14.5 Specific Latent Heat

Definition: Specific latent Heat ( $L$)

• The thermal energy required per unit mass to change the state of a substance at constant temperature.

Constant Temperature

• When a substance changes state, thermal energy is supplied (or removed) but the temperature remains constant.
• The energy is used to overcome intermolecular forces, not to increase the kinetic energy of the particles.

Formula

$\Delta E=mL$ Where:

• $\Delta E$ = Thermal energy required to change state (J)
• $m$ = Mass (kg)
• $L$ = Specific latent heat (J kg${}^{-1}$)

1. Specific Latent Heat of Fusion ($L_f$)

Definition: Specific Latent Heat of Fusion

• Thermal energy per unit mass
• To change state from solid to liquid at constant temperature

Important

• Intermolecular forces are partially overcome
• Melting occurs

2. Specific Latent Heat of Vaporisation ($L_v$)

Definition: Specific Latent Heat of Vaporisation

• Thermal energy per unit mass
• To change state from liquid to gas at constant temperature

Important

• Intermolecular forces are completely overcome
• Boiling occurs

Difference Between $L_f$ and $L_v$

Why is $L_v$ much larger than $L_f$

• During boiling, energy is supplied to completely overcome intermolecular forces, during melting, intermolecular forces are partially overcome.
• There is a greater increase in volume during boiling, therefore more work has to be one against atmospheric pressure so that the liquid can expand into a gas.