Separating substances under lower atmos press

  • Thread starter Thread starter chiefy
  • Start date Start date
Click For Summary

Discussion Overview

The discussion revolves around the separation of a cyclohexane-toluene mixture under reduced atmospheric pressure (350 mm Hg) compared to standard atmospheric pressure (760 mm Hg). Participants explore the implications of lower boiling points at reduced pressure and the methods of distillation applicable to this scenario, including vacuum distillation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that lowering atmospheric pressure will lower the boiling points of cyclohexane and toluene, potentially allowing for separation.
  • One participant mentions that vacuum distillation is typically used for compounds with high boiling points that may decompose at higher temperatures.
  • Another participant calculates the boiling points of cyclohexane and toluene at 350 mm Hg, suggesting that the difference in boiling points is sufficient for effective separation.
  • Some argue that simple distillation may not work effectively under reduced pressure, while fractional distillation could be viable even at normal pressures.
  • There is a discussion about the necessity of cryogenic cooling in vacuum distillation, with some asserting that it is not always required.
  • One participant shares personal experience indicating that distillation at atmospheric pressure yielded better results in their case, where the boiling point difference was smaller.
  • Concerns are raised regarding the flammability of both compounds, which may influence the choice of distillation method.

Areas of Agreement / Disagreement

Participants express varying opinions on the effectiveness of separation under reduced pressure, with some suggesting it is possible while others remain uncertain. There is no clear consensus on the best distillation method or the conditions under which separation is optimal.

Contextual Notes

Participants reference specific boiling points and calculations, but there are unresolved assumptions regarding the effectiveness of different distillation methods and the impact of pressure on separation outcomes.

Who May Find This Useful

This discussion may be of interest to those studying chemistry, particularly in the areas of distillation techniques and the effects of pressure on boiling points.

chiefy
Messages
14
Reaction score
0
Can a cyclohexane-toluene mixture be separated if the external pressure is 350mm HG instead of 760 mmHG ?

My guess is yes, lowering the atmospheric pressure will only lower the boiling point, it won't affect whether separation will occur. By the way, is this what we call vacuum distillation? I need a little greater explanation. DOes anyone have any ideas?
 
Last edited:
Chemistry news on Phys.org
chiefy said:
Can a cyclohexane-toluene mixture be separated if the external pressure is 350mm HG instead of 760 mmHG ?
My guess is yes, lowering the atmospheric pressure will only lower the boiling point, it won't affect whether separation will occur. By the way, is this what we call vacuum distillation? I need a little greater explanation. DOes anyone have any ideas?

yeah, but I think that for the most part vacuum distillation is used for compounds with high boiling points, which decompose under such temperatures.

http://www.Newton.dep.anl.gov/askasci/chem00/chem00635.htm
 
The boiling point of cyclohexane at atmospheric pressure is 81.4 and that of toluene 110.8 which means that they can be separated easily. At reduced pressure,the difference in boiling points will probably be large enough to allow for separation.
 
if you're meaning through simple distillation probably not, I think what they usually do with vacuum distillation is to trap the gaseous organics in a "cryogenic chamber", that is if the organics are gases at that temperature. Fractional distillation may work even at normal pressures.
 
I am not quite sure understand your post. The only reason for vacuum distillation as opposed to atmospheric pressure is that because the boiling points are lowered, it is possible to operate with less thermal decomposition.
 
Mr.Chiefy:
According to my Handbook of Chemistry and Physics,the change in boiling point with pressure is given by:
[tex]\deltat=(273.1+t)(2.8808-logp)/phi+.15(2.8808-logp)[/tex]
where t is temperature,p is pressure and phi can be obtained from a graph. I picked 4.7 for cyclohexane and 4.6 for toluene.

Assuming my arithmetic is fairly correct,this is what transpires:
Cyclohexane at 350mm boils at 58.5 centigrade
Toluene at 350mm boils at 85.5 centigrade.
Consequently distillation at 350mm ought to separate the compounds very well.
 
vacuum distillation is something I just through in there, I simply wanted to know if they separate under a lower pressure, and I think the consensus is that they will. Thanks for the replies.
 
renerob said:
I am not quite sure understand your post. The only reason for vacuum distillation as opposed to atmospheric pressure is that because the boiling points are lowered, it is possible to operate with less thermal decomposition.

thermal decomposition of organics? cyclohexane and toulene?
 
Personally I have had a couple of compounds that I have tried to purify by distillation and I got better results by distilling at atmospheric pressure than at low temperature. Then again, the bp difference was a bit smaller in my case.

Also, you don't necessarily need cryogenic cooling when you do vacuum distillation. A simple, water-cooled distillation apparatus is often good enough when you are dealing with high boiling materials. There definitely are cases where you need to condense the distillate at very cold temps, but not always.
 
  • #10
GCT:
Good point. In this particular case one other good reason is that both compounds are very flamable.
 

Similar threads

  • · Replies 1 ·
Replies
1
Views
5K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 28 ·
Replies
28
Views
5K
  • · Replies 13 ·
Replies
13
Views
22K
  • · Replies 46 ·
2
Replies
46
Views
20K
  • · Replies 6 ·
Replies
6
Views
6K
Replies
17
Views
66K
Replies
3
Views
12K
  • · Replies 10 ·
Replies
10
Views
7K
  • · Replies 1 ·
Replies
1
Views
3K