# Question about JANAF tables

1. Jan 30, 2008

### engineer23

All the JANAF tables I have present values for 1 atm. But what if the reactants or products are at a temperature other than 1 atm (say 10 atm or .5 atm)? Is there a way to scale the JANAF tables for different pressures?

2. Jan 31, 2008

### Astronuc

Staff Emeritus
Yes, I believe there are. I did some calculations on chemical rocket propulsion which used JANAF data, and the code had equations for temperature and pressure. I thought JANAF tables were available at different pressures, much like steam tables. Unfortunately, it's been more than 20 years since I did that work, and I haven't kept up with it.

I think JANAF and NIST have cooperated in maintaining the tables.

3. Jan 31, 2008

### stewartcs

Perhaps NIST will help...

http://webbook.nist.gov/chemistry/fluid/

CS

4. Apr 7, 2008

### engware

Hi there:

This is a good question, but it is a tricky one.

With dealing with steam, you have steam tables and there is no problem. Physical properties are given as a function of both pressure and temperature. Such values have been experimentally measured.

When dealing with gases, combustion, it is another ball game.

Knowing that for ideal gas one can use the state equation as pv = RT, enthalpy values are given as a function of temperature only -- h = u + pv = u + RT = cpT.

For basic combustion calculations this is just fine when calculating the flame temperature.

However, when dealing with combustion products over a huge pressure and temperature range, this is not acceptable. In order to get aceptable numerical values, the minimization of Gibbs free energy for the combustion products is used. Such values are given as function of both pressure and temperature. Such calculations cannot be done by hand.

Here are s few URLs that can be of some help to you:

Technical background information on the physical properties
http://members.aol.com/engware/properties.htm [Broken]

Physical properties online calculator
http://members.aol.com/engware/calc1.htm [Broken]

Technical background information on combustion
http://members.aol.com/engware/components.htm [Broken]

Combustion online calculator
http://members.aol.com/engware/calc4.htm [Broken]

Thanks,

Gordan

PS

If you need additional explanation on the subject matter, I am here to help you out and share a few sample problems with you. Again, this is not an easy subject matter and it takes time to realize and digest what is at stake ...

Last edited by a moderator: May 3, 2017
5. Jan 18, 2009

### vilas4485

Hi,

Suppose we have been given the substace O2. The state 1 is at 400 K and 3 atm. The state 2 is at 1000 K and 5 atm. Assume that ideal gas law applies. Now the JANAF thermochemical tables give the values only at 1 atm, so how can I find the enthalpy difference, entropy difference and internal energy change between states 1 and 2.

Thank you,
Vilas V jangale