Boiling point

The boiling point (abbreviation: bp ), evaporation point or boiling point (abbreviation: bp ) of a pure substance is a pair of values ​​in the phase diagram and is composed of two quantities: the saturation temperature (especially the boiling point ), and the saturation vapor pressure (especially boiling pressure ) at the phase boundary between gas and liquid. He therefore made ​​up of the two state variables of pressure and temperature in the transition of a substance from a liquid to a gaseous state.

The boiling point represents the conditions which exist at the phase transition of a substance from a liquid to a gaseous phase, which is referred to as boiling or evaporation. In addition, he is responsible for the reverse process of condensation, but only for pure substances, identical to the condensation point. Upon evaporation of a mixture, there is a change in boiling behavior, and it is observed a boiling range rather than a single boiling point. At a phase transition from a liquid to a gaseous phase downstream of the boiling point is referred to as evaporation.

In table works the boiling temperatures at atmospheric pressure are given, so at 1013.25 hPa This boiling point is called the normal boiling point, the specified boiling temperature than Normalsiedetemperatur ( TSied ). A method for its assessment is the Pailhes method while the Guldberg rule establishes a relationship with the critical temperature. The term boiling point is frequently used as shorthand for the Normalsiedetemperatur and is therefore in common usage usually its synonym is, but this would reduce the boiling point of only a single pair of values ​​and therefore is formally incorrect.

In a pressure cooker to make, for example, exploit the fact that the boiling point and the boiling pressure depend on each other. By increasing the pressure of usually a bar (1000 hPa) is obtained in this way an increase in the boiling temperature of water of 100 ° C to about 120 ° C. Both represent boiling temperature, but only the value of 100 ° C and the boiling temperature under atmospheric pressure and thus the Normalsiedetemperatur. A mixture of the two terms is therefore nonspecific, not self-evident and should be avoided.

• 6.1 Boiling point elevation
• 6.2 boiling ranges 6.2.1 Zeotropic mixtures
• 6.2.2 Azeotropic mixtures

Boiling

Below and above the boiling point of the heating of the liquid or the gas only results in an increase in the temperature. The supplied energy is converted into kinetic energy of the particles. During the phase transition of the liquid to the gas, however, the temperature remains constant, provided that the pressure remains constant. All supplied thermal energy is invested in the change of state.

The boiling point is reached, with further supply of energy, the chemical and physical interactions between the particles to be dissolved - the particles pass into the gas phase. The temperature of the liquid is stagnant, since the thermal energy supplied is used for the complete solution of the intermolecular bonds. The energy that is required when one mole of the substance for this purpose, also referred to as enthalpy of vaporization and their non-substance -related quantitative counterpart as heat of vaporization. Only when all the particles are in the gas phase, the temperature of the system increases again.

Water, hydrogen peroxide or alkali (for example sodium hydroxide solution) with no dust particles or gas bubbles can be heated in a clean vessel and about the boiling point of addition, without causing boiling. Smallest disturbances, such as vibrations, which involve a mix by itself, can lead to an explosive separation of the liquid from the vapor phase, which is called boiling. Because of this, one adds in the chemical liquids that are at risk of a delay in boiling, so-called boiling chips made ​​of clay or pumice which will be unaffected by the chemical, but facilitate the porous structure, the formation of small bubbles so that it does not come to defervescence.

See also: evaporation, gasification, evaporation, transpiration, Pictet- Trouton rule

Boiling point curve

All temperature-pressure pairs of values ​​at the phase boundary gas - liquid in a phase diagram, taken together, give the boiling point curve, there is a thermodynamic equilibrium on it. It refers to the boiling point curve in this case often called boiling curve, boiling, or boiling point curve Siededruckkurve. This curve is limited by two points:

• Triple point Pt: Is the pressure-temperature pair of values ​​lower than the triple temperature or the triple - pressure, it is only a transition between solid and gaseous state, ie a sublimation or Resublimation possible.
• Critical point Pc: If the pressure-temperature value pair is higher than the critical temperature or critical pressure, there is between the density of the liquid and the gaseous state, no difference, which is why they are also no longer separated by a phase boundary, and the fabric therefore referred to as a supercritical fluid in this state.

The balance of the boiling point curve is a dynamic equilibrium. From a liquid occur resistant particles in the gas phase - they evaporate. On the other hand, these particles enter again into the liquid phase - condense. The ratio of the emerging from the liquid phase particles and the re- entering it particles is hereby dependent on both the temperature and the pressure, the higher the temperature is, the more particles evaporate due to its higher speed (see Maxwell -Boltzmann distribution). However, if the vapor pressure, the more particles evaporate, the higher and the more particles also condense. A balance then sets when just about are much more particles in the gas phase, like stepping back again into the liquid phase. In this state, the gas phase is saturated, one speaks then of the saturation vapor pressure. The thermodynamic laws from which the boiling point curve derived quantitatively, is known as the Clausius- Clapeyron equation. For water can this relationship between saturation vapor pressure and saturation temperature also determine the approximate equations of the type of Magnus formula.

