Monday, April 10, 2006

HW #3 - P4 - Determining Internal Energy Change Using Heat Capacity Polynomials

Determine the change in the specific internal energy of hydrogen (H2), in kJ/kg, as it is heated from 400 to 1000 K, using:
a.) the empirical specific heat equation (Shomate Equation) from the back of your book.
b.) The Cv(IG) value at the average temperature. (Use theat capacity polynomial to determine this CoV value.)
c.) The Cv(IG)value at room temperature, 25oC. (Use theat capacity polynomial to determine this Cv(IG)value.)

7 comments:

Anonymous said...

for 4a... i get Cv = -16.8782 J/mol. Can someone confirm if this number is right or wrong?

Anonymous said...

Since i am pretty sure this is wrong, i'll ask two more questions.

1) Is it possible to get a negative number here and still have deltaH be positive?

2) I am having trouble seeing where this R fits in. What does the Cv equation look like before integration?

Dr. B said...

Anon 6:43
You don't calculate Cv in this problem. You must INTEGRATE the polynomial of Cv. Cv = Cp - R for IG's and Cp = Shomate polynomial.

We are NOT assuming a constant Cv value. No, Cv cannot be negative even if we DID assume it was constant.

Look at the example problem online for doing integral of Cp dT. Here, just do INT{ Cp-R } dT.

I hope this helps !

Anonymous said...

I have done this calculation about 3 times and I keep getting a wrong number. I am using Cv = Cp - R, where R is (7/2)*8.31434 J/mole-K. My A-E values are the ones I read from the chart in the range 298-1500K. Is it possible I am using the wrong values for R or A-E?

Dr. B said...

carina 9:42 PM
R = 8.314 J/mole*K. You don't need the (7/2). Yes, you get the A-E values from Appendix E, pg 337. Cv = Cp - R and then you integrate. I think the R issue was the problem.
Best of luck !

Anonymous said...

What are we supposed to do in parts B and C? Is all we're doing is solving for Cv? I don't think I'm quite understanding.

Dr. B said...

Somebody 10:12 PM
In parts (b) and (c) you are supposed to use a CONSTANT value of Cv.

In (b), you determine the constant value by evaluating the polynomial for Cv at the average of the initial and final temperatures.

In (c), you determine the constant value by evaluating the polynomial for Cv at 25 degC.