Latent Heat

Definition of Latent Heat

Latent heat is the energy transferred to or from a substance during a change of state without changing its temperature. This energy is used to break or form the bonds between particles rather than increasing their kinetic energy.

For example, when ice melts to water, energy is absorbed to break the bonds holding the solid structure together, but the temperature remains constant until all the ice has melted. This is because the energy changes the potential energy of the particles by breaking bonds, so their kinetic energy (which relates to temperature) does not increase during the phase change.

Similarly, when steam condenses to water, energy is released as bonds form, again with no temperature change during the phase change.

Latent Heat of Fusion and Vaporisation

Latent heat of fusion is the energy required to change a substance from solid to liquid (melting) or released when it changes from liquid to solid (freezing).

Latent heat of vaporisation is the energy required to change a substance from liquid to gas (boiling or evaporation) or released when it changes from gas to liquid (condensation).

In both cases, energy is absorbed or released without a change in temperature because it is used to overcome or form the forces between particles.

Specific latent heat values differ between substances and depend on the type of change of state (fusion or vaporisation). Tables of these values may be provided in exams.

Calculating Latent Heat

The amount of energy transferred during a change of state can be calculated using the formula:

$$Q = m \times L$$

• Q is the energy transferred in joules (J)
• m is the mass of the substance in kilograms (kg)
• L is the specific latent heat in joules per kilogram (J/kg)

The specific latent heat value depends on the substance and the type of change of state (fusion or vaporisation).

Example: For instance, if 2 kg of ice melts, and the latent heat of fusion of ice is 334,000 J/kg, the energy required is:

$$Q = 2 \times 334,000 = 668,000 \text{ J}$$

Applications and Examples

Latent heat plays an important role in everyday processes such as:

• Melting ice: Energy is absorbed to change ice at 06C to water at 06C without temperature change.
• Boiling water: Energy is absorbed to convert water at 1006C to steam at 1006C.
• Condensation: Steam releases energy as it condenses back to liquid water.

This energy transfer is crucial in natural and industrial processes, such as climate regulation, cooking, and refrigeration.

Example: Calculate the energy needed to boil 0.5 kg of water at 1006C if the latent heat of vaporisation of water is 2,260,000 J/kg.

Using the formula $Q = m \times L$:

$$Q = 0.5 \times 2,260,000 = 1,130,000 \text{ J}$$

So, 1,130,000 joules of energy are needed to turn 0.5 kg of water into steam at 1006C.