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Homework Help: Clapeyron equation

  1. Jan 10, 2006 #1
    why's it positive for most materials?
  2. jcsd
  3. Jan 10, 2006 #2
    I'm not sure exactly what you mean. The equation says, for a phase transition:
    [tex]\frac{dP}{dT}=\frac{\Delta H}{T\Delta V}[/tex]
    I think you're talking about dP/dT, but correct me if I'm wrong. Suppose you have a liquid that is being converted into a gas. This process is endothermic so [itex]\Delta H>0[/itex] and since the volume of the gas is greater than that of the liquid [itex]\Delta V>0[/itex]. Since the LHS has all positive terms, then [itex]\frac{dP}{dT}>0[/itex]. If you go in reverse and condense a gas then both [itex] \Delta H[/itex] and [itex]\Delta V[/itex] change sign so the RHS remains positive.

    If you want to know if it is positive for most substances, you have to think of it as "are delta H and delta V the same sign for most substances and most phase transitions". There are exceptions, like the liquid-solid transition of water, but think about most substances and the answer will come to you.
  4. Jan 13, 2006 #3
    thank you very much!!! :)
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