Melting point - Punto de fusión

Ice cubes placed in the water will reach 0 ° C melting.

The melting point (or, rarely, the liquefaction point ) is the temperature at which a substance changes from a solid to a liquid state . [ 1 ] At the melting point, the solid and liquid phase exist in thermodynamic equilibrium . The melting point of a substance is pressure dependent and is generally specified at normal pressure, such as 1 atmosphere or 100 kPa .

When considered as the temperature of the reverse change from liquid to solid, it is known as the freezing point or the crystallization point . Due to the ability of some substances to supercool, the freezing point is not considered a characteristic property of a substance. When determining the "characteristic freezing point" of a substance, in fact, the actual methodology is almost always "the principle of observing disappearance rather than ice formation", that is, the melting point. [ 2 ]

Examples

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

For most substances, the melting and freezing points are approximately the same. For example, the melting point and freezing point of mercury is 234.32 kelvins (−38.83 ° C or −37.89 ° F). [ 3 ] However, certain substances have different solid-liquid transition temperatures. For example, agar melts at 85 ° C (185 ° F) and solidifies from 31 ° C (88 ° F; 304 K); such direction dependence is known as hysteresis . The melting point of ice at 1 atmosphere pressure is very close [ 4 ]At 0 ° C (32 ° F; 273 K); This is also known as the ice point. In the presence of nucleating substances, the freezing point of water is not always the same as the melting point. In the absence of nucleators, water can exist as a subcooled liquid to −48.3 ° C (−55 ° F, 224.8 K) before freezing.

The chemical element with the highest melting point is tungsten, at 3414 ° C (6177 ° F; 3687 K); [ 5 ] This property makes tungsten excellent for use as filaments in light bulbs. The frequently cited coal does not melt at ambient pressure, but sublimates at about 3,726.85 ° C (6,740.33 ° F; 4,000.00 K); Only one liquid phase exists above pressures of 10 MPa (99 atm) and is estimated to be 4,030–4,430 ° C (7,290–8,010 ° F; 4,300–4,700 K) (see carbon phase diagram). Tantalum Hafnium Carbide (Ta 4 HfC 5 ) is a refractory compound with a very high melting point of 4215 K (3942 ° C, 7128 ° F). [ 6 ]At the other end of the scale, helium does not freeze at all at normal pressure, even at temperatures close to absolute zero; A pressure of more than twenty times normal atmospheric pressure is necessary.

List of common chemicals
Chemical [ Note 1 ] Density ( g/cm³) Melting Point ( K ) [ 7 ] Boiling point ( K )
Water 1 0 degrees Celsius (273.2 K) 100 degrees Celsius (373.2 K)
Solder (Pb60Sn40) 183 degrees Celsius (456.2 K)
Cacao butter 34.1 degrees Celsius (307.3 K) -
Paraffin wax 0.9 37 degrees Celsius (310.2 K) 370 degrees Celsius (643.2 K)
Hydrogen 0.00008988 14.01 20.28
Helio 0.0001785 - [ note 2 ] 4.22
Beryllium 1.85 1560 2742
Carbon 2.267 3800 4300
Nitrogen 0.0012506 63.15 77.36
Oxygen 0.001429 54.36 90.20
Sodium 0.971 370.87 1156
Magnesium 1.738 923 1363
Aluminum 2.698 933.47 2792
Sulfur 2.067 388.36 717.87
Chlorine 0.003214 171.6 239.11
Potassium 0.862 336.53 1032
Titanium 4.54 1941 3560
Iron 7.874 1811 3134
Nickel 8.912 1728 3186
Copper 8.96 1357.77 2835
Zinc 7.134 692.88 1180
Galio 5.907 302.9146 2673
The payment 10.501 1234.93 2435
Cadmium 8.69 594.22 1040
Indio 7.31 429.75 2345
Iodine 4.93 386.85 457.4
Tantalum 16.654 3290 5731
Tungsten 19.25 3695 5828
Platinum 21.46 2041.4 4098
Oro 19.282 1337.33 3129
Mercury 13.5336 234.43 629.88
Lead 11.342 600.61 2022
Bismuth 9.807 544.7 1837

Melting temperature of chemical elements

The following table shows the melting temperatures of the elements in ° C (atmosphere pressure): [ 8 ]

