Kinetic explanation of evaporation of boiling/Melting.

[al_201314]

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.

 

[Andrew Mason]

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

 

[al_201314]

Thanks for the help!