Monday, April 10, 2006

HW #3 - P6 - Vapor Pressure of Solids and the Clausius-Clapeyron Equation

Using the Clapeyron-Clausius equation and the triple- point data of water, estimate the sublimation pressure of water at -30°C and compare to the actual value of 38.02 Pa. ΔHsub = 2838.4 kJ/kg .

5 comments:

Dr. B said...

If you got something like 52 kPa for your pressure, you used the latent heat of vaporization. Instead, you need to use the latent heat of sublimation. I just added this value to the problem statement on the course website and to my blog entry.

Anonymous said...

When the question says to compare one value to another, what exactly does that mean? Are we to say one is greater than the other, or calculate the percent error from the actual:
(predicted-real)/real

Anonymous said...

Where would we find the latent heat of sublimation had you not given it to us?

Just a minor concept question:
Why is C irrelevant here when using double values? Is there a derivation somewhere? Or a hint to a derivation? I understand that we can use the equation of a line, so is C just the "y-intercept" and it magically becomes 0?

Dr. B said...

compare 7:47
It would be best to calculate the %error using the formula you provided. Good work.

Dr. B said...

jolene 8:05
I thought it was on the website, but I did not find it there. You can find data like this in Handbboks like the Handbook of Chemistry and Physics or Perry's Chemical Engineer's Handbook. But these days, I am inclined to do a Google search first because it is fast...if it works.

Good question.

In the Clausius-Clapeyron Eqc, C is the y-intercept if you plot Ln{P*} vs 1/T(K). The derivation of the C-C Eqn is in Thermo-CD on page 3E-11. But, C is not zero. It just isn't very interesting. You can calculate C if you want to and then use it to help answer this question. Basically, you have 2 eqns in 2 unknowns: P*2 and C. I just did not bother to evaluate C. I solved for P*2 and stopped.

I hope this is helpful. Check out 3E-11 in TCD.