Why does the pressure-volume-constant of Helium increase?

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Summary:

Why does the pressure-volume-constant of Helium increase with increases of external pressure; when the pressure-volume-constant decreases with increases of pressure for other real gasses?
Im doing a lab on an online software called beyond labs. On this software I am able to test gas laws by adding ideal and real gasses to a balloon in a pressure chamber.

When I am conducting the test I have a consistent temperature of 298K and .300 moles across all the tests; the only variables being manipulated is the pressure as the independent variable, and volume as the dependant variable.

For all my tests on real gasses as the pressure increases the volume decreases; Avargadro's Law: V=kn.
Subsequently, the pressure-volume-constant decreases because real gasses do not act idealistically. however with Helium gas as I increase the pressure, the pressure-volume- constant is on an increasing trend.

Why is this the case?
 

Answers and Replies

  • #2
hutchphd
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It shows up in the Van der Waals modification to the ideal gas law. The two positive coefficients a and b

[P + a(n/V)2] (V/n - b) = RT

take into account interparticle long range attraction and short term repulsion (size effects) respectively. It is not hard to see that they work in different directions on the PV product. For Helium the a is very small,. I think that explains it.
Physically this is because the long distance attraction is electric dipole-dipole and the noble gases are very small and spherical.
 
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  • #3
Borek
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Can't say I ever heard about the "pressure-volume-constant".

Do you mean value of the pressure*volume product? It is constant only for ideal gases, or real gases in the range where they can be treated as ideal.
 
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  • #4
TeethWhitener
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This is a general behavior for real gases. The compression factor:
$$Z=\frac{pV_m}{RT}$$
for real gases generally dips below 1 at low temperatures and reverses direction to rise above 1 at higher temperatures (for an ideal gas, ##Z=1##). The point at which ##Z## re-crosses the value ##Z=1## is known as the Boyle temperature. It’s about 320K for N2 and about 20K for helium. So if you’re doing experiments at 300K, you’ll be (far) above the Boyle temperature for helium but still below the Boyle temperature for a gas like N2.
 
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