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Latent heat (also known aslatent energy orheat of transformation) is energy released or absorbed, by a body or athermodynamic system, during a constant-temperature process—usually afirst-order phase transition, like melting or condensation.
Latent heat can be understood as hidden energy which is supplied or extracted tochange the state of a substance without changing its temperature or pressure. This includes thelatent heat of fusion (solid to liquid), thelatent heat of vaporization (liquid to gas) and thelatent heat of sublimation (solid to gas).[1][2]
The term was introduced around 1762 by Scottish chemistJoseph Black. Black used the term in the context ofcalorimetry where a heat transfer caused a volume change in a body while its temperature was constant.
In contrast to latent heat,sensible heat is energy transferred asheat, with a resultant temperature change in a body.
The termssensible heat andlatent heat refer to energy transferred between a body and its surroundings, defined by the occurrence or non-occurrence of temperature change; they depend on the properties of the body. Sensible heat is sensed or felt in a process as a change in the body's temperature. Latent heat is energy transferred in a process without change of the body's temperature, for example, in a phase change (solid/liquid/gas).
Both sensible and latent heats are observed in many processes of transfer of energy in nature. Latent heat is associated with the change of phase of atmospheric or ocean water,vaporization,condensation,freezing ormelting, whereas sensible heat is energy transferred that is evident in change of the temperature of the atmosphere or ocean, or ice, without those phase changes, though it is associated with changes of pressure and volume.
The original usage of the term, as introduced by Black, was applied to systems that were intentionally held at constant temperature. Such usage referred tolatent heat of expansion and several other related latent heats. These latent heats are defined independently of the conceptual framework of thermodynamics.[3]
When a body is heated at constant temperature by thermal radiation in a microwave field for example, it may expand by an amount described by itslatent heat with respect to volume orlatent heat of expansion, or increase its pressure by an amount described by itslatent heat with respect to pressure.[4]
Latent heat is energy released or absorbed by a body or athermodynamic system during a constant-temperature process. Two common forms of latent heat arelatent heat of fusion (melting) andlatent heat of vaporization (boiling). These names describe the direction of energy flow when changing from one phase to the next: from solid to liquid, and liquid to gas.
In both cases the change isendothermic, meaning that the system absorbs energy. For example, when water evaporates, an input of energy is required for the water molecules to overcome the forces of attraction between them and make the transition from water to vapor.
If the vapor then condenses to a liquid on a surface, then the vapor's latent energy absorbed during evaporation is released as the liquid'ssensible heat onto the surface.
The large value of theenthalpy of condensation of water vapor is the reason that steam is a far more effective heating medium than boiling water, and is more hazardous.
Inmeteorology, latent heat flux is theflux of energy from the Earth's surface to theatmosphere that is associated withevaporation ortranspiration of water at the surface and subsequentcondensation ofwater vapor in thetroposphere. It is an important component of Earth's surface energy budget. Latent heat flux has been commonly measured with theBowen ratio technique, or more recently since the mid-1900s by theeddy covariance method.
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In 1748, an account was published inThe Edinburgh Physical and Literary Essays of an experiment by the Scottish physician and chemistWilliam Cullen. Cullen had used anair pump to lower the pressure in a container withdiethyl ether. No heat was withdrawn from the ether, yet the ether boiled, but its temperature decreased.[5][6] And in 1758, on a warm day inCambridge, England,Benjamin Franklin and fellow scientistJohn Hadley experimented by continually wetting the ball of a mercurythermometer with ether and usingbellows to evaporate the ether.[7] With each subsequentevaporation, the thermometer read a lower temperature, eventually reaching 7 °F (−14 °C). Another thermometer showed that the room temperature was constant at 65 °F (18 °C). In his letterCooling by Evaporation, Franklin noted that, "One may see the possibility of freezing a man to death on a warm summer's day."[8]
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The English wordlatent comes from Latinlatēns, meaninglying hidden.[9][10] The termlatent heat was introduced into calorimetry around 1750 byJoseph Black, commissioned by producers ofScotch whisky in search of ideal quantities of fuel and water for their distilling process to study system changes, such as of volume and pressure, when the thermodynamic system was held at constant temperature in a thermal bath.
