Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Ideal gases thermodynamic enthalpy and internal energy change

  1. Jan 21, 2012 #1
    1. The problem statement, all variables and given/known data
    See attachment ecxample001.

    2. Relevant equations

    See attachment D11.

    3. The attempt at a solution
    In the first equation (Cp/R = a+bT+cT^2...etc.), Cp/R is the constant pressure specific heat. The general formula for enthalpy change is h2-h1 = integral[Cp]dT, so does Cp/R = Cp except Cp/R is the constant pressure specific heat on a molar basis?

    Then in the next line of formula in the solution (h2-h1 = R/M integral (a+bT...)dT) how did it go from Cp/R to R/M? I'm totally lost in this. And by the formula do I seriously plug in these huge numbers (from D11)? I tried plugging it in and get keep getting somethign to the 17th power. I'm also very confused on the units, as there's a lot of variables and I'm not very clear on them and what they represent. Any help would be appreciated. Thanks!

    Attached Files:

  2. jcsd
  3. Jan 21, 2012 #2
    It looks like the numbers in the chart are intended to be between 1 and 25 in absolute value. Values for b are premultiplied by 1000. C by 1M and d by 1B. So the value d for CO2 would be 2.002X10^-9.
  4. Jan 21, 2012 #3
    thanks i know that from the chart. So how did the problem go from cp/R to R/M?
  5. Jan 21, 2012 #4
    Enthalpy change is integral of CpdT. They give you Cp/R as function of temperature on a molar basis. So Cp is R times the polynomial. The M comes from the fact it is a mass basis.
  6. Jan 21, 2012 #5
    So R/M is merely a conversion factor to get Cp molar basis back to the regular Cp?
    Thanks a lot man.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook