Boiling point - Punto de ebullición

Boiling water

The boiling point of a substance is the temperature at which the vapor pressure of the liquid is equal to the pressure surrounding the liquid and it transforms into vapor. [ 1 ] [ 2 ]

The boiling point of a liquid varies depending on the surrounding ambient pressure. A liquid in a partial vacuum has a lower boiling point than when that liquid is at atmospheric pressure . A high pressure liquid has a higher boiling point than when that liquid is at atmospheric pressure. For example, water boils at 100 ° C (212 ° F) at sea level, but at 93.4 ° C (200.1 ° F) at 1,905 meters (6,250 feet) elevation. For a given pressure, different liquids will boil at different temperatures. [ 3 ]

The normal boiling point (also called atmospheric boiling point or atmospheric pressure boiling point ) of a liquid is the special case where the vapor pressure of the liquid is equal to the defined atmospheric pressure at sea level, 1 atmosphere . [ 4 ] [ 5 ] At this temperature, the vapor pressure of the liquid becomes sufficient to overcome the atmospheric pressure and allow steam bubbles to form within the bulk of the liquid. The standard boiling point has been defined by IUPAC since 1982 as the temperature at which boiling occurs under a pressure of 1 bar. [ 6 ]

Heat of vaporization is the energy required to transform a given quantity (a mole, kg, pound, etc.) of a substance from a liquid into a gas at a given pressure (often atmospheric pressure).

Liquids can transform into vapor at temperatures below their boiling points through the process of evaporation. Evaporation is a surface phenomenon in which molecules located near the edge of the liquid, which are not contained by sufficient liquid pressure on that side, escape to the surroundings as vapor. On the other hand, boiling is a process in which molecules in any part of the liquid escape, resulting in the formation of vapor bubbles within the liquid.

Melting points in blue and boiling points in pink of the first eight carboxylic acids (in ° C ).
Animation of boiling water.

The temperature of a substance or body depends on the average kinetic energy of the molecules . At temperatures below the boiling point , only a small fraction of the molecules on the surface have enough energy to break the surface tension and escape. This increase in energy constitutes a heat exchange that leads to an increase in the entropy of the system (tendency to disorder of the material points that make up its body).

The boiling point depends on the molecular mass of the substance and the type of intermolecular forces of this substance. This requires determining whether the substance is polar covalent, covalent nonpolar, and determine the type of links ( dipole permanent - dipole induced or hydrogen bridges -).

The boiling point cannot be raised indefinitely. As the pressure is increased, the density of the gas phase increases until, finally, it becomes indistinguishable from the liquid phase with which it is in equilibrium; this is the critical temperature, above which there is no clear liquid phase. The helium has the lowest normal boiling point (-268.9 ° C) corresponding to any substance and tungsten , the highest (5930 ° C).

Standard boiling point

In the thermodynamic tables of chemicals, the entire phase diagram is not indicated, only the boiling temperature in the standard state, that is, with a pressure of one atmosphere (1013.25 hPa ). This boiling point is called the Normal Boiling Point and the Normal Boiling Temperature . The term boiling point is often used to refer to the normal boiling point.

The following table shows the boiling temperatures [ 7 ] in the standard state (1 atm ) in ° C:

