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Effect of impurities on the boiling point of ethyl ethanoate

  1. Feb 7, 2018 #1

    Daniel2244

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    After carrying out reflux and distillation to produce ethyl ethanoate (ethyl acetate) I measured the boiling point and I got 71 degrees Celsius whereas the true value is 77.1 degrees Celsius. Impurities increase the Boiling point (BP) as well as concentration. However my calculates BP is lower than the true value so what is affecting the boiling point?
     
    Last edited: Feb 7, 2018
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  3. Feb 7, 2018 #2

    DrDu

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    Maybe an Azeotrope?
    https://en.wikipedia.org/wiki/Azeotrope
     
  4. Feb 7, 2018 #3

    Borek

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    Nope, that's not true. Impurities with a volatility lower than the main component increase the boiling point.
     
  5. Feb 7, 2018 #4

    Daniel2244

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    Yeah, that makes more sense. However, is the reason my boiling point is lower than the true value because of an azeotrope?
     
  6. Feb 7, 2018 #5

    DrDu

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    Not necessarily. However, if it is not, the boiling point should not be constant, but rise in the course of the distillation as the more volatile substance will boil off first.
     
  7. Feb 7, 2018 #6

    HAYAO

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    You should also mention what the main component is. Ethyl acetate forms azeotrope with several liquid, e.g. water, methanol, ethanol, acetonitrile.
     
  8. Feb 7, 2018 #7

    Daniel2244

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    Could you explain differently because I do not understand
     
  9. Feb 7, 2018 #8

    Borek

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    Boiling temperature of the mixture is never constant, it always goes up during the distillation.
     
  10. Feb 7, 2018 #9

    Daniel2244

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    So essentially could I say: the ethyl acetate I made has a lower BP then the true value which shows it contains impurities. The ethyl acetate contains an azeotrope for example water or un-reacted ethanol which is more volatile than ethyl acetate which causes the Bp to decrease . I'm not sure if this is correct or not, the research I am doing only tells me why the bp increase whereas i'm look for why my bp is lower then the true value.
     
  11. Feb 7, 2018 #10

    HAYAO

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    Ethyl acetate is more volatile than water or ethanol.
     
  12. Feb 7, 2018 #11

    Daniel2244

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    So why is my boiling point lower than the true. I know you're probably annoyed with me keep asking but I do not understand why my BP is lower.

    So far I have gathered that, Impurities that are less volatile than the main component in my case (ethyl ethanoate) increases the boiling point. So, impurities which are more volatile than ethyl ethanoate will decrease the boiling point?
     
    Last edited: Feb 7, 2018
  13. Feb 7, 2018 #12

    HAYAO

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    You are overthinking. DrDu and Borek are simply stating the general idea for you to understand the underlying concepts, so that it will help you draw out conclusions based on those concepts.

    The general idea is that low-volatile impurity will raise the boiling point of the mixture, as Borek said.
    Another general idea is that there are positive and negative azeotropes, which means the boiling point of the mixture at the azeotrope composition is lower or higher than any of the component alone, as implicitly stated by DrDu.

    What do you think is the contributing factor for your case, based on your obtained experimental results?


    I'm not annoyed :smile:, and most likely neither are everyone else.
     
  14. Feb 7, 2018 #13

    Daniel2244

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    Well, due to my temperature being lower than true value I believe it is a positive azeotrope because the boiling point is lower than the constituents. If I am right the new interaction between ethyl acetate and water are weaker and can escape into vapour easier. But it still doesn't explain how impurity affects the bp.

    Would un-reacted chemicals affect the boiling point such as un-reacted ethanoic acids or ethanol? just curious due to water being immiscible with ethyl acetate
     
  15. Feb 7, 2018 #14

    Borek

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    Sure thing.
     
  16. Feb 7, 2018 #15

    Daniel2244

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    Ok, so could I say impurities such as unreacted chemicals would have caused my decrease in temperature due to different volatility of ethonal and ethanoic acids. Additionally, they could be an positive azeotrope in the ethyl acetate I produced which also causes the Bp to decrease due to weaker interactions between molecules therefore vapor can escape more easily.
     
  17. Feb 7, 2018 #16

    HAYAO

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    At this point, I don't know how you define "impurity". Since you are talking about distillation of ethyl acetate, even ethanol and water in this distilled ethyl acetate are impurities.

    Like Borek said, yes.

    Very good point. The mixture have two layers. However, having two layers does not mean that each layer is completely pure in composition. The upper layer is ethyl acetate rich, and the lower layer is water rich. As an azeotrope, upper and lower layer contain 97 wt% and 9 wt% of ethyl acetate, respectively (Azeotrope Tables by Wikipedia).

    If the impurity(ies) has lower volatility than ethyl acetate then the bp rises, generally speaking. However, positive azeotrope causes bp to lower. In this particular case, the latter is the main contribution over the former.
     
  18. Feb 7, 2018 #17

    DrDu

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    My preferred hypothesis is an azeotrope with water. You could check drying your liquid with e.g. a molecular sieve and re-measuring its boiling point.
     
  19. Feb 10, 2018 #18

    Daniel2244

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    Thanks for the help :)
     
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