What are 3 examples of evaporation processes

Specific heat of vaporization (latent heat)

The specific heat of evaporation (enthalpy of evaporation) is the heat energy required for evaporation of a liquid per kilogram of the substance!

Evaporation process

If a liquid is heated more and more, the boiling point will be reached at some point. At this point the physical state changes and the liquid finally begins to evaporate (also called Boil designated). No further temperature increase is observed for pure substances during evaporation, despite the heat energy that is still being supplied. During the evaporation, the energy obviously no longer benefits the increase in the kinetic energy of the particles, which would otherwise mean an increase in temperature (see also the article on temperature and particle movement).

In the event of evaporation, the supplied energy leads to an increase in the internal energy in the form of changed binding energies between the liquid and gaseous state. The intermolecular bonds of the liquid state are, so to speak, created by the supplied thermal energy broken up and thus allow the transition to the gaseous state. In the gaseous state, the molecules are only relatively weakly bound to one another due to the lower binding forces.

During the evaporation, energy has to be applied to break the intermolecular bonds. In the case of pure substances, the temperature of the liquid remains constant until the evaporation process is complete!

More detailed information can also be found in the article Why does the temperature remain constant when the physical state changes?

The fact that an evaporation process requires permanent heat to break the intermolecular bonds can be seen, for example, when boiling water. If water is brought to the boil in a saucepan, the water will only evaporate as long as the hotplate remains switched on. However, if the supply of heat is interrupted, the water will also stop boiling.

The question arises as to how much heat has to be added in order to completely evaporate a certain amount of a liquid. The heat required for this is also called Heat of evaporation or Enthalpy of evaporation designated. The amount of heat required for heating to the boiling point is not taken into account in this heat of evaporation. The heat of evaporation therefore only contains the heat energy to be supplied during evaporation when the liquid has already been heated to boiling temperature.

The heat of evaporation (enthalpy of evaporation) is the heat energy to be added to a liquid at the boiling point in order to completely evaporate a certain amount of the substance!

For the difference between the terms heat and enthalpy, see the article Difference between heat of vaporization and enthalpy of vaporization.

Since the supplied heat of evaporation is not directly noticeable as a temperature increase during evaporation, but can still be found in the form of internal energy in the evaporated substance, the heat of evaporation is also known as latent heat. The term “latent” comes from Latin and means “to be hidden” or “not to appear directly”.

Experimental determination of the heat of vaporization

Experimental setup

Using the example of water, the heat of evaporation is to be determined experimentally in the following, which is necessary for the evaporation of a certain amount of water. For this purpose, water is first heated to boiling temperature with an immersion heater. The evaporation of the water mass over time is then observed using a scale on which the test setup is located.

The supplied heat of evaporation can be determined from the electrical output of the immersion heater, which is completely converted into thermal output. The thermal energy Q supplied at a power PV (= Heat of evaporation) results over the operating time t of the immersion heater:

\ begin {align}
\ label {q}
Q_ \ text {V} = P \ cdot t \ [5px]
\ end {align}

Carrying out the experiment

First, the water is heated to boiling temperature with the immersion heater. If the water begins to evaporate, the experiment can now be started at any time. To do this, the scale is reset to zero and the time measurement is started. The water gradually evaporates and the evaporated mass is displayed on the scales. The displayed value of the scales is recorded at regular time intervals. At any point in time t, the heat of vaporization Q supplied up to that point can be calculated using the formula (\ ref {q})V be determined. In this way you get a statement which amount of heat for evaporation which mass mV led.

Trial evaluation

If the evaporated mass is represented as a function of the heat of evaporation (evaporation curve), a proportional relationship is shown. This clearly means that, for example, the evaporation of twice the amount of water also requires twice the heat of evaporation. The evaluation of the experiment shows that evaporation of a water mass of 48 g obviously requires a heat of evaporation of around 120 kJ. With an added thermal energy of around 240 KJ, twice the water mass of 96 g of water has evaporated.

Especially with regard to the comparability of the evaporation heats of different liquids, it therefore makes sense to determine the evaporation heats QV always on a uniform amount of liquid to be evaporated mV available (e.g. 1 kilogram). This constant ratio between the heat of evaporation and the mass to be evaporated is called specific heat of vaporization or as specific enthalpy of vaporization qV designated:

\ begin {align}
& \ boxed {q_ \ text {V} = \ frac {Q_ \ text {V}} {m_ \ text {V}}} ~~~ [q_ \ text {V}] = \ frac {\ text {J} } {\ text {kg}} ~~~~~ \ text {specific heat of vaporization} \ [5px]
\ end {align}

The experiment finally gives a specific heat of vaporization of around 2500 kJ / kg for water. This clearly means that for the evaporation of a water mass of 1 kilogram, a thermal energy of 2500 kJ is necessary. With the experimentally determined heat of evaporation of the water on the basis of the described test setup, however, it must be noted that the heat given off by the immersion heater does not fully benefit the evaporation of the water. The heat is partly used to warm the vessel and thus goes as a Heat loss to the surrounding area. For the pure evaporation of the water, a smaller amount of heat is therefore required than was based on the formula (\ ref {q}). The literature value for the specific heat of vaporization of water is therefore somewhat lower at 2257 kJ / kg.

The specific heat of evaporation is the heat of evaporation to be added per kilogram of a liquid to be evaporated!

conclusion

The specific heat of vaporization qV describes the relationship sought between the mass to be evaporated mV and the heat of vaporization Q to be supplied for thisV:

\ begin {align}
& \ boxed {Q_ \ text {V} = q_ \ text {V} \ cdot m_ \ text {V}} ~~~ \ text {heat of vaporization} \ [5px]
\ end {align}

In the case of water, the heat of evaporation to be added is more than five times the amount of heat that had to be used to heat the water from 0 ° C to 100 ° C. This relatively large heat of vaporization is one of the reasons why a fire can be extinguished with water.

Specific heat of vaporization of selected liquids

If the experiment described above is carried out instead of with pure water with a water-alcohol mixture or with other liquids (e.g. metal melts that are evaporated), it becomes apparent that the substances differ fast evaporate. As a result, more or less heat energy is required to vaporize a certain mass of the respective substance. The specific heat of vaporization is therefore dependent on the substance.

The greater the specific heat of vaporization of a substance, the more heat is required to vaporize a certain mass. Substances with a large specific heat of vaporization do not vaporize in this way when heat is supplied fast. The evaporation curves in the diagram are correspondingly flatter. The table below shows the specific heat of vaporization of selected liquids.

It must be noted that the specific heat of evaporation is indirectly influenced by the external air pressure, as this changes the boiling temperatures (see also the article Why does water boil earlier at high altitudes?)! However, since most of the evaporation processes are carried out at an ambient pressure of 1 bar, the specific heat of evaporation usually relates to the boiling temperature at 1 bar.

Note that the temperature does not remain constant during evaporation with all liquids! For example, petroleum as a mixture of different substances does not evaporate at a fixed boiling point, but within a boiling range. In the case of petroleum, this is between 180 ° C and 330 ° C. The heat supplied during the change in physical state is therefore used both for increasing the temperature and for the evaporation process. This means that it is not possible to precisely assign which proportion of the heat supplied will benefit the temperature increase or evaporation. This means that no (specific) heat of vaporization can be assigned to such a mixture of substances. Generally one occurs Boiling range at Mixtures of substances on, while however Basic substances usually one boiling point exhibit.