Calculating Specific Latent Heat
- Latent (meaning hidden) heat describes the energy required to change the state of a substance
- The specific part again refers to 1 kg of a substance
- The amount of energy Q required to melt or vaporise a mass of m with latent heat L is:
Thermal energy required for a change of state = Mass × Specific latent heat
Q = mL
- Where:
- Q = thermal energy required for a change in state, in joules (J)
- m = mass, in kilograms (kg)
- L = specific latent heat, in joules per kilogram (J/kg)
- This equation can be rearranged with the help of a formula triangle:
- For context, the values of latent heat for water are:
- Specific latent heat of fusion = 330 kJ/kg
- Specific latent heat of vaporisation = 2.26 MJ/kg
- Therefore, evaporating 1 kg of water requires roughly seven times more energy than melting the same amount of ice to form water
Worked example
Calculate the energy transferred to the surroundings as 0.60 kg of stearic acid changed state from liquid to solid.
The specific latent heat of fusion of stearic acid is 199 000 J/kg.
Step 1: List the known quantities
-
- Mass, m = 0.60 kg
- Specific latent heat of fusion, L = 199 000 J/kg
Step 2: Write down the relevant equation
Q = mL
Step 3: Substitute in the values
Q = 0.60 × 199 000 = 119 400 J
Examiner Tip
Remember that L is used as the symbol of specific latent heat of fusion or vaporisation. This equation will be given on your equation sheet, however, it is important you know how to use it!