Equilibrium change in the example of the water

Example output point: in the steady state there is water in equilibrium with its vapor at the boiling point 80 ° C and a pressure of 474 hPa:

The system's response to the changes of individual state variables amount to a shift of the equilibrium position: It runs one phase transition from amplified, which makes the fault undone (see principle from the smallest coercion).

Material dependence of the boiling point

Examples of normal boiling points of pure substances

Chemical elements

• The lowest Normalsiedetemperatur all elements with -269 ° C has helium, although it has a larger molar mass than hydrogen with a Normalsiedetemperatur of -253 ° C. This is due to the fact that the hydrogen molecule is somewhat easier to polarize as helium, and therefore also forms somewhat stronger Van der Waals interactions.
• The highest Normalsiedetemperatur has rhenium with 5596 ° C.
• A comparison group of inert gases, non-metals, semi-metals and metals is that metals have a much higher boiling point than non-metals, since the metal bond is the strongest bond ( in addition to ions and atomic bonding ). exceptions:

Compounds

One of the lowest Normalsiedetemperaturen has on carbon monoxide with -191.6 ° C, the highest point metal carbides such as titanium (IV ) carbide (TiC, 4820 ° C) and tungsten (IV ) carbide (WC, 6000 ° C).

A special feature is sulfur trioxide ( SO3) before: The melting point of one of its modifications is located at 62.3 ° C. on the Normalsiedetemperatur of 44.8 ° C of the liquid sulfur trioxide.

If the critical pressure is below the atmospheric pressure, it can not be specified Normalsiedetemperatur. In order to bring the liquid to a boil yet, you must do so at a lower pressure. In this case, the boiling pressure must be specified when specifying the boiling temperature, which is another reason to strictly separate the terms Normalsiedetemperatur and boiling point.

If the pressure of the triple point to the normal pressure, the Normalsublimationstemperatur or a boiling temperature is specified at a higher boiling pressure instead of Normalsiedetemperatur. For example, sulfur hexafluoride SF6 sublimates under atmospheric pressure at -63 ° C.

Many, especially organic and all macromolecular compounds decompose when heated prior to reaching the boiling point, since its evaporation enthalpy is greater than the individual binding energies in the molecule. Here you can not specify a boiling temperature, but only the decomposition temperature. However, some may be under reduced pressure, and brought to the boil at a lower temperature thereby.

Homogeneous multi-component systems

The boiling points of homogeneous mixtures such as alloys, gas mixtures or aqueous solutions have compared to the pure substances to changing boiling points and a modified boiling behavior.

Boiling point elevation

If a substance is dissolved in a solvent, then the boiling point of the mixture is increased compared to the pure solvent; is referred to in relation to the saturated vapor pressure of the solution effect. According to Raoult's Law François Marie Raoult (1830-1901), this increase ΔTSdp is proportional to the number of moles of solute:

The individual symbols stand for the following sizes:

• ΔTSdp - boiling point elevation
• Ke - ebullioscopic constant
• B - molality of solute
• K - molar boiling point elevation
• N - amount of substance

The proportionality factor as explained above, the ebullioscopic constant ( and boiling point constant KS), that the change in the boiling point of one kg of the solution of the pure solvent, wherein the molar amount of the dissolved substance is a mole or the molar elevation of boiling point, which is less common, and no information on the mass strikes.

For example, the boiling point rises one kilogram of water by 0.51 K to 100.51 ° C when exactly one mole of any other substance dissolves therein, provided that the substance dissolves in water and is non-volatile. Solving two moles in a kilogram of water, so the water does not boil at 100 ° C 2 × 0.51 ° C = 101.02 ° C.

It is to be observed that salts dissociate in aqueous solution. Sodium chloride, for example, decomposes into the ions Na and Cl-. The boiling point elevation is therefore ( in dilute solutions ) expects twice as high as at first.

A practical example: pasta water has a typical salt content of 10 g / kg. At a molecular weight of 58.4 g / mol corresponding to this, along with the abovementioned doubling, 0.34 mol / kg ions. By the salt concentration thus yields a boiling point increase of only about 0.17 K.

The Raoult's law applies only to "ideal" solutions, which are solutions in which a substance is released only physically. For " non-ideal " solutions occur during mixing energetic phenomena ( heating or cooling ), which caused the formation of hydrogen bonds or by Protolysen. As a result, deviations from Raoult's law arise. Only in very great dilution the formula also applies to " non-ideal " solutions approach, which is why it is called in the case of the ideal solution by an infinitely dilute solution. The boiling point elevation is also a colligative property, and therefore depends on the true number of particles of the solute, but not of the type from. About a changed using the above formula can also be used for molecular weight determination, the boiling point elevation, which is called ebullioscopy.

Likewise, depending on the concentration of solutes is the melting point, which is why it is also called a melting point depression. Cause of these effects is also a lowering of the chemical potential. Combine boiling point elevation and melting point depression, then shows an overall expansion of the thermodynamic state of the liquid region at the expense of the other states of matter.