H
−252,8
He
−268,9
Li
1342
Be
2471
B
4000
C
3825
N
−195,8
O
−183
F
−188,1
Ne
−246.1
Na
882,9
Mg
1090
Al
2519
Si
3265
P
280,5
S
444,6
Cl
−34
Ar
−185.8
K
759
Ca
1484
Sc
2836
Ti
3287
V
3407
Cr
2671
Mn
2061
Fe
2861
Co
2927
Ni
2913
Cu
2562
Zn
907
Ga
2204
Ge
2833
As
616
See
685
Br
58.8
Kr
−153.3
Rb
688
Sr
1382
Y
3345
Zr
4409
Nb
4744
Mo
4639
Tc
4265
Ru
4150
Rh
3695
Pd
2963
Ag
2162
Cd
767
In
2072
Sn
2602
Sb
1587
Te
988
I
184,4
Car
−108.1
Cs
671
Ba
1897
*
Hf
4603
Ta
5458
W
5930
Re
5627
Os
5012
Ir
4428
Pt
3825
Au
2856
Hg
356,6
Tl
1473
Pb
1749
Bi
1564
Po
962
At Rn
−61,7
Fr
677
Ra
1737
**
Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts And
*
La
3464
Ce
3443
Pr
3520
Nd
3074
Pm
3000
Sm
1794
Eu
1529
Gd
3273
Tb
3230
Dy
2567
Ho
2700
Er
2868
Tm
1950
Yb
1196
Lu
3402
**
Ac
3198
Th
4788
Pa
4027
U
4131
Np
4273
Pu
3228
Am
2011
Cm
3100
Bk Cf Is Fm Md No Lr


Saturation temperature and pressure

Demonstration of the lower boiling point of water at a lower pressure, obtained by using a vacuum pump.

A saturated liquid contains as much heat energy as it can without boiling (or, conversely, a saturated vapor contains as little heat energy as possible without condensation).

The saturation temperature means boiling point. Saturation temperature is the temperature for a saturation pressure corresponding to which a liquid boils in its vapor phase. It can be said that the liquid is saturated with thermal energy. Any addition of thermal energy results in a phase transition.

If the pressure in a system remains constant ( isobaric ), a vapor at saturation temperature will begin to condense into its liquid phase as thermal energy (heat) is removed. Similarly, a liquid at saturation temperature and pressure will boil in its vapor phase as additional thermal energy is applied.

The boiling point corresponds to the temperature at which the vapor pressure of the liquid equals the surrounding ambient pressure. Therefore, the boiling point depends on the pressure. Boiling points can be published relative to NIST, US standard pressure of 101,325 kPa (or 1 atm), or IUPAC standard pressure of 100,000 kPa. At higher elevations, where the atmospheric pressure is much lower, the boiling point is also lower. The boiling point increases with increasing pressure to the critical point, where the properties of the gas and the liquid become identical. The boiling point cannot be increased beyond the critical point . Similarly, the boiling point decreases with decreasing pressure until the triple point is reached. The boiling point cannot be lowered below the triple point.

If the heat of vaporization and the vapor pressure of a liquid at a certain temperature are known, the boiling point can be calculated using the Clausius-Clapeyron equation , therefore:

where:
= is the normal boiling point in Kelvin
= Is the gas constant , 8.314 J · K -1 · mol -1
= is the vapor pressure at the given temperature, atm
= is the enthalpy of vaporization , J / mol
= the temperature at which the vapor pressure is measured, K
= is the natural logarithm

The saturation pressure is the pressure for a corresponding saturation temperature at which a liquid boils in its vapor phase. The saturation pressure and the saturation temperature have a direct relationship: as the saturation pressure increases, so does the saturation temperature.

If the temperature in a system remains constant (an isothermal system ), steam at saturation pressure and temperature will begin to condense into its liquid phase as the pressure of the system increases. Similarly, a liquid at saturation pressure and temperature will tend to flash in its vapor phase as system pressure decreases.

There are two conventions regarding the standard boiling point of water: the normal boiling point is 99.97 ° C (211.9 ° F) at a pressure of 1 atm (that is, 101.325 kPa). The IUPAC recommended standard boiling point of water at a standard pressure of 100 kPa (1 bar) is 99.61 ° C (211.3 ° F). . [ 8 ] [ 9 ] As a comparison, at the top of Mount Everest, at an elevation of 8,848 m (29,029 ft), the pressure is about 34 kPa (255 Torr) and the boiling point of water is 71 ° C (160 ° F). . [ 10 ]The Celsius temperature scale was defined until 1954 by two points: 0 ° C was defined by the freezing point of water and 100 ° C was defined by the boiling point of water at standard atmospheric pressure.