H
−259
He
−272
Li
181
Be
1287
B
2075
C
3500
N
−210
O
−219
F
−219
Ne
−249
Na
98
Mg
650
Al
660
Si
1414
P
44
S
115
Cl
−102
Ar
−189
K
64
Ca
842
Sc
1541
Ti
1668
V
1910
Cr
1907
Mn
1246
Fe
1538
Co
1495
Ni
1455
Cu
1085
Zn
420
Ga
30
Ge
938
As
817
Se
221
Br
−7
Kr
−157
Rb
39
Sr
777
Y
1522
Zr
1858
Nb
2477
Mo
2623
Tc Ru
2333
Rh
1964
Pd
1555
Ag
962
Cd
321
In
157
Sn
232
Sb
631
Te
450
I
114
Car
−112
Cs
29
Ba
727
*
Hf
2233
Ta
3017
W
3422
Re
3185
Os
3033
Ir
2446
Pt
1768
Au
1064
Hg
−39
Tl
304
Pb
327
Bi
271
Po
254
At
302
Rn
−71
Fr
27
Ra
696
**
Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts And
*
La
920
Ce
799
Pr
931
Nd
1016
Pm
1042
Sm
1072
Eu
822
Gd
1313
Tb
1359
Dy
1412
Ho
1472
Er
1529
Tm
1545
Yb
824
Lu
1663
**
Ac
1050
Th
1750
Pa
1572
U
1135
Np
644
Pu
640
Am
1176
Cm
1345
Bk
986
Cf
900
Es
860
Fm
1527
Md
827
No
827
Lr
1627


Melting point measurements

Kofler bench with samples for calibration.

There are many laboratory techniques for the determination of melting points. A Kofler bench is a metal strip with a temperature gradient (from room temperature to 300 ° C). Any substance can be placed in a section of the strip, revealing its thermal behavior at the temperature at that point. The differential scanning calorimetry provides information on the melting point along with its melting enthalpy .

Automatic digital melting point meter

A basic apparatus for melting point analysis of crystalline solids consists of an oil bath with a transparent window (the most basic design: a Thiele tube ) and a simple magnifying glass. The various grains of a solid are placed in a thin glass tube and partially immersed in the oil bath. The oil bath is heated (and shaken) and with the help of the magnifying glass (and the external light source) the melting of the individual crystals can be observed at a certain temperature. In large / small devices, the sample is placed in a heating block and the optical detection is automated.

Measurement can also be performed continuously with an operational process. For example, oil refineries measure the freezing point of diesel fuel online, which means that the sample is taken from the process and automatically measured. This allows for more frequent measurements as the sample does not have to be collected manually and taken to a remote laboratory.

Techniques for refractory materials

For refractory materials (e.g. platinum, tungsten, tantalum, some carbides and nitrides, etc.), the extremely high melting point (generally considered to be above 1800 ° C) can be determined by heating the material in a black body furnace and black body temperature measurement with a pyrometeroptical. For higher fusion materials, this may require extrapolation by several hundred degrees. The spectral luminosity of an incandescent body is known to be a function of its temperature. An optical pyrometer combines the brightness of a body under study with the brightness of a source that has been previously calibrated based on temperature. In this way, measurement of the absolute magnitude of radiation intensity is unnecessary. However, known temperatures should be used to determine the calibration of the pyrometer. For temperatures above the source's calibration range, an extrapolation technique should be employed. This extrapolation is achieved using Planck's radiation law. The constants in this equation are not known precisely enough, which causes errors in extrapolation to get larger at higher temperatures. However, standard techniques have been developed to perform this extrapolation.

Consider the case of using gold as the source (mp = 1063 ° C). In this technique, the current through the pyrometer filament is adjusted until the intensity of light from the filament matches that of a black body at the melting point of gold. This sets the primary calibration temperature and can be expressed in terms of current through the pyrometer lamp. With the same current settings, the pyrometer is seen in another black body at a higher temperature. An absorbent medium of known transmission is inserted between the pyrometer and this black body. The temperature of the black body is adjusted until there is a match between its intensity and that of the pyrometer filament. The true highest blackbody temperature is determined from Planck's Law. The absorbent medium is then removed and the current through the filament is adjusted to match the intensity of the filament with that of the black body. This establishes a second calibration point for the pyrometer. This step is repeated to bring the calibration to higher temperatures. Now, the temperatures and their corresponding pyrometer filament currents are known and a temperature versus current curve can be drawn. This curve can be extrapolated at very high temperatures. The temperatures and their corresponding pyrometer filament currents are known and a temperature versus current curve can be drawn. This curve can be extrapolated at very high temperatures. The temperatures and their corresponding pyrometer filament currents are known and a temperature versus current curve can be drawn. This curve can be extrapolated at very high temperatures.

