What Is the Joule Thompson Effect and How Is It Calculated for Different Gases?

[Snoop06]

Hey all, I'm not sure this is a homework problem, more a problem I'm having with equations and this effect.

Experimentally we measured dT and dp, I plotted them, then determined the Joule Thompson coefficients for 3 gases (He, CO2, N2). These values came out to be

μJT(CO2) = .815 bar/K
μJT(N2) = .1319 bar/K
μJT(He) = -.0949 bar/K

which seemed reasonable enough to me.

Now, I need to calculate theoretical values using various equations of state. I began with van der Waals, and got

μJT = 1/Cp((2a/RT)-b)

I thought this would be simple, just looking up the Cp, and using known van der Waals coefficients, but my calculation for CO2 comes out to be something like -4 or so, so something is wrong, but I can't figure out what it is. Also, I could not find anywhere theoretical μJT values, which I am supposed to put in my lab report.

Thanks for any tips!

 

[member 553083]

It is possible that the equation you are using to calculate the Joule-Thomson coefficient is not applicable for the particular conditions of your experiment. The ideal gas law equation is used to calculate the Joule-Thomson coefficient for an ideal gas, but in real gases the coefficients can be different. You should look for literature on the Joule-Thomson coefficient specific to the type of gas you are working with and its conditions.