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Old Feb 16th 2015, 05:24 PM   #1
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Enthalpy-thermo problem

Hi, I have a practice problem in thermo that I have some difficulties with:

The PVT relation for helium is given by the following equation of state:

P=RT/V-b, where b=2.3*10^5 m^3/mol

Through an isenthalpic (H=0) throttling process, the gas is let to expand from a pressure of 10^8 Pa at 300 K to a pressure of 10^5 Pa. Calculate the final temperature of the gas. Cp canbe considered constant at a value of 5/2R.


I started by using the eq. for enthalpy

dH=0=CpdT + (V-T(dV/dT))dP and finding (dV/dT) from the PVT relation given which resulting in R/P

Replacing R/P in the dH eq. we get:

0=CpdT + (V-T(R/P))dP

Wouldn't this cancel the dP term since TR/P =V.

Or should I not cancel them and just integrate?

I'm a bit confused and could use some help.

Thank you in advance
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Old Feb 17th 2015, 09:31 AM   #2
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Hi,

Looks like you have the answer...

Having 0=CpdT + (V-T(R/P))dP and P=RT/V-b or TR/p = V-b

Then 0=CpdT + b dP

So integrate this.

TSH for PHF
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Old Feb 22nd 2015, 05:22 PM   #3
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Thank you
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