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Vapor pressure

Discussion in 'MCAT Study Question Q&A' started by inaccensa, Aug 17, 2011.

  1. inaccensa

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    I'm trying to understand what exactly the term high and low vapor pressure refer to.

    Methanol vs Ethanol vs Hexane

    The intermolecular forces between Ethanol is higher than that of methanol (greater dispersion forces, so more intermolecular bond) At an given temp, ethanol will vaporize less easily than methanol. Similary, at a given temp, hexane will reach the vapor pressure equlibrium more easily. Now my question is, Does Ethanol have a lower VP?? (at a given temp, it less easily vaporizes and needs more KE or time, if it is been heated to reach the surrounding atmospheric pressure) When we say low vapor pressure, we simply mean that a particular solute has less tendency to evaporate? Its more strongly held in solution. Addtionally, if I add some hexane in ethanol, the vapor pressure will further decrease. The basic premise is that Ethanol has lower VP, higher BP, more stable?
     
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  3. liveoak

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    Say you take a tube of water and fill it halfway. The top of the tube is closed, and you let the tube stand vertically (with the bottom not exposed to the atmosphere).

    The pressure of the air above the liquid in the tube is the vapor pressure of that fluid at that temperature.

    Thus, those that easily evaporate will have a higher vapor pressure. And those that evaporate easily have weaker intermolecular bonds.

    yep
     
  4. docntrainin

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    Also important to know, if something has a high vapor pressure, will have a low heat (enthalpy) of vaporization.
     
  5. justhanging

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    It doesn't mean that Ethanol is more stable, VP and BP have to do with the intermolecular forces not any sort of stability. Something else that you need to clarify is that if you add hexane to ethanol the vapor pressure of ethanol will decrease because there is a smaller mole fraction of ethanol but the total vapor pressure of the solution will increase because your adding a more volatile component.
     
  6. inaccensa

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    I meant Ethanol is more stable than the other two, but err you are right, stability is not something I'd want to use inthis context. Now,I have a little trouble understanding this whole volatile and nonvolatile solutes in solution.

    I do the idea that adding something like hexane in pure ethanol will reduce the vapor pressure of alcohol, since the surface area available for ethanol is less and it cant evaporate as easily as it did before. But the fact that the total vapor pressure above the solution increases is really throwing me off. The total P = XePe +XhPh but how does this increase the overall pressure?
     
  7. justhanging

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    What is said holds true always in ideal solutions, but if the solution is not ideal there could be a possibility that the solute and solvent interact favorably and reduce the total vapor pressure. One way to realize this is to draw a graph.

    http://www.chemguide.co.uk/physical/phaseeqia/idealpd.html

    This website explains it well. There is one graph on the bottom with 3 stright lines. 2 corresponding to roaults law for each individual component, then the blue line on top to the total vapor pressure.

    From your equation P = XePe +XhPh you have to realize that Ph is larger then Pe because that corresponds to the VP of the pure substance. So when you decrease Xe in favor of Xh you are actually gaining pressure because your multiplying by a bigger factor (Ph) even though you are reducing Xe and the whole XePe term.The XhPh term will increase more then the XePe term will decrease.
     

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