It was known that when the air temperature rises above freezing—air then becoming the obvious heat source—snow melts very slowly and the temperature of the melted snow is close to its freezing point.[5] In 1757, Black started to investigate if heat, therefore, was required for the melting of a solid, independent of any rise in temperature. As far Black knew, the general view at that time was that melting was inevitably accompanied by a small increase in temperature, and that no additional heat was needed beyond what this increase in temperature would require in itself. Soon, however, Black was able to show that much more heat was required during melting than could be explained by the increase in temperature alone.[11][12] He was also able to show that heat is released by a liquid during its freezing; again, much more than could be explained by the decrease of its temperature alone.[13]
Black would compare the change in temperature of two identical quantities of water, heated by identical means, one of which was, say, melted from ice, whereas the other was heated from merely cold liquid state. By comparing the resulting temperatures, he could conclude that, for instance, the temperature of the sample melted from ice was 140 °F lower than the other sample, thus melting the ice absorbed 140 "degrees of heat" that could not be measured by the thermometer, yet needed to be supplied, thus it was "latent" (hidden). Black also deduced that as much latent heat as was supplied into boiling the distillate (thus giving the quantity of fuel needed) also had to be absorbed to condense it again (thus giving the cooling water required).[14]
In 1762, Black announced the following research and results to a society of professors at the University of Glasgow.[15] Black had placed equal masses of ice at 32 °F (0 °C) and water at 33 °F (0.6 °C) respectively in two identical, well separated containers. The water and the ice were both evenly heated to 40 °F by the air in the room, which was at a constant 47 °F (8 °C). The water had therefore received 40 – 33 = 7 “degrees of heat”. The ice had been heated for 21 times longer and had therefore received 7 × 21 = 147 “degrees of heat”.[a] The temperature of the ice had increased by 8 °F. The ice had thus absorbed 8 “degrees of heat”, which Black calledsensible heat, manifest as a temperature increase, which could be felt and measured. In addition to that, 147 – 8 = 139 “degrees of heat” were absorbed aslatent heat, manifest as phase change rather than as temperature change.[16][17]
Black next showed that a water temperature of 176 °F was needed to melt an equal mass of ice until it was all 32 °F. So now 176 – 32 = 144 “degrees of heat” seemed to be needed to melt the ice. The modern value for the heat of fusion of ice would be 143 “degrees of heat” on the same scale (79.5 “degrees of heat Celsius”).[18][15]
Finally, Black increased the temperature of a mass of water, then vaporized an equal mass of water by even heating. He showed that 830 “degrees of heat” was needed for the vaporization; again based on the time required. The modern value for the heat of vaporization of water would be 967 “degrees of heat” on the same scale.[19]
Later,James Prescott Joule characterised latent energy as the energy of interaction in a given configuration of particles, i.e. a form ofpotential energy, and the sensible heat as an energy that was indicated by the thermometer,[20] relating the latter tothermal energy.
Aspecific latent heat (L) expresses the amount of energy in the form of heat (Q) required to completely effect a phase change of a unit of mass (m), usually1kg, of a substance as anintensive property:
Intensive properties are material characteristics and are not dependent on the size or extent of the sample. Commonly quoted and tabulated in the literature are the specific latent heat of fusion and the specific latent heat of vaporization for many substances.
From this definition, the latent heat for a given mass of a substance is calculated by
where:
The following table shows the specific latent heats and change of phase temperatures (at standard pressure) of some common fluids and gases.[citation needed]
| Substance | SLH of fusion (kJ/kg) | Melting point (°C) | SLH of vaporization (kJ/kg) | Boiling point (°C) | Boiling point (K) | SLH of Sublimation (kJ/kg) |
|---|---|---|---|---|---|---|
| Ethyl alcohol | 108 | −114 | 855 | 78.3 | 351.45 | |
| Ammonia | 332.17 | −77.74 | 1369 | −33.34 | 239.81 | |
| Carbon dioxide | 184 | −78 | 574 | −78.46 | 194.69 | 570[21] |
| Helium | 21 | −268.93 | 4.22 | |||
| Hydrogen(2) | 58 | −259 | 455 | −253 | 20.15 | |
| Lead[22] | 23.0 | 327.5 | 871 | 1750 | 2023.15 | |
| Methane | 59 | −182.6 | 511 | −161.6 | 111.55 | |
| Nitrogen | 25.7 | −210 | 200 | −196 | 77.15 | |
| Oxygen | 13.9 | −219 | 213 | −183 | 90.15 | |
| RefrigerantR134a | −101 | 215.9 | −26.6 | 246.55 | ||
| RefrigerantR152a | −116 | 326.5 | −25 | 248.15 | ||
| Silicon[23] | 1790 | 1414 | 12800 | 3265 | 3538.15 | |
| Toluene | 72.1 | −93 | 351 | 110.6 | 383.75 | |
| Turpentine | 293 | 154 | 427.15 | |||
| Formic Acid[24] | 275.46 | 8.35 | 1010 | 100.75 | 373.9 | 1300 |
| Water | 334 | 0 | 2264.705 | 100 | 373.15 | 2840 |
The specific latent heat of condensation of water in the temperature range from −25 °C to 40 °C is approximated by the following empirical cubic function:
where the temperature is taken to be the numerical value in °C.
Forsublimation anddeposition from and into ice, the specific latent heat is almost constant in the temperature range from −40 °C to 0 °C and can be approximated by the following empirical quadratic function:
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As the temperature (or pressure) rises to thecritical point, the latent heat of vaporization falls to zero.
I am inclined to believe that both of these hypotheses will be found to hold good,—that in some instances, particularly in the case of sensible heat, or such as is indicated by the thermometer, heat will be found to consist in the living force of the particles of the bodies in which it is induced; whilst in others, particularly in the case of latent heat, the phenomena are produced by the separation of particle from particle, so as to cause them to attract one another through a greater space., Lecture on Matter, Living Force, and Heat. May 5 and 12, 1847
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