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Chemistry- boiling point

  1. Apr 28, 2009 #1
    find the boiling point of HF according to the following reaction

    HF(L) --> HF(G)
    ΔH=7.53 [KJ/mol]
    ΔSsys=0.0257[KJ/(mol*K)]

    i used the equation

    ΔH=-ΔS*T
    >>T=-ΔH/ΔS
    >>T=-0.0257/7.53

    T=-292.996°K

    surely this is meant to be positive ???
    have i used the wrong equation?? which other equation should i have used?
     
  2. jcsd
  3. Apr 28, 2009 #2

    Mapes

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    Where does ΔH=-ΔS*T come from? Surely ΔG=0 at a phase equilibrium at constant temperature and pressure.
     
  4. Apr 28, 2009 #3
    i dont know, i have it written in my workbook, could it be a mistake??

    if ΔG=0
    ΔG=H-ΔS*T
    H=ΔS*T

    and then it all works fine,,, but how do i know that ΔG=0,?

    is there no equation that ΔH=-ΔS*T?/
     
  5. Apr 28, 2009 #4

    Mapes

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    This is a condition that applies to any two systems at equilibrium at constant pressure and temperature. It's derived in intermediate thermodynamics as a consequence of the first and second laws.

    There could be, but I haven't seen it, and it doesn't apply here.
     
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