latent heat of fusion is also referred to as heat of fusion and enthalpy of fusion.

At constant temperature and pressure, the heat per unit mass required to transform the state of a substance from solid to liquid is called the latent heat of fusion. Latent heat of fusion is denoted by LF.

The change in enthalpy is positive when a substance changes its state from a solid to a liquid. Therefore, the change in enthalpy is negative when a substance changes its state from a liquid to a solid.

During the latent heat of the fusion process, the heat applied to the substance breaks apart the molecules holding their bonds to form a liquid.

The heat or thermal energy required for a substance to change its state from solid to liquid or from liquid to solid is known as the latent heat of fusion.