Specific Latent Heat Capacity (CIE A Level Physics)

• Energy is required to change the state of a 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

Changes of state

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 specific 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)
• It is important to recognise the difference between latent heat and specific latent heat
  • latent heat is the energy required to change a materials state
  • specific latent heat is the energy required to change the state of 1 kg of a material

Types of latent heat

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