Why is there a difference in kinetic energy among water molecules?

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Discussion Overview

The discussion centers on the differences in kinetic energy among water molecules, particularly focusing on the distribution of kinetic energy and the behavior of surface molecules compared to those in the bulk. The scope includes theoretical considerations and conceptual clarifications regarding molecular motion and energy distribution.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that there is always a distribution of energies among molecules due to constant collisions and energy exchanges, which is not unique to water.
  • One participant questions the assertion that surface molecules have more kinetic energy than bulk molecules, suggesting that surface molecules might actually have lower temperatures due to evaporative cooling.
  • Another participant argues that the energy dynamics at the surface involve surface energy, noting that while there is a difference in total energy between surface and bulk molecules, the average kinetic energy does not significantly depend on their location.
  • There is a suggestion that the movement hindrance of bulk water molecules affects energy transfer to surface molecules, potentially influencing their kinetic energy.

Areas of Agreement / Disagreement

Participants express differing views on the kinetic energy of surface versus bulk water molecules, with no consensus reached on whether surface molecules have higher or lower kinetic energy. The discussion remains unresolved regarding the implications of surface energy on kinetic energy distribution.

Contextual Notes

Some assumptions about the relationship between kinetic energy and temperature are not fully explored, and the discussion does not resolve the complexities of molecular interactions at the surface versus in the bulk.

Elsa1234
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why's there unequal distribution of kinetic energy in water molecules?
 
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Because that's the way it is - there is always some distribution of energies of molecules, it doesn't hold for water only. Molecules collide all the time, partially exchanging their kinetic energies. Some get faster, some get slower in the process. Total energy stays constant, but individual speeds/energies don't.

See for example https://en.wikipedia.org/wiki/Maxwell–Boltzmann_statistics
 
Borek said:
Because that's the way it is - there is always some distribution of energies of molecules, it doesn't hold for water only. Molecules collide all the time, partially exchanging their kinetic energies. Some get faster, some get slower in the process. Total energy stays constant, but individual speeds/energies don't.

See for example https://en.wikipedia.org/wiki/Maxwell–Boltzmann_statistics
Why do the surface molecules have more kinetic energy than the other molecules?
 
Why do you think they do?
 
Elsa1234 said:
Why do the surface molecules have more kinetic energy than the other molecules?

Is this even true in the first place? If anything would expect surface molecules to have a lower temperature because of evaporative cooling.
 
Borek said:
Why do you think they do?
I think because the lower bulk of water is not free to move around and due to movement hindrance by the upper molecules their energy gets transferred to the surface.
 
Nope - if anything, the more pronounced effect will be the one suggested by Ygg.

There exists a phenomenon called surface energy, and yes, the total energy of a molecule on the surface is slightly different from the total energy of a molecule in the bulk. But the difference is pretty small and not directly related to the kinetic energy of the molecules. As a first approximation (and a quite good one), average kinetic energy of the molecule doesn't depend on whether it is in the bulk or on the surface.
 

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