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Internal energy is summation of potential and kinetic energies of a substance

Using this interpretation of internal energy, how can I explain boiling?

Kinetic remains constant. Potential should decrease because of an increase in average spacing. But internal energy should increase.

Can somebody explain what flaw I am making?

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  • $\begingroup$ You forgot about entropy... $\endgroup$
    – Zhe
    May 9 '17 at 12:27
  • $\begingroup$ Well I am not taught how to explain changes in internal with respect to entropy. I am just taught that if I can denote changes in kinetic and potential, I can work out changes in internal. I know this can also be explained by thermodynamic law, but I am not interested in that. $\endgroup$ May 9 '17 at 12:28
  • $\begingroup$ Well, this process is dominated by entropy, so you not going to understand it without considering entropy. Enthalpy is only half the picture. $\endgroup$
    – Zhe
    May 9 '17 at 12:32
  • $\begingroup$ Can you please then write an answer which includes entropy(I have studied the concepts of entropy so I will be able to understand the answer)? And can you please also relate why internal energy of an ICE cube increases even though volume is decreasing?(maybe this confusion is arising due to the fact that I am taught potential energy in terms of average separation instead of strength of electrostatic attraction) $\endgroup$ May 9 '17 at 12:35
  • $\begingroup$ You can do this in terms of enthalpy, too. How many hydrogen bonds do you need to break to vaporize water? $\endgroup$
    – Zhe
    May 9 '17 at 14:41
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You are wrong when you say the potential energy of particles decrease when they move apart.

The forces between the particles are attractive not repulsive.

Recall the definition of potential energy. It is the work done by external force on a charged particle under influence of electric field.

Therefore, the potential energy for particles, which attract each other, increases when they move apart.

So when to particles get close their potential energy decreases, hence their stability increases. This also happens to be one of the reasons why they come closer.

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  • $\begingroup$ Can you please also relate why internal energy of an ICE cube increases even though volume is decreasing?(maybe this confusion is arising due to the fact that I am taught potential energy in terms of average separation instead of strength of electrostatic attraction) $\endgroup$ May 9 '17 at 13:05
  • $\begingroup$ Ice is less dense than water. Ice has quite some space between its molecules and when it melts the space between the molecules decrease. Perhaps this could be the answer. Water molecules attract each other by H-bonding. $\endgroup$
    – Mitchell
    May 9 '17 at 14:04
  • $\begingroup$ Yes my question is exactly this. You stated increase in potential is explained by an increase in average separation. Here however, the average spacing is decreasing yet potential is increasing. Why is this happening? $\endgroup$ May 9 '17 at 14:44
  • $\begingroup$ There are other factors at work. Potential energy cannot be held accountable for every observation we make. This is more related to thermodynamics and out of the scope of the main question. You should consider asking another question. $\endgroup$
    – Mitchell
    May 9 '17 at 14:57
  • $\begingroup$ So you're saying that Internal energy is not equal to only potential and kinetic energy? $\endgroup$ May 9 '17 at 14:58

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