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I am a physics and a chemistry student who needs help with a contradictory statement made by my books on the matter.

I was under the impression that internal potential energy of water would increase when it is brought to a boil, as the intermolecular bonds get broken, and thus the stability and forces between the molecules drop, causing an increase in the internal potential energy. My physics book reaffirms this. However, my chemistry books with the same example in mind says that "Since evaporation breaks weak bonds, the water molecules have a lower internal potential energy." This, to me, does not make sense.

Neither my physics teachers, nor my chemistry teachers can agree on the matter at my school. Help is greatly appreciated.

The question word for word is "We heat a tight container with water so that the water evaporates. Has the internal potential energy increased or decreased?"

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  • $\begingroup$ Which Physics /Chemistry books + page references might help? $\endgroup$
    – Farcher
    Nov 23, 2022 at 16:55
  • $\begingroup$ I'm from Norway, so both books would be in norwegian. Basically, the physics book says that "As the distance between the molecules increases, the potential energy increases just as your potential energy in the gravitational field increases when you walk up a flight of stairs. We call the potential energy associated with the bonds between the molecules in a substance internal potential energy. The further apart the molecules are, the weaker the forces between them. It is therefore logical to choose that the potential energy is zero when the molecules are so far apart that no forces act." $\endgroup$
    – Rekeren1
    Nov 23, 2022 at 17:24
  • $\begingroup$ The Physics book sounds right as work needs to be done to break bonds. $\endgroup$
    – Farcher
    Nov 23, 2022 at 22:36

1 Answer 1

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The confusion arises because internal energy and potential energy are two different things. Internal energy is the sum of kinetic and potential energy: $$U = E_\text{K} + E_{P}$$ In liquids the potential energy is negative due to attraction. When a liquid is converted into vapor at the same temperature the kinetic energy stays the same but the potential energy increases from something fairly negative to something fairly close to zero. That's because gas phase molecules interact more weakly due to larger intermolecular distances.

We may conclude that (a) the internal energy of the vapor is higher than that of the liquid , and (b) that the potential energy of the vapor is smaller in absolute value than that of the liquid but higher in true value, because 0 is is smaller in absolute value but larger in true value than a negative number.

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