I need professional help on this physics question.?

Question

Consider an equal mass of water at liquid state and water vapour both at the same temperature (say 300K).
Is the internal energy of the water at liquid state higher than the internal energy of water vapour of the same temperature?
I need clear justification on this.
All i know is, internal energy of a system is defined as the sum of microscope KE and PE of the molecules in the system.
In gas such as water vapour, intermolecular forces of attraction is weak and hence it has low PE.(correct me if i'm wrong)
Since they are at the same temperature, both systems have the same KE?
So my conclusion is water in liquid is at a higher energy state. I'm very skeptical about this conclusion. A gas should be at a higher energy state....

Answer 1

It's good to see people actually thinking about what the terms they're using mean, rather than just plugging them into equations.

Basically, the place where you seem to be confused is here:

"In gas such as water vapour, intermolecular forces of attraction is weak and hence it has low PE"

Actually, this means the exact opposite, that the system has very high potential energy. Think about gravity for a minute - if a rock placed at, say, 1 foot above the ground were to fall to Earth, it would not release very much energy. Conversely, if the rock were placed 1 mile above the Earth, it would release a considerable amount of energy. The higher the rock is, the greater it's potential energy. However, the further the rock is away from Earth, the weaker the force of gravity acting on it - thus weak gravitational attraction corresponds to higher potential energy. Similarly with the particles in a gas - if they were to "fall" into a partially bound state such as that of a liquid, they would release energy, and thus the unbound state where the particles are too far away from each other to form bonds is the higher energy state.

This is not to say that a weaker force always corresponds to a higher energy state - if the reason that the force is weaker is that there is less mass/charge/whatever producing it, that really will lower the potential energy. But in the transition from a liquid to a gas does not change the charge of any of the particles - the reduction in intermolecular forces comes from the fact that the particles are moving out of each other's electrical potential wells, a process which, just like lifting up a rock, raises the potential energy.

Answer 2

Hi,

You make a good point, but a water vapor is not the same as a gas. You said that the water vapor is at 300K, so it's not a gas, it's a suspension of water molecules in air.

As for your assumption that the vapor has low PE. I don't know that is true. Think about this, what happens when you place a piece of glass (or metal, or whatever as long as the temp is the same) inside the vessel that holds the vapor? I'm pretty sure that the vapor would condense into water onto the glass spontaneously which shouldn't happen if water vapor has lower PE than Liquid water.

This is an interesting question, Sorry I don't have more insight, it's been a while since I took thermo.

Good luck,
Matt

Answer 3

The energy of changing the water to vapor can be considered as the energy necessary for each molecule to break surface tension of the liquid gas interphase.
Therefore, the energy is released when the water returns to the liquid state is the source of the missing PE.

Answer 4

yea, according to thermodynamics, a gas should be at a higher energy state since the molecules are more free to move.
the enthalpy of a gas is higher than that of a liquid. use the equation to solve for internal energy (delta E) of a hypothetical substance, and try it for both the liquid and gas.
lol sorry this is as much as i know to answer your question...i just had a test on thermodynamics...but i dont know if this is anything relevant
hope it helps :)

Answer 5

If the vapor came from the water there is a refrigeration effect that takes place. Each cc of water that is changed to vapor there will be 60 calories absorbed. That is how the swamp coolers work in the desert.

Answer 6

Just consider this. Since you have to add heat to water to turn it to a gas or make steam ( heat of vaporization) the water in the gaseous state has more energy.

Answer 7

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