# Why does a gas have more potential energy than a liquid and even more than a solid?

Discussion in 'Physics & Math' started by pluto2, Mar 15, 2014.

1. ### pluto2Registered Senior Member

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Why does a gas have more potential energy than a liquid and even more than a solid?

Does this have something to do with internal energy?

3. ### wellwisherBannedBanned

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A gas has higher entropy. Entropy will add additional degrees of freedom to molecules, by the absorption of energy. When we condense the gas into a liquid, the heat of condensation that is given off, reflects, in part, the lowering of entropy and the retrieval of the energy once tied up in the entropy/ The loss of entropy also allows for more order, allowing the EM forces to act more efficiently.

Going from liquid to a solid, does the same thing. The liquid has higher entropy and when it freezes this disorder is lowered. This frees up energy, while the loss of entropy/disorder in the liquid, allows the higher degree of order that is needed to overlap orbitals via the EM force.

Some materials, like CO2, go directly from solid to gas at standard conditions. This is due to spike in entropy that skips the EM attractions of the liquid state, unless we add pressure so we can limit the entropy spike allowing the CO2 to form the liquid.

For numbers; the entropy of vaporization (gas-liquid) of water is 108.951 J mol-1 K-1, while the entropy of fusion (liquid-solid) 22.00 j mol-1 k-1. Once the water releases this energy within the entropy, the molecular disorder goes down, allowing the EM forces to act more efficiently.

5. ### pluto2Registered Senior Member

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894
But why does entropy add more degrees of freedom to molecules by the absorption of energy? And also how exactly is entropy related to potential energy?

7. ### exchemistValued Senior Member

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If you consider the extra energy a gas contains, compared to a liquid at the same temperature, the difference is the Enthalpy of Vaporisation (or Latent Heat of Vaporisation). This can be broken down into two parts:

a) the increase in "Internal Energy", ΔU, which is needed to do work to separate the molecules against the intermolecular attraction between them and
b) the PΔV work needed to push the atmosphere out of the way, to make space for the newly formed gas to occupy.

You can I suppose look on the separation of the molecules as a kind of chemical potential energy, due to the "broken" bonds of van der Waal's attraction, which is released once more when a vapour condenses and the "bonds" are re-formed.

So yes, it is an Internal Energy difference.

8. ### OnlyMeValued Senior Member

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Is this even an accurate statement/question?

It is true that at a given temperature both a liquid and gas have greater kinetic energy, that is the individual atoms and molecules have a greater degree of freedom of movement. But potential, energy is another animal entirely. It can involve more than just the kinetic state of the gas, liquid or solid. Given equal volumes the solid will have a greater mass which would lead to both a greater gravitational energy potential and a greater mass to energy potential than either the gas or liquid. Consider on the last point $E = mc^2$ where the total mass would be considered potential energy.

And then, it will take more energy to raise the temperature of most solids than equal volumes of most gasses or liquids, the added heat then represents a greater potential energy for the solid over the gas or liquid. A hot rock has more heat energy to dissipate than would an equal volume of either a gas or liquid at the same temperature.

For the question to be answered with any clarity more information about what kind of potential energy is required. Many of the answers above seem to be including inherent kinetic energy as potential energy.

The OP seems to me to be somewhat unclear as to the specifics of what is intended by the use of the phrasing, "potential energy".

9. ### exchemistValued Senior Member

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OnlyMe, have a look at my reply. If we say that there is chemical potential energy released when bonds form, then the breaking of van der Waal's attractions could be thought of as adding chemical potential energy. It is an unusual way to express what happens but the internal energy does go up when a liquid evaporates.

I assume, from the context, that pluto2 has chemical potential energy in mind.

10. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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The OP is a good question and exchemist is correct. Exchemist answered the question as a chemist would and I will try to give some insight as a chemical engineer would answer it.

First lets look at the change of enthalpy as temperature changes water from ice to steam.

There is a nice linear relationship between enthalpy (energy of the substance) and temperature for a single phase. For liquid water the increase in the energy is seen as an increase in the vibrations of the water molecules so it is proportional to the temperature increase. This is intuitive because we all know that there is a big difference between putting your hand in 20C water and 90C water. The higher energy content of the hot water will cause damage your to your hand. At the phase changes the energy increases with out a corresponding change in the temperature. As exchemist said this is due to the breaking of the polar bonds that hold the water together when forming steam for example. So this means there is much more energy in a kg of 100C steam than there is in a kg of 100C of water. This is in fact potential energy. Work can be extracted from the steam as it phase changes back to water without having a change in the temperature. The potential energy is extracted from the steam and the steam molecules combine to form liquid water.

Here is a description of the different types of potential energy.

The difference between a kg of steam and a kg of water is pretty amazing. 1 kg of water at 1 atm and 100C contains about 419 kj of energy and 1 kg of steam at 1 atm and 100C contains about 2676 kj of energy. The 2257kj difference is the potential energy that can be extracted with out a temperature change.

Here are steam tables that show this information.

11. ### brucepValued Senior Member

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Great post. Felt like I was reading a training manuel at the refinery. So what industry do you do 'your' chem engineering in?

12. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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I worked in a metal fabrication plant where I was in charge of several Chemical Vapor Depostion reactors and I have recently switched jobs and am now working in a ceramics division on ceramic cement developement.

The bulk of my hands on experince with steam was on board a nuclear submarine before I went to college. I was an enlisted 'nuke' machinist mate, radiation control tech and a radiochemistry tech. Great job except for the whole going to sea and going underwater for months at a time (blech).

13. ### brucepValued Senior Member

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4,098
Thanks for sharing that with me. Living at sea for long periods is something I was familiar with when I was a commercial oilfield [saturation] diver working in the Gulf of Mexico for JR McDermott. Many moons ago in the 70's. I got my steam engineers license [Boiler operation license, LOL] early on in my refining career. Kept it until I retired. Because the sub duty is so long you probably felt like you were born on the sub.

Later

14. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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I have never worked in the oil refining business, but I a make beer and wine, and I have been seriously thinking about making a distilation tower and trying my hand at making vodka. As far as working as a saturation diver I am on the fence. I am torn between thinking whoa what a stud, and thinking are you freaking nuts???