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Phase Changes and internal energy

  1. Jul 23, 2011 #1
    The internal Energy E is this: [itex]\Delta[/itex] E = q + w = q - P[itex]\Delta[/itex]V

    where q is heat, w is work, and V is volume

    Phase changes occur when you change internal energy of the system, right?

    I am assuming that this means when you go from gas to liquid to solid, you must decrease the internal energy (remove heat).

    And when you go from a solid to liquid to a gas, you must add internal energy (add heat).

    That obviously makes sense, but what happens when you involve the work part of internal energy.
    When you do work on the system (volume decreases), you increase its internal energy, yet compression causes a gas to turn to a liquid.
    How can this be since you are increasing the internal energy?

    Or am i understanding this wrong? Is it the increase in internal energy or the increase in heat that determines phase changes?
  2. jcsd
  3. Jul 23, 2011 #2
    Have you ever pumped up a tyre?

    What happens?

    The tyre becomes hotter.

    This is because the work you do in increasing the gas pressure appears as heat according to the gas law

    PV/T = constant

    Since the volume does not change the temperature must rise.

    Also the boiling point rises with increase in pressure. (Really we should call this the condensation point since we are talking about a gas to liquid phase change.)
  4. Jul 23, 2011 #3
    So why when we compress a gas, it turns to a liquid, even though you increase its internal energy?

    that seems counter intuitive, since removal of energy is what causes a gas to turn into a liquid not addition of energy
  5. Jul 24, 2011 #4
    The increase in internal energy due to compression happens before the phase transition.
    During the actual transition the gas must be able to transfer energy (heat) to the environment.
    The increase in pressure brings the gas at the right conditions for molecular interactions to become strong enough for a condensed phase. This may be done by decreasing temperature, increasing pressure or both.
    If you cool down the gas rather than compressing it, it looses some energy but the energy lost during cooling is not related to the energy lost during the phase transition.
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