Enthalpy of vaporization

The heat of evaporation or vaporization ΔQv is the amount of heat required to bring a certain quantity of a liquid from a liquid to a gaseous state ( evaporation ) without changing the temperature. The heat of evaporation is an important parameter in the boiling point curve. When one speaks of evaporation heat of evaporation.

The usually quite high amount of heat is applied technically to cool.

In the reverse process (condensation) this amount of heat is released as condensation heat exactly.

Separation work

Need for the transition from liquid to gaseous state - even if the liquid is already at boiling point - energy should be supplied. This separation work serves to overcome the attractive forces between the liquid particles. The energy applied is not lost due to the energy conservation law, but will become a part of the energy contained in the inner gas and

When liquefied noble gases, the separation work is smallest, since only van der Waals forces must be overcome in other liquids are added dipole or hydrogen bonding. Even higher heat of vaporization is the metals (strong metallic bond ) and the salts at the highest due to the relatively extremely strong ionic bond.

Example: In order to evaporate a kilogram of water at 100 ° C and 1013 mbar, the separation work is spend DELTA.U = 2088 kJ. The separation work is relatively high for water because of the hydrogen bonds between water molecules.

Shift work in the isobaric case

In addition, the amount of the supplied heat of evaporation depends on the process conditions. Takes the vaporization or evaporation isobaric at a constant pressure P, as is often the case, it is necessary, the resulting gas to expand the volume of fluid Vf, the gas volume VG, to the external pressure P, the displacement work p * (VG -VF ) = p afford? V. The supplied energy is thus consumed both for separation work as well as for shift work: ΔQv = DELTA.U p ·? V.

Example: At 100 ° C and 1013 mbar, a kilogram of water in the liquid state has a volume of 1.04 dm3 and in the gaseous state a volume of 1,673 m3. The water concentration in the gas phase is at 100 ° C then 598 g / m³. (see vapor-pressure curve )

The volume increase during evaporation is therefore 1,672 m3 and the work done in the expansion against the external air pressure shift work 169 kJ. The supplied under isobaric conditions at 100 ° C and 1013 mbar per kg water evaporation heat is therefore ΔQv = DELTA.U p ·? V = 2088 kJ 169 kJ = 2257 kJ = 2.26 MJ.

Under other conditions, such as Evaporation into vacuum evaporation at constant volume, etc. are other laws.

Heat of vaporization and enthalpy of vaporization

The state variable H = U p · V is formed from the state variables U, p and V is called enthalpy. U, p and V change by the amounts DELTA.U, and Ap? V, then H changes by the amount AH = DELTA.U V p · · Ap? V. If the pressure remains, as in the case considered here, constant, then AH = DELTA.U p ·? V.

In the isobaric case is supplied and distributed to separation and shift work energy ΔQv = DELTA.U p · DELTA.V thus equal to the enthalpy change of the system

And enthalpy of vaporization? Hv is then called.

Using the formula character ΔQv emphasized that the power supply is done in the form of heat, using the formula character? Hv emphasized that the state variable enthalpy of the system is changed.

Numerous daily evaporation and evaporation processes take place under isobaric conditions, because the relevant systems are exposed to atmospheric air pressure. The expended heat of vaporization is then a particular enthalpy of vaporization and is tabulated under that name for many substances.

The substance-specific enthalpy of evaporation depends on the temperature, but not from the external air pressure. Table values ​​are usually found for the boiling temperature of the substance (vapor pressure of the substance is then 1013 mbar). For any temperature, the molar enthalpy of vaporization can the measured vapor pressure ( of the substance to be distilled ) are calculated with the relationship between the Clausius - Clapeyron (saturation vapor pressure).

Temperatures calculated enthalpy of vaporization for water

The molar enthalpy of vaporization ( in kJ / mol) can be converted into the specific enthalpy of vaporization ( in kJ / g) by placing them by the molar mass ( here: 18.02 g / mol for water) shares.

The molar enthalpy of vaporization can ( 0-200 ° C ) can be calculated using the following empirical relationship in the temperature range 273- 473K:

Heat of condensation

Condenses the gas under the same conditions again, the vaporizing heat of vaporization expended in the form of identical magnitude condensation heat is also released. One speaks then clear from the fact that this energy was stored in the form of not sensible latent heat in the gas. This expression is however misleading, because the heat of vaporization during the evaporation in part converted into internal energy and is discharged in part as a mechanical energy to the environment. None of these forms of energy is heat.

Heat of sublimation

For substances that sublimate ( phase change from solid to gaseous form, for example: iodine), one speaks of a heat of sublimation, which takes into account in addition to the heat of vaporization and the heat of fusion of the substance.

Heat of vaporization

Vaporizes a liquid into the gas phase of another substance as a result of falling below its saturation vapor pressure in the gas phase, it is called evaporation instead of evaporation, eg in water in the gas mixture of air. An evaporation takes place, if there is no evaporation heat is supplied from the outside, since it is driven by the evaporation of the molecules associated with the entropy. The heat of vaporization of the liquid is then withdrawn, so you yourself also called the process of evaporative cooling.

Applications

One application is the evaporative cooling of thermally loaded components.

The liquid cooling by evaporation, for example, the basic function of a cooling tower. In a widely used type ( heated ) cooling water is fed from the top into the cooling tower. Tennis Ball Large Keramikfüllkörper in the cooling tower create a large surface area over which the water down into a catch basin " trickles ". Before reaching the tailings blowing large fans from below ambient air into the cooling tower. Considerable amounts of water are evaporated and cooled the cooling water. The main quantity of the necessary heat of vaporization comes from the incoming heated cooling water, only a small subset is supplied externally of the fan motors and electrical energy outside air.

On the principle of evaporation also based

• Normally operated " Evaporative refrigerators "
• Wine Cooler
• The cooling of the human body, for example, by the evaporation of sweat.

Overview of vaporization of the chemical elements

Specific enthalpy of vaporization? Hv [ kJ / g] and the molar enthalpy of vaporization [ kJ / mol] of the pure chemical elements for the boiling temperature of the element and a pressure of 1013 hPa All data were taken from the respective data overviews of these elements in detail.

Main group elements:

Subgroup elements: (all consistent with the data mentioned in the individual elements, the specific enthalpy of vaporization was calculated from the molar enthalpy of vaporization, and apply at the boiling point of the elements. )