Kinetic explanation of evaporation of boiling/Melting.

In summary, the conversation discusses the concept of latent heat in the kinetic explanation for evaporation and melting. It is explained that the latent heat supplied in causing evaporation represents the potential energy gain of the escaping molecules and the work needed to expand against the atmosphere. Likewise, for melting, the latent heat is regarded as increasing the potential energy of the molecular internal energy. However, it is clarified that at the molecular level, a moving molecule only has kinetic energy and it is at the macroscopic level that this energy can be used to perform work. The conversation concludes with a thank you for the explanation.
  • #1
al_201314
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0
Hi guys

I am slightly confuse about a statement in the kinetic explanation for evaporation as well as melting.

Firstly for the evaporation: "Thus the latent heat supplied in causing evaporation represents the potential energy gain of the escaping molecules plus the work needed to expand against the atmosphere"

I understand that the mean KE drops after some of the molecules go into the air and latent heat is supplied to maintain the mean KE. But how does this extra heat represent the gain in PE of the escaping molecules? My thought is that since the molecules have escaped, how does it gain PE?


Similary, how does the latent heat for melting being regarded as increasing the PE of component of the molecular internal energy? I would have thought that the extra heat would continue to raise the KE to increase the molecules' vibrations for melting.

Thanks for any explanation.
 
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  • #2
al_201314 said:
Hi guys

I am slightly confuse about a statement in the kinetic explanation for evaporation as well as melting.

Firstly for the evaporation: "Thus the latent heat supplied in causing evaporation represents the potential energy gain of the escaping molecules plus the work needed to expand against the atmosphere"

I understand that the mean KE drops after some of the molecules go into the air and latent heat is supplied to maintain the mean KE. But how does this extra heat represent the gain in PE of the escaping molecules? My thought is that since the molecules have escaped, how does it gain PE?
Pressure x volume = (Force / Area) Volume = Force x distance = energy

It is a matter of semantics whether one refers to this energy as Potential Energy or Kinetic Energy. It is kinetic energy at the molecular level. But at the macroscopic level, PV represents stored energy that can be used (not completely) to do work (eg steam engine).

Similary, how does the latent heat for melting being regarded as increasing the PE of component of the molecular internal energy? I would have thought that the extra heat would continue to raise the KE to increase the molecules' vibrations for melting.
You are right if you are looking only at the molecular level. A moving molecule has only kinetic energy.

But at the macroscopic level, the translational Kinetic energy of the gas molecules give them an ability to perform work at the macroscopic level. If the molecules are stuck to each other and are merely vibrating, they do not have (much) ability to do PdV work.

AM
 
  • #3
Thanks for the help!
 

1. What is the kinetic explanation of evaporation?

The kinetic explanation of evaporation is a process in which the molecules of a liquid gain enough energy to break free from the surface and enter the gas phase. This process is driven by the kinetic energy of the molecules, which causes them to move faster and escape into the air.

2. How does boiling occur?

Boiling occurs when the temperature of a liquid reaches its boiling point and the molecules have gained enough energy to overcome the intermolecular forces holding them together. This causes the liquid to change into a gas, or vapor, and escape into the air.

3. What is the role of kinetic energy in melting?

Kinetic energy plays a crucial role in melting as it is the energy that causes the molecules in a solid to vibrate and eventually break free from their fixed positions. As the molecules gain more kinetic energy, they are able to move more freely, causing the solid to change into a liquid state.

4. How does the temperature affect the rate of evaporation and boiling/melting?

The temperature directly affects the rate of evaporation and boiling/melting. As the temperature increases, the molecules gain more kinetic energy and move faster, causing the rate of evaporation and boiling/melting to increase. Conversely, as the temperature decreases, the rate of these processes slows down.

5. What other factors can affect the rate of evaporation and boiling/melting?

Apart from temperature, other factors that can affect the rate of evaporation and boiling/melting include the surface area of the substance, the strength of the intermolecular forces, and the pressure of the surrounding environment. Additionally, the presence of impurities or solutes in the liquid can also affect these processes.

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