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Distillation question~

  1. Oct 7, 2008 #1
    Hi,
    I think this is a simple enough question for all the chemists out there~ but for me it's tough... :(

    so we did a simple distillation experiment. We were given an unknown alcohol to identify. I found the boiling point to be about 89degrees which matched the boiling point of the '1-propanol + water'. Now... what else is there to explain to prove that it was indeed that alcohol????

    my first drop of distillate was obtained at around 89degrees and my last one was obtained at around 92.5ish... and the initial volume was 5ml whereas the final was like 3.7ish? - -;;
    and the liquid that was left in the container; the one that wasn't dissolved; turned to pink!

    What I think is that of course it matches the boiling point, that's one reason and also since it has a relatively high boiling point, it shows that it's less volatile, so it needed more heat to distill??

    What else is there to discuss???
    Please help in analyzing this situation...!!!

    Thanks!!
     
  2. jcsd
  3. Oct 7, 2008 #2

    symbolipoint

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    You would help be more certain if you could use a gas chromatograph for the sample and compare the detection time with a known sample of the alcohol. I do not know what to tell you about the residual stuff that turned pink; maybe contamination from inadequately washed glassware + exposure to alkali?
     
  4. Oct 7, 2008 #3
    oh... so the colour's not supposed to change......? - . - uhoh
    uh.. i don't know what gas chromatograph is... we just have to write up a whole one-page discussion on the data collected from distillation....
     
  5. Oct 8, 2008 #4

    symbolipoint

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    You imply without realizing that you have not yet studied a course on Organic Chemistry. I assumed that you at least are at that level. What course are you in right now? High school or college? Is the course for Chemistry majors, or not?
     
  6. Oct 8, 2008 #5
    i'm in org chem right now.. but what we've learned has nothing to do with distillation... we're doing isomers right now...- . -
    i'm in uni and it's just a first year course.
     
  7. Oct 8, 2008 #6

    symbolipoint

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    I suggested applying gas chromatography because you wanted to "prove" the identity of what you distilled. Since this is a bit early in your course, you will soon have instruction and exercise (laboratory) for fractional distillation, which allows finer separation of distillable materials. You can check temperatures of each portion which comes through the fraction/reflux column. It would not really be a proof, but at least an indication based on temperature that a certain portion of material conforms to the boiling point of your 1-propanol. Still, even in simple distillation, if all you had was a solution of the 1-propanol plus water, detecting by temperature which liquid your receiver flask is receiving should be easy; you know the water will not come over until 100 C.
     
  8. Oct 9, 2008 #7
    so we know that 1-propanol was distilled out of the mixture... I measured the temperature change every 5 drops... and is 89 to 92.3 a big temperature change? maybe I could talk about that?? I need to fill up one whole page... but all I have now is the boiling point. . . .
     
  9. Oct 9, 2008 #8

    chemisttree

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    Not true. Water azeotropes with 1-propanol at a temperature of 88oC and has a molar fraction of 72% 1-propanol. The other 28% is water. No matter how carefully you distill this mixture, the azeotrope will always boil at 88oC and have a molar ratio of 72:28 1-propanol:water.

    Once the water has been azeotroped out with the 1-propanol, the remaining 1-propanol will distill at what temperature? Why did you see the temperature rise at the end of your distillation? What is the regular boiling point for 1-propanol? What is it for water? Why would a mixture of water and 1-propanol boil at a temperature less than either of the individual components alone? Discuss these points in your write-up and you will likely receive full credit.

    The pink color is a contaminant, probably from a red sharpie, extracted during the distillation or present in the original solution in extremely dilute form. See a lot of handwritten labels in the lab using red ink? Alkali by itself does not cause a reaction with 1-propanol to produce red stuff, but if phenolthalein is present with alkali, that's a different story....
     
  10. Oct 10, 2008 #9

    ok
    so once azotrope is distilled, the other component which has a higher bp should be distilled right...? then shouldn't water distill after azeotrope? or is it that azeotrope gets distilled then 1-propanol then water? because that is the order of increasing b.p...
    The temperature rose at the end in order to separate the other less volatile componenet....
    hmm the boiling point of mixture is lower than either of the two components because.... ?? i'm not sure about this one... is it because they are mixed in differnet ratios???
    we weren't given the ratios of the components... how does that affect in the interpretation of the data??
    also isn't there something that has to do with the fact that the boiling point of each individual component are both high and close to each other?
     
  11. Oct 10, 2008 #10

    chemisttree

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    That would depend on if any water remains. If water is a minor contaminant in the propanol, the water will all distill at around 88oC. What would happen if water were present in large amounts?

    You need to research that one on your own. Useful search term is 'azeotrope' and 'codistillation'.

    Are you telling me that you know nothing about the composition of the distillate of the azeotrope that boils at 88oC? C'mon! Please tell me that my effort to respond to your question wasn't wasted...

    You will know that when you research the azeotrope effect.
     
  12. Oct 12, 2008 #11
    all we were given was the boiling point of 1-propanol + water...
    and does all this also happen in simple distillation???
     
  13. Oct 20, 2008 #12

    chemisttree

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    Yep. Even if you were to perform a more complex distillation, you would still get the low boiling mixture. What is the defining property (boiling point-wise) of an azeotrope?
     
  14. Nov 9, 2008 #13
    I preformed the same experiment except my mixture of 1-propanol + water began boiling at 23oC then stopped then again at 88oC. Why would this happen?? It makes no sense at all.

    ->Major Problem #2: In the final minutes of the distillation, the temperature changed from 88 to 102oC. Does it matter when this change happens? (I sorta lost count during the lab) Is there any significance to when it changes temperature?

    ->One last questions: Why is there that final increase in temperature?

    I can't see how the chemical compositon of the solution changes
     
    Last edited: Nov 10, 2008
  15. Nov 10, 2008 #14
    help? anyone? please
     
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