To determine the melting points of a refractory substance by this method, it is necessary to have black body conditions or to know the emissivity of the material being measured. Containment of high-melting material in a liquid state can introduce experimental difficulties. The melting temperatures of some refractory metals have been measured by observing radiation from a black body cavity in solid metal samples that were much longer than they were wide. To form such a cavity, a hole perpendicular to the long axis is drilled in the center of a rod of the material. These rods are heated by passing a very large current through them, and the radiation emitted from the hole is observed with an optical pyrometer. The melting point is indicated by the darkening of the hole when the liquid phase appears, destroying the black body conditions. Today, containerless laser heating techniques are employed, combined with fast pyrometers and spectrometers, to allow precise control of how long the sample is held at extreme temperatures. Such experiments lasting less than one second address several of the challenges associated with more traditional melting point measurements performed at very high temperatures, such as vaporization of the sample and reaction with the container. to allow precise control of how long the sample is held at extreme temperatures. Such experiments lasting less than one second address several of the challenges associated with more traditional melting point measurements performed at very high temperatures, such as vaporization of the sample and reaction with the container. to allow precise control of how long the sample is held at extreme temperatures. Such experiments lasting less than one second address several of the challenges associated with more traditional melting point measurements performed at very high temperatures, such as vaporization of the sample and reaction with the container.

Thermodynamics

Pressure dependence of the melting point of water.

For a solid to melt, heat is required to raise its temperature to the melting point. However, more heat needs to be supplied for fusion to take place: this is called heat of fusion and is an example of latent heat.

From the point of view of thermodynamics, at the melting point, the change in the Gibbs free energy (G) of the material is zero, but the enthalpy (H) and entropy (S) of the material are increasing (ΔH, ΔS> 0). The phenomenon of fusion occurs when the Gibbs free energy of the liquid becomes lower than the solid for that material. At various pressures this happens at a specific temperature. It can also be shown that:

 

Here T, ΔS and ΔH are respectively the melting point temperature, the melting entropy change and the melting enthalpy change.

The melting point is sensitive to extremely large changes in pressure, but in general this sensitivity is an order of magnitude less than that of the boiling point, because the solid-liquid transition represents only a small change in volume. [ 9 ] [ 10 ]If, as observed in most cases, a substance is denser in the solid than in the liquid state, the melting point will increase with increasing pressure. Otherwise the reverse behavior occurs. In particular, this is the case for water, as graphically illustrated to the right, but also for Si, Ge, Ga, Bi. With extremely large changes in pressure, substantial changes in melting point are observed. For example, the melting point of silicon at ambient pressure (0.1 MPa) is 1415 ° C, but at higher pressures than 10 GPa at 1000 ° C decreases [ 11 ]

Melting points are often used to characterize organic and inorganic compounds and to determine their purity. The melting point of a pure substance is always higher and has a smaller range than the melting point of an impure substance or, more generally, of mixtures. The greater the amount of other components, the lower the melting point and the wider the melting point range, often referred to as the "pasty range." The temperature at which melting begins for a mixture is known as " solidus ", while the temperature where melting is complete is called " liquidus.". Eutectics are special types of mixtures that behave like single phases. They melt abruptly at a constant temperature to form a liquid of the same composition. Alternatively, cooling a liquid with the eutectic composition will solidify as uniformly dispersed mixed crystals, small (fine-grained) with the same composition.

Unlike crystalline solids, glasses do not have a melting point; on heating they undergo a smooth transition from glass to a viscous liquid. For further heating, they gradually soften, which can be characterized by certain softening points.

Freezing point depression

The freezing point of a solvent is lowered when another compound is added, which means that a solution has a lower freezing point than a pure solvent. This phenomenon is used in technical applications to prevent freezing, for example by adding salt or ethylene glycol to the water.

Carnelley's rule

In organic chemistry , Carnelley's rule , established in 1882 by Thomas Carnelley, states that high molecular symmetry is associated with a high melting point . [ 12 ] Carnelley based his rule on examining 15,000 chemical compounds. For example, for three structural isomers with molecular formula C 5 H 12 , the melting point increases in the series isopentane −160 ° C (113 K) n-pentane −129.8 ° C (143 K) and neopentane −16.4 ° C ( 256.8 K) [ 13 ] Similarly, in xylenes and also in dichlorobenzenes, the melting point increases in the order meta, ortho and then para. The pyridine has a lower symmetry than benzene , hence its lower melting point, but the melting point increases again with diazine and triazine . Many cage-like compounds such as adamantane and highly symmetrical Cuban have relatively high melting points.

A high melting point results from a high heat of fusion, a low entropy of fusion, or a combination of both. In highly symmetric molecules, the crystalline phase is densely packed with many efficient intermolecular interactions that result in a greater enthalpy change in the fusion.

Prediction of the melting point of substances (Lindemann criterion)

An attempt to predict the bulk melting point of crystalline materials was first made in 1910 by Frederick Lindemann . [ 14 ] The idea behind the theory was the observation that the average amplitude of thermal vibrations increases with increasing temperature. Fusion begins when the amplitude of the vibration becomes large enough that adjacent atoms partially occupy the same space. The Lindemann criterion states that fusion is expected when the root mean square amplitude of vibration exceeds a threshold value.

