Latent heat

In thermochemistry, latent heat is the heat released or absorbed by a body during a change of state without change of temperature.[1][2][3][4] The term most often refers to a phase transition such as the melting of ice or the boiling of water.[5][6] The term was introduced around 1750 by Joseph Black. It is derived from the Latin latere (to lie hidden). In its original context of calorimetry set by Black, besides phase changes, the term referred in particular to the heat transferred to a body upon change of volume at constant temperature without phase change.

Thermodynamics

In meteorology, latent heat flux is the flux of heat from the Earth's surface to the atmosphere that is associated with evaporation or transpiration of water at the surface and subsequent condensation of water vapor in the troposphere. It is an important component of Earth's surface energy budget. Latent heat flux has been commonly measured with the Bowen ratio technique, or more recently since the mid-1900s by the eddy covariance method.

Contents

Usage

Two of the more common forms of latent heat (or enthalpies or energies) encountered are latent heat of fusion (melting or freezing) and latent heat of vaporization (boiling or condensing). These names describe the direction of energy flow when changing from one phase to the next: from solid to liquid, and to gas.

In both cases, the change is endothermic, meaning that the system absorbs energy on going from solid to liquid to gas. The change is exothermic (the process releases energy) for the opposite direction. For example, in the atmosphere, when a molecule of water evaporates from the surface of any body of water, energy is transported by the water molecule into a lower temperature air parcel that contains less water vapor than its surroundings. Because energy is needed to overcome the molecular forces of attraction between water particles, the process of transition from a parcel of water to a parcel of vapor requires the input of energy causing a drop in temperature in its surroundings. If the water vapor condenses back to a liquid or solid phase onto a surface, the latent energy absorbed during evaporation is released as sensible heat onto the surface. The large value of the enthalpy of condensation of water vapor is the reason that steam is a far more effective heating medium than boiling water, and is more hazardous.

The terms sensible heat and latent heat are not special forms of energy; instead they characterize the same form of energy, heat, in terms of their effect on a material or a thermodynamic system. Heat is thermal energy in the process of transfer between a system and its surroundings or between two systems with a different temperature.

Both sensible and latent heats are observed in many processes while transporting energy in nature. Latent heat is associated with the phase changes of atmospheric water vapor, mostly vaporization and condensation, whereas sensible heat is energy transferred that affects the temperature of the atmosphere.

History

The term latent heat was introduced into calorimetry around 1750 by Joseph Black. James Prescott Joule characterized latent energy as the energy of interaction in a given configuration of particles, i.e. a form of potential energy, and the sensible heat as an energy that was indicated by the thermometer,[7] relating the latter to thermal energy.

Specific latent heat

A specific latent heat (L) expresses the amount of energy in form of heat (Q) required to completely affect a phase change of a unit of mass (m), usually 1kg, of a substance as an intensive property:

L = \frac {Q}{m}

Intensive properties are material characteristics and are not dependent on the size or extend 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

Q = {m} {L}

where:

Q is the amount of energy released or absorbed during the change of phase of the substance (in kJ or in BTU),
m is the mass of the substance (in kg or in lb), and
L is the specific latent heat for a particular substance (kJ-kgm−1 or in BTU-lbm−1), either Lf for fusion, or Lv for vaporization.

Table of latent heats

The following table shows the latent heats and change of phase temperatures of some common fluids and gases.

Substance Latent Heat
Fusion
kJ/kg		 Melting
Point
°C		 Latent Heat
Vaporization
kJ/kg		 Boiling
Point
°C
Alcohol, ethyl 108 −114 855 78.3
Ammonia 339 −75 1369 −33.34
Carbon dioxide 184 −78 574 −57
Helium     21 −268.93
Hydrogen(2) 58 −259 455 −253
Lead[8] 24.5 327.5 871 1750
Nitrogen 25.7 −210 200 −196
Oxygen 13.9 −219 213 −183
R134a   −101 215.9 −26.6
Toluene   −93 351 110.6
Turpentine     293  
Water 334 0 2260 100

Latent heat for condensation of water

The latent heat of condensation of water in the temperature range from −40 °C to 40 °C is approximated by the following empirical cubic function:

L_{water}(T)=-0.0000614342 T^3%2B0.00158927 T^2-2.36418 T%2B2500.79[9] [J / g]

with a determination coefficient of R^2=0.999988, where T is in °C.

See also

References

  1. ^ Partington, J.R. (1949). An Advanced Treatise on Physical Chemistry, Volume 1, Fundamental Principles. The Properties of Gases, Longmans, Green, and Co., London, pages 155-157.
  2. ^ Prigogine, I., Defay, R. (1950/1954). Chemical Thermodynamics, Longmans, Green & Co, London, pages 22-23.
  3. ^ Adkins, C.J. (1975). Equilibrium Thermodynamics, second edition, McGraw-Hill, London, ISBN 0-07-084057-1, Section 3.6, pages 43-46.
  4. ^ Landsberg, P.T. (1978). Thermodynamics and Statistical Mechanics, Oxford University Press, Oxford, ISBN 0-19-851142-6, page 11.
  5. ^ Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-856552-6. 
  6. ^ Clark, John, O.E. (2004). The Essential Dictionary of Science. Barnes & Noble Books. ISBN 0-7607-4616-8. 
  7. ^ J. P. Joule (1884), The Scientific Paper of James Prescott Joule, The Physical Society of London, p. 274, "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
  8. ^ Textbook: Young and Geller College Physics, 8e, Pearson Education
  9. ^ Cubic fit to Table 2.1,p.16, Textbook: R.R.Rogers & M.K. Yau, A Short Course in Cloud Physics, 3e,(1989), Pergamon press
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