Latent heat: melting, vaporization, solidification, condensation

The latent heat is one that is not “sense” because it represents the heat which 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.

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

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

Sensitive heat refers to heat that is “ felt” or measured in the process through changes in body temperature. On the other hand, latent heat refers to the moment when energy is absorbed or released without generating changes in temperature.

Latent heat of fusion

Melting 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 in enthalpy which results from the absorption of energy and which causes the substance in question to pass from the solid phase to the liquid phase at constant pressure.

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

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

This enthalpy change can be applied to any amount of substance that achieves fusion, however small, and is a constant value (the same amount of energy) that is expressed in kJ / kg units when you want to query mass units

It is always a positive amount, except in the case of helium, which means that helium freezes on absorption of heat. The latent heat of the melting value of water is 333.55 kJ/kg.

Latent heat of vaporization

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

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 temperature dependent, although it can be assumed that it remains constant over small temperature ranges and at temperatures well below one.

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

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

The first term (energy increase) will be the energy needed to overcome the intermolecular interactions that exist in the liquid, where substances with higher strengths between bonds (water, for example) will have higher latent heat of vaporization (2257 kJ / Kg ) than than those with little strength 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 earlier, molecules of a substance in the liquid phase have greater internal energy than solid molecules, so solidification releases energy rather than absorbing it, as in fusion.

So, 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 value of the latent heat of fusion of water is 333.55 kJ / kg, the value of the latent heat of solidification or freezing of 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.

As far as the energy of each molecule is concerned, in gases it is even greater than in liquids, so that an energy release also occurs when passing from the first phase to the second.

Related:   Fermionic Condensate: properties, applications and examples

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 condensation value for water will be equal to -2257 kJ / Kg.

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

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