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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: Pearin on July 21, 2009, 02:03:37 AM

Title: Viscosity of nitrogen and helium
Post by: Pearin on July 21, 2009, 02:03:37 AM

Hello everyone,

i have a question regarding the viscosity of nitrogen and helium.

From the data i have, i found that viscosity of helium is greater than nitrogen at varies temperature. However, when comparing the size and the interaction between their respective molecules, nitrogen has the greater size and stronger interaction.

Can anyone help me on this problem?

Thanks alot.

Kind regards.

Title: Re: Viscosity of nitrogen and helium
Post by: Johnny010 on August 17, 2009, 05:15:10 PM

Well...assuming these were even remotely comparable:

The electron 'cloud' of N is much larger and has a much greater repulsion than that of He(2 electrons).
Thus the repulsion of N would be much greater so the density (from the van der waals gas equation) would be lower ie. less viscous.

Thats my thought anyway, it make sense.
Have a look at the Van der Waals gas eqation (it basically takes in to account the area taken up by the actual molecules and also the repulsion between them to gain a more 'real' gas set of equations).

Title: Re: Viscosity of nitrogen and helium
Post by: renge ishyo on August 17, 2009, 06:12:22 PM

It depends on whether you are comparing liquid nitrogen to liquid helium or gaseous nitrogen to gaseous helium. In general, the rules for viscosity are slightly different depending on which phase you are considering. For instance, in the liquid phase intermolecular interactions are important so that the viscosity tends to *decrease* as the temperature is raised. On the other hand, in the gas phase the intermolecular interactions don't seem to matter, and it is the velocity or degree of motion of the molecules that causes the viscosity of a gas to *increase* with increasing temperature. And there are situations that are sort of in between the two extremes where the two tendancies compete with each other more or less.

My guess is the confusion is arising by comparing apples to oranges. For instance, comparing the viscosities of liquid nitrogen to gaseous helium at a given temperature would be pointless as the two viscosities are in general related to two completely different sets of molecular phenomena. In one instance the viscosity of liquid nitrogen would be getting lower with increasing temperature and in the other case the viscosity of the gaseous helium would be getting higher as the temperature increased.

That's my answer anyways. Hope it helped.