Sublimation heat is the heat absorbed by a unit mass of material during sublimation, which is also equal to the sum of melting heat and vaporization heat of the same material under the same conditions.When matter sublimates, particles directly change from lattice structure to gas molecules. Therefore, on the one hand, work must be done to overcome the binding force between particles, and on the other hand, work must be done to overcome the external pressure.Because substances absorb a lot of heat when subliming, they can be used for cooling.For example,dry ice(Solid statecarbon dioxide）It is a widely used refrigerant[1]
In the process of sublimation, on the one hand, particles must overcome the binding force between particles to do work, on the other hand, they must overcome the external pressure to do work.according toconservation of energyAt this time, it must absorb heat from the outside.Therefore, the heat of sublimation is numerically equal to the sum of heat of fusion and heat of vaporization.The relationship is r=λ+L.
In winter, the frozen clothes hanging outdoors will dry, and the camphor balls placed in the suitcase will become smaller and disappear after a long time. These phenomena indicate that substances can be vaporized directly from the solid phase without the liquid phase.The direct transformation of substances from solid phase to gas phase is calledsublimationThe opposite process is called sublimation. For example, frost formed on the ground in winter is ice crystals formed by condensation of water vapor at night.[2]
Principle and application
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The latent heat of phase change absorbed by a substance in the process of sublimation is called sublimation heat. It is the heat absorbed by a substance of unit mass when it transforms from a solid phase to a gas phase under a certain pressure and temperature.During sublimation, solid particles are directly transformed into gas molecules. On the one hand, work must be done against the binding force between solid particles, and on the other hand, work must be done against the external pressure, so the sublimation heat of matter is very large.E.g. solid COtwo(dry ice), at latm, when the temperature is - 78.5 ℃, the sublimation heat is 573 kJ/kg.It can be seen that the sublimation heat of dry ice is very large, and it is often used in food freezing and as a refrigerant for artificial rainfall in industry.One of the measures for modern high-speed aircraft to overcome the "thermal barrier" (the harm caused by the friction between aircraft and air during high-speed flight) is to use the sublimation of graphite coated on the aircraft surface to cool down.
The sublimation heat of crystal and the vaporization heat of liquid also decrease with the increase of temperature.The sublimation heat of crystal is equal toHeat of fusionAndHeat of vaporizationThe sum of.The heat required for the melting of a unit mass of crystalline solids at a certain temperature is called the heat of melting.It is also equal to the heat released by the crystallization of a unit mass of liquid matter into a solid at a constant temperature.The heat required for vaporization (evaporation) of a unit mass of liquid at a certain temperature is called heat of vaporization.For example, the heat of vaporization of water is 607kcal/kg, the heat of fusion is 80 kcal/kg, and the heat of sublimation is 687 kcal/kg.[2]
Sublimation heat of closed container
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If sublimation is carried out in a closed container, after a period of time, sublimation and coagulation reach dynamic equilibrium, at this time, the pressure of solid gas two phase equilibrium coexistence isSaturated vapor pressureThe corresponding temperature is called the condensation temperature.The saturated vapor pressure is related to the temperature. The higher the temperature, the greater the saturated vapor pressure.Generally, the saturated vapor pressure of solids, such as metals, is very low at room temperature. For example, the saturated vapor pressure of aluminum at the melting point of 962 ℃ is only 2 × 10-3MmHg. The saturated vapor pressure of some solid substances is very high. For example, the saturated vapor pressure of dry ice at - 78.5 ℃ is 760 mmHg, and the saturated vapor pressure of iodine at 114 ℃ is 90 mmHg. Compared with metal, the saturated vapor pressure of ice is also relatively high, 4.6 mmHg at 0 ℃, 4.22 mmHg at - 1 ℃, and 1.95 mmHg at - 10 ℃.
It should be pointed out that the saturated steam also needs crystal nucleus in the process of sublimation.The steam that does not crystallize when it is lower than the condensation temperature is called supercooled steam orSupersaturated steamThe supercooled steam is also a metastable state. Once there is a crystal nucleus, it can be condensed into crystals quickly.[2]
Sublimation heat of some substances
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Fresh water ice sublimation heat:
Sublimation heat of fresh water ice[3]
Sublimation heat of trihalide:
Sublimation heat of trihalide[4]
significance
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The discovery and calculation of sublimation heat have helped us to achieve temperature control in science, technology and life, such as practical operations such as aircraft wing deicing, cooling and heating. Using the principle of sublimation heat, the development of science and technology has made more progress, and our science and technology has also made continuous progress.
