Hidden heat
Converting states of matter from one form into another requires the involvement of heat energy. For example, converting water at 100°C into steam at 100°C requires the input of 2260 kJ per kg of water, whereas to convert 1 kg of ice at 0°C into water at 0°C involves the input of 334 kJ.
Heat and latent heat
In this video, Dr Eric Scharpf, an energy management specialist, explains the physics meaning of the term ‘heat’, which differs from the general usage of this term. He then defines the term ‘latent heat’ and gives examples of its everyday use in household devices such as heat pumps and refrigerators.
This ‘hidden heat’ (so called because, as the change occurs, there is no change in temperature) is referred to as ‘latent heat'.
Note that the change liquid → gas requires far more energy than the change solid → liquid, because the liquid particles have to be given sufficient energy to completely break free from one another to get into the gaseous state. In the change solid → liquid, the particles are freed from fixed positions into a more mobile arrangement, allowing for movement in between and over one another.
Heat and change of state
This diagram shows the heat energies needed to convert 1 gram of ice at 0°C to its liquid and gaseous states. The unit of energy used is the calorie, which is defined as being the amount of heat energy needed to raise the temperature of 1g of water by 1°C.
Latent heat of vaporisation
The heat energy needed to convert a liquid at its boiling point to a gas at the same temperature is referred to as latent heat of vaporisation.
The reverse process – latent heat of condensation – results in an expulsion of heat energy from the system.
The amount of latent heat of vaporisation for any given liquid is dependent on two main factors:
The molecular/atomic structure of the liquid.
The mass of the liquid undergoing the change.
Latent heat of vaporisation values
Substance
Latent heat of vaporisation kJ/kg
Boiling point
C
Ethanol
855
78
Ammonia
1369
-33
Tetrafluoroethane
216
-26
Water
2260
100
Propane
356
-42
Water has a very high value. This is because attractive forces between the water molecules in the liquid state, known as hydrogen bonds, need to be overcome to release the molecules into the gaseous state.
Latent heat graph
Graph of temperature versus energy supplied for water showing the relative sizes of the latent heats of fusion and vaporisation.
Making use of latent heat
The liquid to gas/gas to liquid cycle has been extensively studied from the scientific energy in/energy out perspective. The technological outcome of this has been the development of highly efficient heating and cooling systems both in the industrial and household settings.
Basic heat pump cycle
On the evaporator side of the heat pump, heat energy is taken in, converting the working fluid into gas. The condenser side liquefies the gas, releasing heat energy to the surroundings as the gas condenses back to liquid.
Fridge and freezer design has the evaporator component on the inside of the cabinet with the condenser component on the outside. This arrangement allows the inside cabinet area to be cooled, with heat from the condenser escaping to the surrounding air on the outside.
Heat pumps operate in the same way but the design is such that the unit can either heat or cool room air depending on the setting.
The working fluid used for most household fridges, freezers and heat pumps nowadays is tetrafluoroethane. It has replaced chlorofluorocarbons (CFCs) because of the harm these chemicals were doing to the ozone layer in the upper atmosphere.
A tetrafloroethane and a dichlorofluoromethane molecule
Tetrafluoroethane is a haloalkane refrigerant with thermodynamic properties and was used to replace dichlorofluoromethane (a colourless odourless gas that was used as a propellant and refrigerant) as it was found to be harmful to the environment – it contributed to ozone depletion. Tetrafluoroethane is now subject to restricted use as it has been discovered it also contributes toward climate change.
Tetrafluoroethane has a relatively low latent heat of vaporisation and low boiling point, but its chemical inertness and low toxicity make it an ideal working fluid. In the industrial setting, ammonia and propane are often used as the working fluids.
Nature of science
The main purpose of science is to explain the natural world, whereas the purpose of technology is to intervene in the world to produce something useful. These two purposes often come together in a supportive and constructive way. The modern heat pump serves as an excellent example of how the basic scientific principles of thermodynamics have underpinned the ever-improving technological design of such a device.
Useful link
Ammonia and propane are excellent working fluids, but the hazardous nature of these chemicals restricts them to the more controlled industrial setting usage. This Suff news article describes a cool store explosion in Hamilton in April 2008 that killed one fireman and injured seven others.
