Gas Law question...

[Ingwe]

Boning up on my gas laws here, which law(s) would state that a gaseous compound is composed of a number of different gases, and given the opportunity these component gases will begin to behave independently?

 

Thanks!

[mississippichem]

The ideal gas law gives a situation where a mixture of gases (not a gaseous compound, is this what you meant?) will behave like a single gas. This is really an outcome of the rather drastic assumptions in the ideal gas law though rather than a valid theoretical insight.

 

In a real gas, dipole moments and Van der Waals interactions will play a role and chemically non-identical gases will have different behavior accordingly. There are a number of ways that you can treat this mathematically, all the practical ones involve empirical parameters that are compound specific. The Van der Waals equation is a great demonstrative example but is not useful without the rather "fudgey" Maxwell correction.

 

See the "virial equation of state" to learn how all these gas laws apply under certain scenarios. You'll learn that all the other gas equations are just approximations that are applicable in some limit (like for example low pressure).

Edited December 26, 2011 by mississippichem

[Ingwe]

Thanks for the quick reply. I am somewhat familiar with the concepts of ideal v. real gases ( somewhat...)but was wondering if Dalton's Law might not apply here somehow.....but I'm not getting it by googling

 

Since total pressure is equal to the sum of the component gases pressures...if left unpressurized would the gases not begin to act independently and stratify by molecular weight??? ( for instance....)

 

Thanks!

[mississippichem]

Thanks for the quick reply. I am somewhat familiar with the concepts of ideal v. real gases ( somewhat...)but was wondering if Dalton's Law might not apply here somehow.....but I'm not getting it by googling

 

Since total pressure is equal to the sum of the component gases pressures...if left unpressurized would the gases not begin to act independently and stratify by molecular weight??? ( for instance....)

 

Thanks!

 

I see your logic, it's a good idea but unfortunately incorrect. Any two gasses or any collection of n-gases will always be miscible and will always mix spontaneously.

 

For an ideal gas the enthalpy of mixing is zero and for real gases it is negligible. The mixing is always spontaneous though as the entropic contribution is positive. You can show this by looking at the number of available microstates before and after mixing, as long as there is a boltzmann distribution IIRC, but really it makes sense intuitively and can be deduced by inspection.

 

Excellent question.

Edited December 26, 2011 by mississippichem

[Ingwe]

OK. Thanks again...I am again somewhat ( operative word) familiar with miscibility, and frankly I think I should have posted this question to more of a fluid dynamics type forum.....but you have been a definite help here, thanks!