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TBR G.Chem Example 7.10 - Vapor Pressure

Discussion in 'MCAT Study Question Q&A' started by ClosetCrammer, Jul 28, 2011.

  1. ClosetCrammer

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    The question regards vapor pressure's relationship with temperature, pressure, and boiling point.

    In the provided solution, they give reason as to why a change in elevation (atmospheric pressure) does not lead to significant change in vapor pressure yet does lead to a significant change in boiling point. Later, they discuss the correct answer and suggest that a lower boiling point increases vapor pressure..

    Is this not a contradiction? Which is true? :confused:

    Thanks!

    EDIT: The correct answer is a comparison between two liquids. Maybe they are suggesting that you are able to compare two different liquids' boiling points to determine vapor pressure, but you cannot compare the same liquid's change in boiling point due to elevation and relate that change to its change in vapor pressure. I don't know?
     
    #1 ClosetCrammer, Jul 28, 2011
    Last edited: Jul 28, 2011
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  3. EBTrailRunner

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    Think of vapor pressure as the proportion of molecules that exist in the gas phase. At a given temperature, a liquid with a lower boiling point will have a higher vapor pressure than one with a higher bp.
     
  4. ClosetCrammer

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    Thanks for your input!

    What about comparing the same liquid at different elevations? At a higher elevation, atmospheric pressure is less therefore the boiling point is depressed. At a lower elevation, boiling point is elevated.

    Yet, TBR claims that vapor pressure is unaffected by the change in elevation?
     
  5. muhali3

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    edit: sdn mcat Q&A errata

    Vapor pressure definitely is affected by change in elevation. But I assume that changes in elevation do not change atmospheric pressure significantly. So at higher elevations, atmospheric pressure would be slightly less, so vapor pressure would increase a little bit. But I think TBR is saying that the amount that it increases is not very significant.

    ...Personally, I think you should just ignore what they're saying, because your intuition is right.

    less external P = higher vapor P = lower BP
     
    #4 muhali3, Aug 1, 2011
    Last edited: Aug 1, 2011
  6. geeyouknit

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    I don't really agree with this. Vapor pressure is the pressure exerted by the particles of the liquid that go into the gas phase. If you increase the temperature, the liquid particles have more energy and more escape into the gas phase, thus increasing the vapor pressure.

    VP is not affected by changes in elevation. What elevation does is change the amount of pressure exerted by the atmosphere on the liquid surface (more atmosphere above = more pressure. Thus higher up means less air above and less atm pressure).

    Remember that increasing temp increases VP. And the boiling point is the temperature at which the VP of a liquid matches the ATM pressure. So, if there is less ATM pressure (at higher altitidues), a lower temperature, ie a lower BP, can create enough VP to match the lower ATM pressure. So VP isn't affected by elevation, but ATM pressure and thus BP is.
     
  7. collegestud2013

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    The vapor pressure over a liquid increases exponentially with temperature (as seen in the Claudius-Clapeyron equation), and the BP of a liquid is the temperature where its VP = atmospheric pressure. So if you're at a lower atmospheric pressure, the actual vapor pressure of your liquid won't really change at a given temperature, but your BP does decrease because a much lower temperature is needed for its VP to reach the surrounding atmospheric pressure.

    And when comparing two liquids, the one with the lower BP would have its VP vs Temperature graph shifted to the left of the one w/ the higher BP (so that its VP = atmospheric pressure at a lower temperature). This makes it that at any given T, the VP of the lower-boiling liquid will be higher.
     
    #6 collegestud2013, Aug 1, 2011
    Last edited: Aug 1, 2011
  8. muhali3

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