14.2.2 Specific Latent Heat Capacity

• Energy is required to change the state of substance
• Examples of changes of state are:
  • Melting = solid to liquid
  • Evaporation/vaporisation/boiling = liquid to gas
  • Sublimation = solid to gas
  • Freezing = liquid to solid
  • Condensation = gas to liquid

 

The example of changes of state between solids, liquids and gases

• When a substance changes state, there is no temperature change
• The energy supplied to change the state is called the latent heat and is defined as:

The thermal energy required to change the state of 1 kg of mass of a substance without any change of temperature

• There are two types of latent heat:
  • Specific latent heat of fusion (melting)
  • Specific latent heat of vaporisation (boiling)

 The changes of state with heat supplied against temperature. There is no change in temperature during changes of state

• The specific latent heat of fusion is defined as:

 The thermal energy required to convert 1 kg of solid to liquid with no change in temperature

• • This is used when melting a solid or freezing a liquid

• The specific latent heat of vaporisation is defined as:

 The thermal energy required to convert 1 kg of liquid to gas with no change in temperature

• • This is used when vaporising a liquid or condensing a gas

Calculating Specific Latent Heat

• The amount of energy Q required to melt or vaporise a mass of m with latent heat L is:

 

• Where:
  • Q = amount of thermal energy to change the state (J)
  • L = latent heat of fusion or vaporisation (J kg^-1)
  • m = mass of the substance changing state (kg)

• The values of latent heat for water are:
  • Specific latent heat of fusion = 330 kJ kg^-1
  • Specific latent heat of vaporisation = 2.26 MJ kg^-1

• Therefore, evaporating 1 kg of water requires roughly seven times more energy than melting the same amount of ice to form water
• The reason for this is to do with intermolecular forces:
  • When ice melts: energy is required to just increase the molecular separation until they can flow freely over each other
  • When water boils: energy is required to completely separate the molecules until there are no longer forces of attraction between the molecules, hence this requires much more energy

Step 1:            Write the thermal energy required to change state equation

 

Step 2:            Rearrange for latent heat

Step 3:            Substitute in the values

m = 530 g = 530 × 10^-3 kg

Q = 0.6 MJ = 0.6 × 10⁶ J

L is the latent heat of vaporisation because the change in state is from liquid to gas (boiling)