# Ideal gas specific heat coefficients

1. Oct 18, 2011

### johnsmith456

Hello,

I'm trying to find the coefficients for the ideal gas specific heat at constant pressure in the form:

Cp = a + bT + cT^2 + dT^3 (kJ/kmol.K)

I need to find these coefficients for Octane. I have found them online in a slightly different form..
Cp = a + bT + cT^2 + dT^3 + eT^4
where I think the units are kJ/kg.K, Its not clear on the website.

I get these values from the hydrocarbons section of this site

http://www.cheric.org/kdb/ [Broken]

Does any body know how to convert, or know a source where I can find these coefficients.

Many thanks guys.

Last edited by a moderator: May 5, 2017
2. Oct 18, 2011

### nasu

For ideal gas the heat capacity is constant.
I suppose you are actually interested in a real gas.
The two "forms" are not different. The second one takes another terms in the power expansion so it is more accurate. I would expect that the coefficient of the T^4 term is (much) smaller than the coefficient of the T^3. So why won't you use the one that goes up to the power 4?

3. Oct 20, 2011

### bbbeard

I suggest you grab a table that covers that desired temperature range from the NIST web site, http://webbook.nist.gov/chemistry/fluid/. This database includes octane. Put the data in Excel or R and do a regression to find the coefficients. BTW the JANAF standard for this kind of cp data includes a term proportional to 1/T2, but not a quartic term.

Last edited by a moderator: May 5, 2017