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Effects of Intermolecular Forces?

  1. Feb 9, 2015 #1
    For a van der Waals gas, there are London dispersion forces causing the gas particles to be attracted to each other while the law of excluded volume provides repulsive forces. How do each of these characteristics effect the pressure, volume, and temperature of the gas?

    I assumed that for the attractive intermolecular forces, pressure and volume would decrease because the particles would clump together (smaller volume) and hit the sides of their container less (less pressure). At the same time, I think that temperature would increase because the molecules would have a greater kinetic energy from bumping into each other. Thus, I believe the opposite would be the case for the repulsive forces.

    Is this actually the case?
     
  2. jcsd
  3. Feb 10, 2015 #2
    The KE would NOT increase if IF forces increased, think about it in terms of particles, if IF increase, then the displacement between the particles decrease, a decrease in dispacement causes a decrease in velocity and thus KE so will tempature would decrease.
    If IF forces increase then
    Volume=decrease
    Pressure=decrease
    know that the if KE>IF the state of matter is a gas, and also that if IF>KE the state of matter is a solid and lastly that if IF>=KE the state of matter is a liquid.
     
  4. Feb 10, 2015 #3

    Simon Bridge

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    Are you sure that a gas with high intermolecular forces would not still fill all the available volume in any container you put it in?
     
  5. Feb 10, 2015 #4
    gases have low intermolecular forces, that is why they are free to move around .A property of a gas is that they fill the volume of the container so of course they would.
     
  6. Feb 10, 2015 #5

    Simon Bridge

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    Sorry we cross posted - the question was intended for OP.
     
  7. Feb 10, 2015 #6
    NP, hope that helped the OP though, funny that you are helping me on my thread I just posted about resonance.
     
  8. Feb 10, 2015 #7
    Do you mean that attractive intermolecular forces will decrease volume, pressure, and temperature, while repulsive molecular forces would increase volume, pressure, and temperature? Why would a decrease in displacement, which would cause a decrease in velocity, outweigh the increase in velocity caused by inelastic collisions between the molecules?
     
  9. Feb 10, 2015 #8
    There are no repulsive intermolecular forces, maybe you mean the repulsive elctromagnetic forces? when we analyze the molecules we assume that their colisions are elastic. A decrease in displacement causes velocity to decrease, if velocity decreases then KE must decrease becasue KE=1/2mv^2. Remember that is KE is greater than IF than the particles can escape the substance via evaporation for example. Furthermore the attractive forces are not the same as intermoleclar forces, they cause other substances like water on a table to want to stick together.
     
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