Latent heat

Latent heat ( " latent" Latin for "hidden" ) refers to the output or input at a phase transition of first order amount of energy (heat). It is called latent because thereby the temperature does not change. By contrast, ie, the applied amount of energy for a temperature increase sensible heat.

Examples

• Phase transition liquid ↔ gas: By the boiling point of heated water has taken sensible heat by then. Upon further heat the water is not hot, but evaporated. As the water vapor has more energy than before in liquid form, although the steam is hotter. It is this hidden energy release it again ( under liquefaction) without cooling it. The same amount of heat is called upon evaporation heat of vaporization, upon liquefaction heat of condensation.
• During the transition fixed ↔ liquid is called the latent heat depending on the direction of heat of fusion and heat of crystallization.

The amount of heat depends on the nature of the substance and of the phase transition, and of course on the amount of the substance. Based on the amount the amount of heat is tabulated as specific latent heat - for elements, see the Periodic Table of Elements.

Units

• The latent heat of a given substance quantity than the energy unit joule.
• The object relating to the mass -specific latent heat has the unit joule / kg.
• On the amount of substance -related ( " molar latent heat" ) is the unit joules / mole.

Cause

The molecular structure of substances provides the explanation of why, despite the heat transport no temperature change takes place. When applied to the heat of vaporization means: The molecules of a liquid are much closer together than in a gas. During evaporation, consequently, the distance between the molecules have to be increased, which is accompanied by an increase in the potential energy ( E pot ). The necessary work is done by the amount of heat supplied. A positive change in temperature corresponds, however, according to the kinetic theory of gases, an increase in the kinetic energy ( Ekin ) of the molecules, which is not associated directly with an increase in distance. The same reasoning also applies to the heat of fusion and the heat of crystallization.

Starts a crystalline substance to melt, then rapidly approaching a new, and indeed the most probable state. This is the macrostate with the largest number of different possible arrangements of the particles ( micro-states ). From the energetic point of view, the divisions between Ekin and E pot is not fixed. But in a state of higher potential energy to the grid blocks can be removed from their places. This enables a very large number of new spatial arrangements and makes it more likely state. More energy supplied is converted into potential so long, so long, this new micro-states can be formed. Since only bumps can transfer energy to a thermometer, the kinetic energy during the melting but does not increase the temperature remains constant.

Importance

The latent heat mainly plays an important role in the field of meteorology, with respect to the phase transitions of water in the atmosphere. On a wet surface, or even water surface, much of the sun's energy is invested in the evaporation of water. Here, about 2450 kilojoules per kilogram of water are reacted at 20 ° C. A change in the air temperature does not occur here, the energy is thus stored in the gaseous state as it were, of the water. Since this storage is reversible, the same amount of energy is released when a rising air parcel reaches the condensation level and the water vapor condenses. Originally on the ground provided by the sunlight energy is therefore at greater heights free again where it contributes to an increase in temperature. This results in the formation of a feuchtadiabatischen temperature, the atmosphere is colder so slowly upward, as without the latent heat would be expected with a trockenadiabatischen gradient. The ratio of sensible to latent heat is the Bowen ratio.

Latent energy is the cause of the extremely high thermal conductivity of a heat pipe.