Changing the internal energy of a material will cause it to change temperature or change state. The thermal energy to:
Each substance has two specific latent heats:
It usually takes more energy to boil a substance than to melt it, so the latent heat of vaporisation for a substance is usually greater than its latent heat of fusion. The table shows some typical values:
|Substance||Specific latent heat of fusion (kJ/kg)||Specific latent heat of vaporisation (kJ/kg)|
The thermal energy for a change in state is calculated using this equation:
thermal energy for a change in state = mass × specific latent heat
This is when:
Calculate the thermal energy needed to freeze 500 g of water at 0°C.
500 g = 500/1,000 = 0.5 kg
specific latent heat of fusion = 334 kJ/kg (from the table above)
specific latent heat of fusion = 334 × 1,000 = 334,000 J/kg
thermal energy = 0.5 × 334,000
= 1,670,000 J (167 kJ)
Latent heat can be measured from a heating or cooling curveline graph. If a heater of known power is used, such as a 60 W immersion heater that provides 60 J/s, the temperature of a known mass of ice can be monitored each second. This will generate a graph that looks like this:
The graph is horizontal at two places. These are where energy is being used to break the bonds between the particles to change the state, rather than increase the speed of the particles (and so to increase the temperature).
The longer the horizontal line, the more energy has been used to cause the change of state. The amount of energy represented by these horizontal lines is equal to the latent heat.
30 minutes = 30 × 60 = 1,800 s
energy transferred = power × time
energy transferred = 60 × 1,800
= 108,000 J
50 g = 50 ÷ 1,000 = 0.05 kg
latent heat of vaporisation = 10,800 ÷ 0.05
= 2,160,000 J/kg (2,160 kJ/kg)