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Adiabatic expansion of a real gas

  1. Feb 18, 2015 #1
    1. The problem statement, all variables and given/known data
    Which of the following changes during the free adiabatic expansion of a real gas?
    (I) internal energy
    (II) temperature
    (III) pressure

    2. Relevant equations
    PV=nRT; ΔU= q + w

    3. The attempt at a solution
    For ideal gases under adiabatic conditions, we know that there is no heat transfer. Thus, the only change is pressure. However, what can be concluded about real gases? Ideal gas law assumes no volume or attraction between molecules but real gases do not act this way, thus under high pressures, there is an increase in attraction due to intermolecular forces. So, if the real gas expands, volume changes but what can be said about internal energy and temperature? The correct answer states II and III but I do not see how temperature is affected under adiabatic conditions. Help please!
     
  2. jcsd
  3. Feb 18, 2015 #2
    I just found out that free adiabatic expansion does not contain a piston (sort of looks like a diffusion process). Therefore, it makes sense that pressure would decrease and temperature would increase from PV=nRT. Internal energy remains the same because no work is being done on the system.
     
  4. Feb 27, 2015 #3
    It might be a little late already, but in a free expansion PV = constant, so T must remain constant too. Pressure decreases but volume increases.
     
  5. Feb 27, 2015 #4
    In free expansion, the gas expands into a vacuum. So no work is done and, since the process is done adiabatically, no heat is transferred. So the change in internal energy is zero. But for a real gas, the internal energy is a function not only of temperature, but also of pressure. So if the internal energy of a real gas is to stay constant while its pressure changes, its temperature must also change.

    Chet
     
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