Assuming all atoms in a crystal vibrate with the same frequency ν, the average thermal energy may be estimated using equipartition theorem as [ 15 ]

where m is the atomic mass , v is the frequency, u is the mean vibration amplitude, k B is the Boltzmann constant, and T is the absolute temperature. If the threshold value of u 2 is c 2 a 2 , where c is the Lindemann constant and a is the atomic spacing, then the melting point is estimated as

Other expressions can be obtained for the estimated melting temperature depending on the estimate of the average thermal energy. Another commonly used expression for the criterion is Lindemann [ 16 ]

From the expression for the Debye frequency for ν, we have

where θD is the Debye temperature and h is Planck's constant . Values ​​of c range from 0.15–0.3 for most materials. [ 17 ]

Melting point prediction

In February 2011, Alfa Aesar released more than 10,000 compound melting points from its catalog as open data. This data set has been used to create a random forest model for melting point prediction [ 18 ] which is now freely available [ 19 ] . Open melting point data is also available from Nature Precedings . [ 20 ] Tetko et al. [ 21 ] published high-quality data from patents and also models [19] developed with these data. [ 22 ]

See also

Grades

  1. Z is the standard symbol for the atomic number; C is the standard symbol for heat capacity; and χ is the standard symbol for electronegativity on the Pauling scale.
  2. Helium does not solidify at the pressure of an atmosphere. Helium can only solidify at pressures above 25 atmospheres, which corresponds to a melting point of absolute zero.

References

  1. ^ Nemer, Beatriz Virginia Cervantes (2006). Pedagogical manual of general chemistry practices on a microscale . Ibeoamerican University. ISBN 978-968-859-594-7 . Retrieved November 26, 2019 .
  2. Ramsay, J. A. (1949). «A new method of freezing-point determination for small quantities». J. Exp. Biol. 26 (1): 57-64. PMID 15406812.
  3. Haynes, p. 4.122.
  4. The melting point of purified water has been measured as 0.002519 ± 0.000002 °C, see Feistel, R.; Wagner, W. (2006). «A New Equation of State for H2O Ice Ih». J. Phys. Chem. Ref. Data 35 (2): 1021-1047. Bibcode:2006JPCRD..35.1021F. doi:10.1063/1.2183324.
  5. Haynes, p. 4.123.
  6. Agte, C.; Alterthum, H. (1930). «Researches on Systems with Carbides at High Melting Point and Contributions to the Problem of Carbon Fusion». Z. Tech. Phys. 11: 182-191.
  7. Holman, S. W.; Lawrence, R. R.; Barr, L. (1 de enero de 1895). «Melting Points of Aluminum, Silver, Gold, Copper, and Platinum». Proceedings of the American Academy of Arts and Sciences 31: 218-233. doi:10.2307/20020628.
  8. David R. Lide (2009). CRC Press Inc, ed. CRC Handbook of Chemistry and Physics (en inglés) (90 edición). p. 2804. ISBN 978-1-420-09084-0.
  9. The exact relationship is expressed in the Clausius @ –Clapeyron relationship .
  10. «J10 Heat: Change of aggregate state of substances through change of heat content: Change of aggregate state of substances and the equation of Clapeyron-Clausius». Consultado el 19 de febrero de 2008.
  11. ^ Tonkov, E. Yu. And Ponyatovsky, EG (2005) Phase Transformations of Elements Under High Pressure , CRC Press, Boca Raton, p. 98
  12. Brown, R. J. C.; R. F. C. (2000). «Melting Point and Molecular Symmetry». Journal of Chemical Education 77 (6): 724. Bibcode:2000JChEd..77..724B. doi:10.1021/ed077p724.
  13. Haynes, pp. 6.153@–155.
  14. Lindemann FA (1910). «The calculation of molecular vibration frequencies». Phys. Z. 11: 609-612.
  15. ^ Sorkin, S., (2003), Point defects, lattice structure, and fusing, Thesis, Technion, Israel.
  16. Philip Hofmann (2008). Solid state physics: an introduction. Wiley-VCH. p. 67. ISBN 978-3-527-40861-0. Consultado el 13 de marzo de 2011.
  17. Nelson, DR, (2002), Defects and geometry in condensed matter physics , Cambridge University Press,
  18. ^ Bradley, JC. And Lang Un.SID (2011) Random Forest model to cast point prediction. onschallenge.wikispaces.com
  19. Forecast to melt point of SMILES. Qsardb.org. Retrieved on September 13, 2013.
  20. ONS Opens Merge Point Collection. Precedings.nature.com. Retrieved on September 13, 2013.
  21. Tetko, Igor V; m. Lowe, Daniel; Williams, Antony J (2016). «The development of models to predict melting and pyrolysis point data associated with several hundred thousand compounds mined from PATENTS». Journal of Cheminformatics 8. doi:10.1186/s13321-016-0113-y.
  22. OCHEM Melting point models. ochem.eu. Retrieved June 18, 2016.

Bibliography

  • Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd edición). CRC Press. ISBN 1439855110.

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