What is heat in evaporation

Heat of evaporation and heat of condensation

Lexicon> Letter V> Heat of Evaporation and Heat of Condensation

Definition: the amount of heat that is required to vaporize an originally liquid substance or that is released again when it condenses

English: evaporation heat, condensation heat

Categories: Basic Terms, Warmth and Cold

Author: Dr. Rüdiger Paschotta

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Original creation: November 8, 2014; last change: 03/20/2021

URL: https://www.energie-lexikon.info/verdampfungswaerme_und_kondensationswaerme.html

The Heat of evaporation is the amount of heat required to vaporize an originally liquid substance, i.e. H. by boiling or evaporation from the liquid to the gaseous state of aggregation. More precisely, only the proportion of heat is meant that is required for the change in physical state and not for an increase in temperature. So it is latent heat. A large part of the energy required to operate a steam boiler comes from the heat of evaporation.

When the resulting gas is then condensed (liquefied) again, exactly the same amount of energy is released as heat that had to be used for evaporation. The amount of heat of condensation corresponds exactly to the heat of evaporation.

Often one speaks of the specific Heat of evaporation or condensation; this is the amount of heat required or released divided by the amount of the substance, which is usually given as a mass or sometimes also a volume. There is also the molar heat of vaporization, which is related to one mole of a substance. You will therefore find different units such as MJ / kg (megajoules per kilogram), MJ / m3 (Megajoules per cubic meter), kWh / kg and kJ / mol (kilojoules per mole).

Evaporation cold means cooling due to the energy required for evaporation.

Evaporation, i.e. evaporation into an overlying gas phase of another substance (e.g. air), can also occur when a liquid is not supplied with heat from the outside. The heat of evaporation is then taken from the liquid itself and the gas above it. So there is a corresponding cooling, which is also called Evaporation cold referred to as. This effect is used, for example, in wet cooling towers.

Heat of vaporization of water

In energy technology, one has to do particularly frequently with the heat of evaporation of water. It amounts to z. B. at 100 ° C (i.e. when boiling at normal pressure) 40.7 kJ / mol or 2.26 MJ / kg = 0.63 kWh / kg. At 20 ° C (e.g. evaporation at room temperature) it is a little more: 44.2 kJ / mol or 2.46 MJ / kg = 0.68 kWh / kg.

Water has a particularly high enthalpy of vaporization.

Compared to many other liquids, water has a particularly high specific enthalpy of vaporization. This is mainly due to the relatively strong forces of attraction between the strongly polar water molecules. In addition, water molecules are quite light, so that 1 kg of water contains a particularly large number of molecules.

Microscopic explanation of the heat of vaporization; Enthalpy of evaporation

The physical reason that the evaporation requires an energy supply can be explained microscopically by two different contributions:

• The individual atoms or molecules are close to each other as long as the substance is still liquid. Mutual forces of attraction occur here. In order to separate the atoms or molecules from each other, a so-called Severing work be done. This increases the internal energy.
• As a rule, the volume of the substance increases very sharply during evaporation. If this expansion has to take place against an external pressure (for example against atmospheric pressure), the substance does work on the environment. The for this Volume work or Move work The additional amount of energy required is the product of the pressure and the increase in volume (p · ΔV).

The sum of both contributions gives the heat of vaporization. More precisely one speaks here of Enthalpy of evaporation. The term Enthalpy emphasizes that external factors such as the ambient pressure are also taken into account, with the pressure being assumed to be constant. The enthalpy of evaporation is therefore the isobaric heat of evaporation.

How can it be that the separation work and thus the enthalpy of evaporation hardly depend on the external pressure?

Interestingly, the enthalpy of vaporization often hardly depends on the pressure. At higher pressure, there is a correspondingly smaller increase in volume, so that the volume work effectively hardly changes. The separation work is also hardly dependent on pressure. At very high pressures, however, the enthalpy of evaporation decreases and ultimately even becomes zero at the critical point. When evaporating just below the critical point, the difference between the liquid and gaseous phase is already quite small. Above the critical point (in supercritical condition) it is no longer possible to differentiate between liquid and gaseous phases.

In addition, the enthalpy of evaporation depends on the temperature. For example, it is 45.0 kJ / mol for water at 0 ° C, but only 40.7 kJ / mol at 100 ° C. This is because the average distance between the molecules is already somewhat increased at a higher temperature. When approaching the so-called critical point, the enthalpy of vaporization even disappears completely.