Relationship between normal boiling point and vapor pressure of liquids

A table of vapor pressure on a logarithmic scale for various liquids

The higher the vapor pressure of a liquid at a given temperature, the lower the normal boiling point (that is, the boiling point at atmospheric pressure) of the liquid.

The vapor pressure graph to the right has graphs of vapor pressures versus temperatures for a variety of liquids. [ 11 ] When it can be seen on the graph, the liquids with the highest vapor pressures have the lowest normal boiling points.

For example, at any given temperature, methyl chloride has the highest vapor pressure of any of the liquids on the graph. It also has the lowest normal boiling point (−24.2 ° C), which is where the vapor pressure curve of methyl chloride (the blue line) crosses the horizontal pressure line of an atmosphere (atm) vapor pressure. absolute.

The critical point of a liquid is the highest temperature (and pressure) at which it will actually boil.

Element Properties

The element with the lowest boiling point is helium . Both the boiling points of rhenium and tungsten exceed 5000K at standard pressure; Because extreme temperatures are difficult to measure accurately without bias, both have been cited in the literature as having higher boiling points. [ 12 ]

Boiling point as a reference property of a pure compound

As can be seen in the graph above of the logarithm of vapor pressure versus temperature for any chemical compoundGiven pure, its normal boiling point can serve as an indication of the overall volatility of that compound. A given pure compound has only one normal boiling point, if any, and the normal boiling point and melting point of a compound can serve as characteristic physical properties of that compound, listed in reference books. The higher the normal boiling point of a compound, the less volatile the compound in general and, conversely, the lower the normal boiling point of a compound, the more volatile the compound in general. Some compounds break down at higher temperatures before reaching their normal boiling point, or sometimes even their melting point. For a stable compound, the boiling point varies from itstriple pointto its critical point, depending on the external pressure. Beyond its triple point, a compound's normal boiling point, if any, is higher than its melting point. Beyond the critical point, the liquid and vapor phases of a compound merge into a single phase, which can be called superheated gas. At any given temperature, if the normal boiling point of a compound is lower, that compound will generally exist as a gas at external atmospheric pressure. If the compound's normal boiling point is higher, then that compound can exist as a liquid or solid at that given temperature at atmospheric external pressure, and will thus exist in equilibrium with its vapor (if volatile) if its vapors are contained. If the vapors of a compound are not contained,

Boiling points of alkanes , alkenes , ethers , haloalkanes , aldehydes , ketones , alcohols, and carboxylic acids as a function of their molar mass

In general, compounds with ionic bonds have high normal boiling points, if they do not decompose before reaching such high temperatures. Many metals have high boiling points, but not all. Generally, in compounds with covalently attached molecules, as the size of the molecule (or molecular mass) increases, the normal boiling point increases. When the molecular size is converted to that of a macromolecule, polymer or otherwise very large, the compound often decomposes at high temperature before the boiling point is reached. Another factor that affects the normal boiling point of a compound is polarity.of its molecules. As the polarity of a compound's molecules increases, its normal boiling point increases, other factors being the same. It is closely related to the ability of a molecule to form hydrogen bonds (in the liquid state), which makes it difficult for the molecules to leave the liquid state and therefore increases the normal boiling point of the compound. The carboxylic acids simple dimerize by forming hydrogen bonds between molecules. A minor factor that affects boiling points is the shape of a molecule. Making the shape of a molecule more compact tends to slightly lower the normal boiling point compared to an equivalent molecule with more surface area.

Comparison of boiling points of the butane isomer
Common name n-butane isobutano
IUPAC Name butane 2-methylpropano
Molecular Form

Butane-3D-balls.png Isobutane-3D-balls.png
Boiling point (° C)

−0.5 −11.7
Comparison of the boiling points of the pentane isomer
Common name n-pentane isopentano neopentano
IUPAC Name pentane 2-methylbutane 2,2-dimethylpropane

Molecular Form

Pentane-3D-balls.png Isopentane-3D-balls.png Neopentane-3D-balls.png
Boiling point (° C)

36.0 27.7 9.5
Binary boiling point diagram of two hypothetical components that only weakly interact without an azeotrope

Most volatile compounds (anywhere close to room temperature) pass through an intermediate liquid phase as they heat up from a solid phase to eventually transform into a vapor phase. Compared to boiling, a sublimation is a physical transformation in which a solid is turned directly into vapor, which occurs in select cases, such as carbon dioxide at atmospheric pressure. For such compounds, a sublimation point is a temperature at which a solid that turns directly to vapor has a vapor pressure equal to the external pressure.