Boiling ranges

Zeotropic mixtures

A zeotropic mixture of two liquids is heated, it starts to boil at a temperature above the boiling point of the low boiler, ie of that component having the lower boiling point. Upon boiling both components at the same time to go into the gas phase. Here, the low-boiling components in the exit vapor has a higher concentration than corresponds to its concentration in the liquid mixture. Therefore accumulates in the fluid of high-boiling and the boiling temperature rises continuously to the boiling point of the high boiler. One speaks in this case of a boiling range (even boiling range, boiling limit ) of the mixture and not more of a boiling point.

The dependence of the boiling point of the ratio of the liquid components and the respective corresponding relationship in the evaporating gas will be in the Siedediagramm ( right). The blank area in the middle, where neither gas nor liquid can exist is called because of its shape Siedelinse. At the right edge of the boiling range of a solution is marked with 50 % mole fraction.

This behavior is used in technical applications, in order to increase concentrations of the individual components in a mixture by distillation or rectification.

Azeotropic mixtures

In azeotropic mixtures, the boiling temperature of the mixture has an extreme value at a given molar ratio. This value is outside the temperature interval that is spanned by the boiling temperatures of the two pure substances. In this particular mixing ratio a boiling point and boiling range is not available.

Examples:

• Water (bp 100 ° C ) and HCl (bp 83 ° C) - azeotropic mixture with 20.2% HCl: bp 108.6 ° C
• Water (bp 100 ° C ) and ethanol (bp 78.3 ° C) - azeotropic mixture with 96 % ethanol: bp 78.2 ° C

Importance for living things

The boiling behavior of the water leads under the physical conditions on the ground means that water exists in large amounts as liquid. This is one of the basic conditions for the development of living beings.

At a lower air pressure or higher temperatures of the water that would be different and would cause water to evaporate within a very short time and thus also an important condition for life in general, namely, liquid water would be much less common. At a higher air pressure or a lower temperature, however, less and less water can evaporate would, and thus would be a prerequisite for precipitation, namely gaseous water in the atmosphere, increasingly rare, for example, which would entail a limitation of the freshwater resources by itself.

Applications

• Analytical Chemistry: The boiling point is a specific material property. Thus, pure substances can be characterized by their boiling point.
• Distillation or fractional distillation, a method for separating a mixture of substances due to different boiling points of the individual components. The low boiling substance is separated from the higher boiling material by evaporation.
• The ebullioscopy (Latin bulla = Siedeblase, gr skopein = consider ) is a method for determining the molar mass by boiling point elevation. Since boiling point increases would be smaller than freezing point depressions, usually the cryoscopic method is preferred. In both methods is a special thermometer its application, which was developed in 1888 by Ernst Otto Beckmann ( 1853-1923 ): the Beckmann thermometer. It has a scale, which includes only about 6 °, but can be read accurately even at 0.01 degrees. The zero point of the scale can be set to the desired temperature in each case.
• Pressure cooker: If water is heated in a hermetically sealed container, the temperature of the liquid water above 100 ° C may rise because the boiling pressure and thus the boiling point increase. This results in a faster fermentation.
• Altimetry: as the atmospheric pressure decreases with altitude, also decreases the boiling point. As a rule of thumb, the boiling point is lowered per 300 m by about one degree. Thus can be explained by the determination of the boiling point of pure water, the respective amount estimated above mean sea level.

Determination of the boiling points of organic substances

The boiling point is a material property. The knowledge of the boiling point allows conclusions to the present material. In reference works (eg CRC Handbook of Chemistry and Physics or paperback for chemists and physicists ) are tables of boiling points of substances and mixtures. From the table values ​​can be estimated the suspected compounds often.

The boiling point can be used as a criterion of purity of material of a known compound. For distillation separations a Vigreux column is suitable.

For mixtures, it may also happen that several agents at the same boiling distil over, the mixture then forms an azeotrope.

Determination of the boiling point by simple means

For simple investigation of the boiling temperature, you need a test tube (or flask) with a lateral extension tube with pierced rubber stopper and thermometer, a piece of rubber tubing, a glass pipe, a paraffin bath, a heat source and a beaker with refrigerated well. According to the imaging apparatus for determining the boiling point of the corresponding distillation apparatus is set up.

The test tube with the nozzle should be one-third filled with the substance to be examined. To prevent bumping, one adds a few boiling chips added. The lower tip of the thermometer should be placed a few centimeters above the liquid surface. For a more accurate temperature determination, a digital temperature sensor ( accuracy: 0.1 ° C), instead of a mercury thermometer be inserted through the rubber stopper.

For a very accurate determination of the boiling point two possible sources of error must be considered:

• 2, if the air pressure of 760 mm of mercury is different, then a second correction factor to the boiling point must be taken into account. Roughly performs a pressure difference of 0.36 % ( 2.4 mm Hg ) at the normal pressure to a boiling point difference of at least 0.1 ° C.