Wednesday, April 25, 2007

HW #7, P10 - DeltaS{Univ} Upon Quenching an Iron Block - ?? pts

A 12 kg iron block initially at 350oC is quenched in an insulated tank that contains 100 kg of water at 22oC. Assuming the water that vaporizes during the process condenses back into the liquid phase inside the tank, determine the entropy change of the universe for this process.

6 comments:

Anonymous said...

Hi professor,
I really don't know how to start this problem. Someone told me that I need to determine the final temperature of the water by setting Qgain = -Qlost. Is this the right way to go? Please help me!

Dr. B said...

Anon:
Someone told you the right thing ! Do it ! Then, use Tfinal to calculate deltaS for the block and then for the water. In this case, the water + the block = the universe.
Good luck !

Anonymous said...

Hi professor,
It's me again, I found the specific heat for iron(0.449 J/g-K) and water(4.186 J/g-K) online. I was wondering if they are right. In any case, I did proceed to solve for T final of water and it is something like 299.3K. Once I got this number do I use the Sat. Temp. table and interpolate for delta S at final temp to find delta S system? Please help!

Anonymous said...

I got Q=delta H= Cp(T1-T2)
but i don't know how to find H1 and H2 at 350C for iron...Please help...

Dr. B said...

help:
I am confused. You got Q = Cp(T1-T2)
So, plug in numbers and carry on with the problem. Why is it important to know H1 and H2 ? You only reallyneed to know deltaH...and you do !

Dr. B said...

Anon 10:51
I gave the specific heats on Moodle, but the ones you found are almost identical. Good work.

Your Tfinal is correct ! Good work.

No, do not use the saturation tables...not for water and definitely not for iron !

Assume water and iron are incompressible and use:
deltaS = Cp * Ln(Tfinal/Tinit)