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I am not even a chem e student. I am a gifted instructor in a public high school. We are trying to work a JETS Challenge problem.

The equation given was Cp = -37357 + 30.529Ө ^0.5 – 4.103Ө² where theta is Kelvin/100.

We were asked to find Cp for carbon dioxide at 350 K. My students and I get the same value, 3.09 KJ/kg-K, but this value is too high when we check handbook sources.

Can you help us?

[Art Montemayor]

Pcroft:

You haven't told us the system pressure; I have to assume you are dealing with atmospheric pressure.

You also haven't told us what handbook value (or source) you are comparing with your calculated value.

When I go to the authoritative source I always go to when dealing with thermo-chemical physical values: http://webbook.nist.gov/chemistry/fluid/

I get 0.89903 J/g*K as the Cp for Carbon Dioxide at 350 oK and 1.0 atmosphere (14.696 psia). This value should be equal to 0.89903 KJ/kg-oK, which are your units.

I haven't used your equation in Excel, but I assume you have no math mistakes. Therefore, your equation is either the wrong relationship, copied wrong (has a typo), or simply does not apply. I challenge its application, based on the results.

I would trust the NIST results before I trust your equation because you haven't offered a source, reference, or the basis for it. NIST is authoritative because it gives you all that and the algorithm for the basic derivations of the related equations.

If you generate a listing of Cp values in the NIST database (for a variety of temperatures) and then regress the data obtained, using a reliable regression program (like DataFit), then you should come up with a reliable equation for the Cp values at different temperatures.

I hope this response helps you and your students out.