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Offline LHM

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Correct statements about samples of ice and liquid water
« on: January 24, 2011, 10:43:27 PM »
Correct statements about samples of ice and liquid water at 0°C include which of the following?
I. Molecules in ice and liquid water have the same kinetic energy.
II. Liquid water has a greater entropy than ice.
III. Liquid water has a greater potential energy than ice.

The answer is all three of them, and while I see why statement II is correct, I fail to see why statements I and III are correct. I've always been under the impression that solids have more potential energy than liquids, so why is III true? And also, from a previous post: http://www.chemicalforums.com/index.php?topic=46246.0 I thought that the liquid water in statement I would have more kinetic energy than the ice? But why not?

Offline DevaDevil

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Re: Correct statements about samples of ice and liquid water
« Reply #1 on: January 25, 2011, 11:30:32 AM »
(I) is correct, because the root-mean-squared velocity of the molecules is expressed in the temperature, so that the kinetic energy of the molecule is 3/2 kB T. (and T is equal in 0C liquid and 0C gas)

Offline rabolisk

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Re: Correct statements about samples of ice and liquid water
« Reply #2 on: January 25, 2011, 12:49:05 PM »
I've always been under the impression that solids have more potential energy than liquids, so why is III true?

You've been wrong. We've already established that ice at 0 degrees and water at 0 degrees will have the same kinetic energy. But you also know that you have to supply heat or energy to convert ice into water. It only makes sense that this energy is stored as potential energy of water. Thus the potential energy of water > potential energy of ice.

Offline LHM

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Re: Correct statements about samples of ice and liquid water
« Reply #3 on: February 03, 2011, 05:36:34 PM »
(I) is correct, because the root-mean-squared velocity of the molecules is expressed in the temperature, so that the kinetic energy of the molecule is 3/2 kB T. (and T is equal in 0C liquid and 0C gas)

Sorry but what's kb, and why is it the same for molecules in ice and molecules in liquid, but not in gases?

Offline rabolisk

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Re: Correct statements about samples of ice and liquid water
« Reply #4 on: February 03, 2011, 06:06:00 PM »
kb is the Boltzmann constant, and it is universal.

Offline LHM

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Re: Correct statements about samples of ice and liquid water
« Reply #5 on: February 19, 2011, 08:55:58 PM »
I've always been under the impression that solids have more potential energy than liquids, so why is III true?

You've been wrong. We've already established that ice at 0 degrees and water at 0 degrees will have the same kinetic energy. But you also know that you have to supply heat or energy to convert ice into water. It only makes sense that this energy is stored as potential energy of water. Thus the potential energy of water > potential energy of ice.

I know yahoo answers aren't exactly trustworthy, so is this wrong then, or is there something else that I'm missing out on? http://answers.yahoo.com/question/index?qid=20090825121808AAxp8iQ

Offline rabolisk

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Re: Correct statements about samples of ice and liquid water
« Reply #6 on: February 21, 2011, 02:39:26 AM »
That answer is simply wrong. Potential energy for water at 0 degrees is greater than potential energy for ice at 0 degrees. You have to put in energy to convert ice to water at 0 degrees. Since energy is conserved, water must have higher energy than ice. Since they have the same kinetic energy, the only other form of energy (potential energy) has to be greater for water than for ice.

Offline rabolisk

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Re: Correct statements about samples of ice and liquid water
« Reply #7 on: February 21, 2011, 02:52:43 AM »
To go deeper into this, there is this misconception that stability (bonds) mean higher potential energy. People speak of potential energy being "stored" in stable bonds (covalent) or weak bonds (intermolecular attractions). That's why you hear people speak of solid, which has more intermolecular interactions, as having higher potential energy. But this is wrong. There is no potential energy "stored" in such bonds. Bonds, whether strong or weak, intramolecular or intermolecular, are REDUCTION of potential energy relative to free particles. For example, if 2 free oxygen atoms have potential energy of zero (with respect to PE, zero is arbitrary), then O2 molecule has NEGATIVE potential energy, not positive potential energy. This is why breaking a bond always requires energy, and formation of a bond releases energy. If you think of solids as having weak bonds between molecules, then it is clear that solids must have less positive or more negative potential energy than liquids and gases.

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