What Happens to Gas in Free Expansion?

[yashar_g]

what would happen to gas in...

Hi there,
I'm working on some experiments regarding to gases. I want to know what'd happen to gas which is uneder high temprature and high pressure when it enteres into vacuum.

it would be very kind of you to answer me in detail.
Many thanks, Yashar.

 

[CompuChip]

First thing that springs to mind is adiabatic expansion (comparable to what happens in a fire extinguisher)... not entirely sure though.

 

[yashar_g]

what would happen to gas in...

Hi there,
I'm working on some experiments regarding to gases. I want to know what'd happen to gas which is uneder high temprature and high pressure when it enteres into space.
is it loose temp.? (adiabatic?)
it would be very kind of you to answer me in detail.

 

[Shooting Star]

(Please don't multi-post the same thing.)

If it's a real gas, it will naturally expand and cool down in the process. If it does not radiate heat, the process should be adiabatic. You have to find the adiabatic law for real gases. Van der Waals' eqn of state may be a good approximation.

 

[Nerd]

As I read your question and the reply I thought of something very interesting. If the hot gas is released into space, we would naturally think that the gas should cool down. However, heat is proportional to the kinetic energy of the gas molecules and since there is no surrounding particles to which the gas molecules can loose their kinetic energy, I would think that it would retain it's temperature. Afterall the only way in which anything can loose heat is by either giving it's kinetic energy to surrounding particles or emmiting it through electromagnetic waves. I don't believe that the gas would start emmiting waves as it is released into space, would it?

 

[dst]

Pressure and temperature are inversely proportional. In vacuum pressure roughly = 0. Gases expand to occupy space. You can work out the rest...

Edit: I think what I wrote is bollocks (haven't slept for about 48 hours). So here's a gas law to cover my arse.

Pressure * volume = number of molecules * gas constant * temperature.

 

[Shooting Star]

As I read your question and the reply I thought of something very interesting. If the hot gas is released into space, we would naturally think that the gas should cool down. However, heat is proportional to the kinetic energy of the gas molecules and since there is no surrounding particles to which the gas molecules can loose their kinetic energy, I would think that it would retain it's temperature. Afterall the only way in which anything can loose heat is by either giving it's kinetic energy to surrounding particles or emmiting it through electromagnetic waves. I don't believe that the gas would start emmiting waves as it is released into space, would it?

For an ideal gas in isolation, your argument is quite correct. However, there are various intermolecular forces in a real gas, which are feeble but attractive when the distances between molecules become large. In order to overcome these forces while expanding, the molecules have to lose a part of their KE, and the gas cools down in the process.

About the loss by EM radiation, that is also bound to happen. If two molecules collide, an electron may be pushed to a higher energy state, and it'll come to the ground state subsequently by emitting a photon.

The a/V^2 term in van der Waals’ eqn of state represents the attractive force, which reduces the observed pressure of a real gas, as compared to an ideal gas.

An ideal gas would not cool down while expanding in space, but any real gas would.

Edit: I think what I wrote is bollocks (haven't slept for about 48 hours). So here's a gas law to cover my arse.

Pressure * volume = number of molecules * gas constant * temperature.

Pardon, your **** is still showing...it's not number of molecules, but number of moles.

 

[russ_watters]

That isn't correct, Shooting Star (though the reasoning for what actually happens is correct). An ideal gas undergoing adiabatic expansion cools down, as the ideal gas equations predict: http://en.wikipedia.org/wiki/Ideal_gas_law
http://en.wikipedia.org/wiki/Ideal_gas_law

Adiabatic cooling occurs when the pressure of a substance is decreased as it does work on its surroundings...

Such temperature changes can be quantified using the ideal gas law...

 

[Shooting Star]

That isn't correct, Shooting Star (though the reasoning for what actually happens is correct). An ideal gas undergoing adiabatic expansion cools down, as the ideal gas equations predict: http://en.wikipedia.org/wiki/Ideal_gas_law

Russ,

An ideal gas expanding in space has no reason to cool down. It does not have any intermolecular force to overcome, nor any work to do by pressing on any outside wall. The average KE per molecule stays constant, since there is no way it can lose heat. (I am, of course, not considering heat loss by radiation, which will happen in a real gas.)

Perhaps I made a mistake by writing the phrase "gas in isolation". I was replyiing to Nerd's comment: "If the hot gas is released into space, we would naturally think that the gas should cool down."

Some further clarification from
http://en.wikipedia.org/wiki/Adiabatic_free_expansion:

Adiabatic free expansion is an irreversible process in which a gas expands without constraint, and during which no heat is exchanged. An example of the process is the release of a gas into a vacuum.

Real gases experience a temperature change (see Joule-Thomson effect) during free expansion. For an ideal gas, the temperature doesn't change, and the conditions before and after adiabatic free expansion satisfy piVi = pfVf, where p is the pressure, V is the volume, and i and f refer to the initial and final states.

 

[russ_watters]

I stand corrected. I didn't think that the "without constraint" part was critical, but I guess it is. If you expand a gas through a nozzle in the atmosphere, the atmosphere will absorb some of the energy of the expansion. In free expansion, the gas will literally expand forever. I'm still not 100% clear on this, though, since the page on the ideal gas law includes adiabatic (but not necessarily free) expansion and a temperature change calculation. I guess with free expansion, though, the volume goes to infinity and the pressure to zero.