In meteorology and supply engineering, latent heat ("latens", Latin for "hidden") is the term given to the enthalpy in the unit joule that is absorbed or emitted during a phase transition of the first order . The technical term used for this in thermodynamics is enthalpy of transformation , since the phase transitions are isothermal and occur at a constant ambient pressure . Depending on the type of phase transition, z. A distinction is made, for example, between sublimation , melting , evaporation or condensation enthalpy . The term latent heat is incorrect from a thermodynamic point of view, since heat is an energy instead of an enthalpy, i.e. it would be defined for processes with a constant volume of the substance.
On the other hand, the energy used to raise the temperature is colloquially called sensible heat . In thermodynamics, the technical term used for this is thermal energy . Their relation to latent heat is called the Bowen relation .
- Phase transition liquid ↔ gaseous: water heated to the boiling point has absorbed thermal energy by then. With further energy supply, the water does not get hotter, but evaporates with a considerable increase in volume. As steam , water contains more energy than before in liquid form, although the steam is not hotter. When the water vapor condenses to liquid water, the energy is released again, whereby the temperature remains constant (isothermal process) and the volume decreases. The amount of energy, which is identical in amount, is called enthalpy of evaporation when evaporating and enthalpy of condensation when liquefying ; only the signs differ.
- In the phase transition solid ↔ liquid, the energy to be expended or released is called the enthalpy of fusion or enthalpy of crystallization, depending on the direction .
Specific latent heat
- the specific latent heat related to the mass has the unit joule / kilogram.
- the specific latent heat related to the amount of substance (“ molar latent heat”) has the unit joule / mol .
The molecular structure of substances explains why no temperature change occurs despite the supply or removal of energy. Applied to the enthalpy of vaporization, this means that the molecules in a liquid are much closer together than in a gas . Consequently, the distance between the molecules has to be increased during evaporation, which is associated with an increase in the potential energy (E pot ). The work required for this is done by the amount of heat supplied.
In contrast , according to the kinetic gas theory , a positive temperature change corresponds to an increase in the kinetic energy E kin of the molecules, which is not directly associated with an increase in distance.
The same reasoning also applies to the enthalpy of melting and crystallization:
When a crystalline substance begins to melt, it very quickly approaches a new one, which is the most likely state. This is the macrostate with the greatest number of different possible arrangements ( microstates ) of the particles . From an energetic point of view, the division between E kin and E pot is not fixed. But in a state of higher potential energy, the lattice building blocks can move away from their places. This enables a very large number of new spatial arrangements and therefore makes this state more likely than one with high kinetic energy. So supplied energy is converted into potential as long as new micro-states can be formed. Since only shocks can transfer energy to a thermometer , but the kinetic energy does not increase during melting, the temperature remains constant.
The latent heat plays an important role in meteorology in relation to the phase transitions of water in the earth's atmosphere . A large part of the solar energy is invested in the evaporation of water on a damp earth's surface or even water . Around 2450 kilojoules per kilogram of water are converted at 20 ° C. A change in the air temperature does not occur here, the energy is so to speak, in the gaseous state of aggregation stored water.
Since this storage is reversible , the same amount of energy is released again when a rising air parcel reaches the condensation level and the water vapor condenses. The energy originally provided on the ground by solar radiation is released again at greater heights and contributes to a rise in temperature there. This leads to the formation of a moist adiabatic temperature gradient , so the atmosphere becomes colder upwards more slowly than would be expected with a dry adiabatic gradient without the latent heat.
- WEBGEO module: Energy sales at the phase transitions of water - WEBGEO - E-learning portal for geography and related sciences