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Fractional Distillation

Discussion in 'MCAT Study Question Q&A' started by cloak25, Apr 27, 2012.

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  1. cloak25


    Feb 28, 2012
    Over time, the effectiveness of fractional distillation is diminished. How can this best be explained?

    A. over time the distilling column heats up, and as the temperature increases, the amount of condensation and re-evaporation decreases.
    B. over time the distilling column heats up, and as temperature increases, the amount of condensation and re-evaporation increases.
    C. over time, the solution becomes poor in the more volatile component, so azeotropes increase.
    D. over time, the solution becomes poor in the more volatile component, so azeotropes decrease.

    Answer: A

    Why is D wrong?
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  3. hellocubed


    Jul 9, 2011
    Wow, I'm seeing a lot of problems I struggled with myself.

    D would be true aside from the azeotropes thingy.
    An azeotrope is composed of compounds with the same vapor pressure, and hence don't change in composition as distillation goes on.

    If it didn't mention azeotropes, then D would also be correct.
  4. MedPR

    MedPR Banned

    Dec 1, 2011
    This description of an azeotrope can be misleading, but still true. An azeotrope is a mixture of two solvents in a specific ratio that cannot be separated by distillation. Typically this is because the BP of the azeotrope is lower than the BP of either pure solvent.
  5. gettheleadout

    gettheleadout MS-3 Moderator Emeritus 7+ Year Member

    Jun 22, 2010
    Why is C wrong? Isn't that the explanation for why fractional distillation only distills aqueous ethanol to a certain point, at which azeotropic distillation must be used?

    Hmm, maybe that's only sometimes true then... If it doesn't work for all solvent/distillate mixtures it wouldn't be completely correct.
  6. DrRichand1

    DrRichand1 2+ Year Member

    May 26, 2008
    The answer is A. this is how fractional distilation differs from simple defraction. The column of silica beads act as a distilation column allowing condensation and evaporation to to occur sevel times as the vapor travels up the column. the more condensation and eveaportion changes that take place the more pure the vapor at the top becomes allowing more time to sperate solutions with very close BP.

    As the column heats up the temp differences between the beads and the vapor get closer to equilirium leading to less condensation and evaporation "areas" when traveling up the column. Therefor leading to decreased affectivness of the column rendering more of both vapors, which we don't want, we trying to separate them.

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