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Real Gases vs Ideal Gases

Discussion in 'MCAT Discussions' started by ssa915, Jul 24, 2006.

  1. ssa915

    ssa915 Member

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    I want to clarify the differences between real and ideal gasses. Any help is appreciated.

    I understand that real gases, b/c they take up volume, have a greater volume than ideal gases. I also understand that real gases, b/c they have attactive forces, these intermolecular forces reduce their pressure, so they have less pressure than ideal gases.

    But it's also true that real gases deviate from ideal gases under LOW temperature and HIGH pressure conditions. This seems to conflict with the above.

    How is this resolved?

    Thanks!
     
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  3. odddodo

    odddodo Member

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    Real gases are contained in a SMALLER volume than ideal gases because they take up volume. The ideal gas law also assumes that there are no particle interactions. High pressure and low temperature increase particle interactions, leading to non-ideal behavior.
     
  4. WilliamsF1

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    Think about it like this:

    Under lower pressure, the molecules are father apart. With higher temps, molecules are farther apart. An ideal gas has no "volume" because the gas takes up no space, density = 0. When the molecules of a real gas spread apart, the density goes down (closer to zero). At least that's the way I remember it, maybe it's actually bad logic. :)
     
  5. photoelectric

    photoelectric Junior Member

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    I think about it this way:

    if you rearrange Ideal Gas Law you see that V = T / P. so the conditions you mentioned (Low T, high P) correspond to smaller volume. a small volume means that...

    1) the actual size of the real molecules becomes a significant factor (Example:: if your real molecules take up 1mL and they are in a volume of 2mL then thats 1/2 of the space - but if they're in a volume of 1000 L then they effectively take up 0%.)

    2) the molecules are squished closer together, so the intermolecs have an exponentially increasing effect (electrostatic interactions vary w/ 1/r^2). This slows them down and so lowers the force that they strike the walls with. You can actually see this effect by just considering that a very low T decreases KE (=3/2RT)
     
  6. photoelectric

    photoelectric Junior Member

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    Oh and..

    Disclaimer: I'm pretty sure that what I posted above is at least partially incorrect, but it's a convenient way to remember the conditions for deviations.
     
  7. Cooolguy

    Cooolguy Senior Member

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    who cares, the test isnt that detailed. just memorize it
     
  8. ssa915

    ssa915 Member

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    Hey, no offense, but if you want do do average, then yea, just memorize it..if you want to rock this exam... know it

    Everyone else...thanks a bunch!
     
  9. pyromatic

    pyromatic Lord of Chaos

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    Ideal systems do not significantly interact. When you turn on interactions, the statistics become much more complicated. For weakly interacting gases, the partition function becomes a product of the ideal partition function and the configuration integral. If you make the assumption that the potential energy of the gas can be written as a sum of interactions between pairs of molecules, then you define a quantity called the mayer function which is a measure of the deviation of each boltzmann factor: f_ij = exp(-b*u_ij) - 1, so that when you substitute this into the configuration integral, you get a sum of integrals of pairs, distinct pairs, etc leading to a pertubative series with each term representing an increase in complexity of the interactions, like one molecule, two molcules interacting, three molecules interacting, and so on.

    You've already seen this in p-chem under the guise of virial coefficients.
     
  10. estairella

    estairella Senior Member

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    I know you didn't want to know something as complicated as what pyromatic said, but the contrast was just too amusing. :laugh:
     
  11. pyromatic

    pyromatic Lord of Chaos

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    Haha I can see why some people want to know just what they need to, but I'm of the opinion that, at least in the physical sciences, if you know the principles you can handle any situation thrown at you. Memorization may work in biology, but I just don't see how it can work in physics. I found the physical science section of the MCAT to draw on few principles but present them in a complicated manner. If you can weed throught the extraneous information and verbose descriptions and then do a simple calculation from first principles, you'll ace it easily. I'd also recommend paying attention to the dimensions of the quantities provided in the question and answers - you'll often get the answer just by noticing that 3 of the 4 answers have wrong units.
     

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