Latent Heat: Fusion, Vaporization, Solidification and Condensation

He latent heat is one that does not"feel", because it represents the thermal energy that is released or absorbed during a phase change, without increasing or decreasing the temperature of the thermodynamic system. There are several types of latent heat, which are governed by the phase changes of a substance.

The types of latent heat are the latent heat of fusion, vaporization, solidification and condensation. In other words, these values ​​are the units of heat per mass that are required to achieve the phase change. In the field of thermodynamics, the study of heat transfer and thermal effects is common.

Latent heat

These effects are involved in any process, even in those that occur at a constant temperature. The two types of heat that can be transferred to a body or substance and to the surrounding environment during a process are then observed, which are governed by the individual properties of the substance involved: the heat sensitive and the heat latent .

The sensible heat refers to the heat that is" feel" or measured in the process through changes in body temperature. In contrast, latent heat refers to the moment in which energy is absorbed or released without generating changes in temperature.

Index

  • 1 Latent heat of fusion
  • 2 Latent heat of vaporization
  • 3 Latent heat of solidification
  • 4 Latent heat of condensation
  • 5 References

Latent heat of fusion

Fusion is a physical process that is represented as the phase transition of a substance from solid to liquid. Therefore, the latent heat of fusion of a substance, or enthalpy of fusion, is the change of enthalpy that results from the absorption of energy and that leads the substance in question to pass from solid phase to liquid phase at constant pressure.

The temperature at which this transition occurs is called the melting temperature, and the pressure is assumed to be 1 atm or 101 325 kPa, depending on the system worked.

Thanks to the difference in the intermolecular forces, the molecules in a liquid phase have a higher internal energy than a solid, so the solids require a positive energy (absorb heat) to melt them and reach the liquid, while the liquids must release heat to freeze (solidify).

This enthalpy change can be applied to any amount of substance reaching the meltdown, no matter how small, and is a constant value (the same amount of energy) that is expressed in units of kJ / kg when you want to refer to units of dough.

It is always a positive quantity, except in the case of helium, which means that helium freezes with the absorption of heat. The latent heat fusion value for water is 333.55 kJ / Kg.

Latent heat of vaporization

Also called enthalpy of vaporization, is the amount of energy that must be added to a substance in liquid phase for it to make a transition to the gas phase. This value is a function of the pressure at which the transformation occurs.

It is usually associated with the normal boiling point of a substance, that is, the boiling point it has when the vapor pressure of the liquid is equal to the atmospheric pressure at sea level (1 atm).

The heat of vaporization is dependent on temperature, although it can be assumed that it remains constant at low temperature ranges and at temperatures much lower than one.

In addition, it is important to note that the heat of vaporization is decreasing at high temperatures, until reaching the so-called critical temperature of the substance, where they are equated. Beyond the critical temperature, the vapor and liquid phases become indistinguishable, and the substance passes into a state of supercritical fluid.

Mathematically, it is expressed as the increase in energy of the vapor phase compared with the energy in the liquid phase, plus the work that must be applied against the atmospheric pressure.

The first term (increase of energy) will be the energy that will be required to overcome the intermolecular interactions that exist in the liquid, where those substances with higher forces between bonds (water, for example) will have higher latent heats of vaporization (2257 kJ / Kg). ) than those with little force between their links (21 kJ / Kg).

Latent heat of solidification

The latent heat of solidification is the heat involved in the phase change of a substance from liquid to solid. As mentioned above, the molecules of a substance in the liquid phase have greater internal energy than the solid ones, so in the solidification the energy is released instead of absorbing it, as in the fusion.

Then, in a thermodynamic system it can be said that the latent heat of solidification is the opposite of that of fusion, since the energy involved is released to the outside when the phase change occurs.

That is, if the latent heat value of the water melt is 333.55 kJ / Kg, then the latent heat value of solidification or freezing of the water will be -333.55 kJ / Kg.

Latent heat of condensation

The latent heat of condensation is that which occurs when there is a phase change from a gaseous substance to a liquid, as in the case of water vapor.

Regarding the energy of each molecule, in gases this is even greater than in liquids, so there is also a release of energy when going from the first phase to the second.

Again, it can be said that the value of the latent heat of condensation will be the same as that of vaporization but with a negative value. Then, a latent heat value of condensation for water will be equal to -2257 kJ / Kg.

At higher temperatures, the heat of condensation will decrease, while the boiling point will increase.

References

  1. Wikipedia. (s.f.) Retrieved from en.wikipedia.org
  2. Smith, J.M., Van Ness, H.C., & Abbott, M.M. (2007). Introduction to Chemical Engineering Thermodynamics. Mexico: McGraw-Hill.
  3. Levine, I. (2002). Physical Chemistry Madrid: McGraw-Hill.
  4. Power, N. (s.f.). Nuclear Power. Retrieved from nuclear-power.net
  5. Elert, G. (s.f.). The Physics Hypertextbook. Retrieved from physics.info


Loading ..

Recent Posts

Loading ..