Change of State and Latent Energy - Learn

Change of State and Latent Energy

During a phase change, the temperature of a substance does not increase or decrease. This is because the thermal energy during a phase change goes into breaking the intermolecular bonds between particles rather than increasing the kinetic energy of the particles. The latent heat is the energy released or absorbed during a change of state. ‘latent’ means hidden or unseen.

The latent heat is calculated using the formula:

$Q=mL$

where:

$Q$ = the heat energy transferred in Joules (J)

$m$ = mass (kg)

$L$ = the latent heat (Jkg⁻¹)

The value of L is dependent on the substance being considered and also the phase change. It takes a lot more energy to change a liquid to a gas than it does a solid to a liquid. This means the latent heat, L, for water to steam will be higher than for ice to water.

When considering a phase change from solid to liquid (melting), $L$ is described as the latent heat of fusion ( ${ L }_{ fusion }$ ). When considering a phase change from liquid to a gas (boiling), $L$ is described as the latent heat of vaporisation ( ${ L }_{ vaporisation }$ ).

Consider water:

Latent heat of fusion ( ${ L }_{ fusion }$ ) = 3.34×10⁵ Jkg⁻¹
Latent heat of vaporisation ( ${ L }_{ vaporisation }$ ) = 22.5×10⁵ Jkg⁻¹

Example 1:

How much energy is required to melt a 5kg block of ice?

Answer:

using: $Q=mL$

where:

$m$ = 5 kg

${ L }_{ fusion }$ = 3.34×10⁵ Jkg⁻¹

$Q=5\times (3.34\times { 10 }^{ 5 })$

$Q=1.67\times { 10 }^{ 6 }J$

$Q=1670\;kJ$