Specific Heat Capacity to Calculate K
Specific Heat Capacity to Calculate K
(OP)
Hi,
I'm trying to calculate K using Cp and the equation K=Cp /(Cp-R).
I looked up Cp in the JANAF tables and in a CRC book and the Cp values were listed at 100 KPa. I'm trying to find K at STP does it matter that Cp is taken at 100 and not 101 KPa? How do I convert it or find it at standard atmospheric pressure?
Also, I'm looking to find Cp values for these gases for a range of 50 - 300 F (283-422K). Can anyone recommend any books where I can look these values up? They aren't in my CRC book or in the JANAF tables that I looked at, and my CRC book only goes down to 298 K.
Hexane
Propylene
iso-Butane
n-Butane
iso-Butylene
Butylene
Trans 2 Butene
CIS 2 Butene
iso-Pentane
n-Pentane
Carbon Tetrachloride
Any help would be appreciated.
Thanks much,
Jesse
I'm trying to calculate K using Cp and the equation K=Cp /(Cp-R).
I looked up Cp in the JANAF tables and in a CRC book and the Cp values were listed at 100 KPa. I'm trying to find K at STP does it matter that Cp is taken at 100 and not 101 KPa? How do I convert it or find it at standard atmospheric pressure?
Also, I'm looking to find Cp values for these gases for a range of 50 - 300 F (283-422K). Can anyone recommend any books where I can look these values up? They aren't in my CRC book or in the JANAF tables that I looked at, and my CRC book only goes down to 298 K.
Hexane
Propylene
iso-Butane
n-Butane
iso-Butylene
Butylene
Trans 2 Butene
CIS 2 Butene
iso-Pentane
n-Pentane
Carbon Tetrachloride
Any help would be appreciated.
Thanks much,
Jesse





RE: Specific Heat Capacity to Calculate K
http://webbook.nist.gov/chemistry/
Best regards
Morten
RE: Specific Heat Capacity to Calculate K
Try what MortenA suggests or go use an equation of state combined with the low pressure specific heats to get K.
Regards
RE: Specific Heat Capacity to Calculate K
Cp-Cv = T(∂P/∂T)V(∂V/∂T)P, which for an ideal gas results in Cp-Cv = (TR/V)(R/P) = R.
Applying the Berthelot equation to express the partial differential quotients, neglecting the terms which appear only at very high pressures, one gets
or when using the van der Waals equation,
or alternatively, if you have Cp and the reduced properties Pr and Tr, one article in ChE (March 14,1977) gave for real gases:
then, k = Cp/Cv = Cp/[Cp-(Cp-Cv)]
RE: Specific Heat Capacity to Calculate K
RE: Specific Heat Capacity to Calculate K
What I'm trying to do is to find K for different mixtures of the gases listed above without knowing the pressure of the gas. I initially wanted to go with K=Cp/Cv, but I could not find Cv, so I tried to use k=Cp/Cp-R. I know this equation is only for ideal gases, but will it not work at all or will it just have an error associated with it?
What is the GPSA book that was referred by dcasto?
Thanks again,
Jesse
RE: Specific Heat Capacity to Calculate K
RE: Specific Heat Capacity to Calculate K
Using
Cp-Cv = T(?P/?T)V(?V/?T)P,
when using the van der Waals equation
Can you provide a reference or indicate major assumptions for
Cp-Cv = R + 2aP/RT^2
Regards
RE: Specific Heat Capacity to Calculate K
To sailoday28,
The van der Waals equation is not the most preferred EOS. Anyway, here are the assumptions, as far as I can remember:
The basic equation: (P+an2/V2)(V-nb) = RT
for one mol (n=1): (P+a/V2)(V-b) = RT
(∂P/∂T)V = R/(V-b)
(∂V/∂T)P ~ V/T - b/T +2a/RT2 - 3abP/R2T3
neglecting 3abP/R2T3
T(∂P/∂T)V(∂V/∂T)P ≈ R + 2a(P+a/V2)/RT2 = R + 2aP/RT2 + 2a2/RV2T2
Again, neglecting the last term
Cp-Cv = T(∂P/∂T)V(∂V/∂T)P ≈ R + 2aP/RT2
The CRC manual (77th ed.) gives the values for a (a measure of intermolecular attractions) and for b (a measure of the volume taken up by the molecules themselves) for most of the gases listed by dummiboi, which when knowing the actual system conditions would enable one to see when the above assumptions are acceptable or have just a qualitative value.
RE: Specific Heat Capacity to Calculate K
I agree that VDW is not the most preferred.
I didn't know your assumptions, but since a and b generally would be small compared to specific volume your derivation makes sense.
However,even for low pressures, the approximation may be inaccurate for low temperatures.
Regards