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Topic: Phase equilibrium: further steps on azeotrope  (Read 3598 times)

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xyxtem

  • Guest
Phase equilibrium: further steps on azeotrope
« on: April 09, 2005, 11:10:01 AM »
I recently came across with an unsolved problem from the past, about phase equilibrium. Here is part of the question.
After the fractional distillation of a 50% water, 50% ethanol mixture, an azeotropic mixture (95.6% ethanol) is obtained from the distillate.
1.  Since the distillate is not pure, it is suggested that the mixture is to be "salted out", dried and then redistillated. What is the meaning of "salting out"?
2.  The drying agent used in (1) should NOT be CaCl2. Why? What should be used instead?
3.  Absolute ethanol may be obtained by adding a metal to the distillate. Which metal may be used? Explain. Can we use Na to remove the last traces of water? Explain.

Here are my thoughs:
1.  Adding conc. H2SO4?? Making precipitate??
2.  CaCl2 may react with ?? to form complex?? CaO+Ca(OH)2 should be used.
3.  Sodium?? But sodium + ethanol --> soluble alkaoxide ion + H2!!!!

I'm sure most of my thoughs are wrong.
Please help me!!!!!!!!!!!! ??? ??? ???

Garneck

  • Guest
Re:Phase equilibrium: further steps on azeotrope
« Reply #1 on: April 09, 2005, 11:58:48 AM »
I recently came across with an unsolved problem from the past, about phase equilibrium. Here is part of the question.
After the fractional distillation of a 50% water, 50% ethanol mixture, an azeotropic mixture (95.6% ethanol) is obtained from the distillate.
1.  Since the distillate is not pure, it is suggested that the mixture is to be "salted out", dried and then redistillated. What is the meaning of "salting out"?
2.  The drying agent used in (1) should NOT be CaCl2. Why? What should be used instead?
3.  Absolute ethanol may be obtained by adding a metal to the distillate. Which metal may be used? Explain. Can we use Na to remove the last traces of water? Explain.

Here are my thoughs:
1.  Adding conc. H2SO4?? Making precipitate??
2.  CaCl2 may react with ?? to form complex?? CaO+Ca(OH)2 should be used.
3.  Sodium?? But sodium + ethanol --> soluble alkaoxide ion + H2!!!!

I'm sure most of my thoughs are wrong.
Please help me!!!!!!!!!!!! ??? ??? ???

1. Not H2SO4. You've got the answer in the second question. You have to use a hygroscopic salt or oxide (such as CaO) that can get all the water out of the mixture
2. You should dry it with CaO, it will get the water out of of the alcohol. CaO + H2O = Ca(OH2)
3. Sodium wil react with both water and ethanol, so it can't be used. As far as I can remember, after you dry it out with CaO, magnesium should be used. But don't quote me.

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