Impurities and mixtures

In the previous section, the boiling points of the pure compounds were covered. Vapor pressures and boiling points of substances can be affected by the presence of dissolved impurities ( solutes ) or other miscible compounds, the degree of effect depending on the concentration of the impurities or other compounds. The presence of non-volatile impurities such as salts or compounds with a volatility much lower than the compound of the main component reduces its mole fraction and the volatility of the solution and, therefore, raises the normal boiling point in proportion to the concentration of solutes. This effect is called boiling point elevation.. As a common example, salt water boils at a higher temperature than plain water.

In other mixtures of miscible compounds (components), there may be two or more components of varying volatility, each with its own pure component boiling point at any given pressure. The presence of other volatile components in a mixture affects the vapor pressures and therefore the boiling and dew points of all components of the mixture. The dew point is a temperature at which a vapor condenses into a liquid. Furthermore, at any given temperature, the composition of the vapor is different from the composition of the liquid in most of these cases. To illustrate these effects between the volatile components of a mixture, a boiling point diagram is commonly used . the distillation it is a process of boiling and [generally] condensation that takes advantage of these differences in composition between the liquid and vapor phases.

See also

References

  1. Goldberg, David E. (1988). 3,000 Solved Problems in Chemistry (1st edición). McGraw-Hill. section 17.43, p. 321. ISBN 0-07-023684-4.
  2. Theodore, Louis, ed. (1999). Pollution Prevention: The Waste Management Approach to the 21st Century. CRC Press. section 27, p. 15. ISBN 1-56670-495-2.
  3. https://www.engineeringtoolbox.com/boiling-points-water-altitude-d_1344.html
  4. General Chemistry Glossary Purdue University website page
  5. Reel, Kevin R.; Fikar, R. M.; Dumas, P. E.; Templin, Jay M. & Van Arnum, Patricia (2006). AP Chemistry (REA) – The Best Test Prep for the Advanced Placement Exam (9th edición). Research & Education Association. section 71, p. 224. ISBN 0-7386-0221-3.
  6. Cox, J. D. (1982). «Notation for states and processes, significance of the word standard in chemical thermodynamics, and remarks on commonly tabulated forms of thermodynamic functions». Pure and Applied Chemistry 54 (6): 1239. doi:10.1351/pac198254061239.
  7. ^ David R. Lide (June 3, 2009). CRC Handbook of Chemistry and Physics (in English) (90 edition). Boca Raton, CRC Press / Taylor and Francis. p. 2804. ISBN 9781420090840 . Archived from the original on January 10, 2016 . Retrieved February 19, 2016 .
  8. Standard Pressure IUPAC defines the "standard pressure" as being 105 Pa (which amounts to 1 bar).
  9. Appendix 1: Property Tables and Charts (SI Units), Scroll down to Table A-5 and read the temperature value of 99.61 °C at a pressure of 100 kPa (1 bar). Obtained from McGraw-Hill's Higher Education website.
  10. West, J. B. (1999). «Barometric pressures on Mt. Everest: New data and physiological significance». Journal of Applied Physiology 86 (3): 1062-6. PMID 10066724. doi:10.1152/jappl.1999.86.3.1062.
  11. Perry, R.H., ed. (1997). Perry's Chemical Engineers' Handbook (7th edición). McGraw-Hill. ISBN 0-07-049841-5.
  12. DeVoe, Howard (2000). Thermodynamics and Chemistry (1st edición). Prentice-Hall. ISBN 0-02-328741-1.