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Homework Help: Phase change?

  1. Oct 9, 2005 #1
    phase change?????

    How do I calculate the values of change in entropy (S) and change in Gibbs free enery (G) for the conversion of n=1 mol of liquid water at 100 C and 1 bar pressure in to vapor at the same temperatue and a pressure of 0.1 bar. Assume ideal behavior for the vapor. the molar enthalpy for vaporization of water at 100 C and 1 bar is 40.6 kJ/mol.

    I know that delta S = delta H/T, but how the change in pressure play into this problem for delta S and delta G????

  2. jcsd
  3. Oct 9, 2005 #2
    I'll give you some hints for the entropy:

    You have 2 thermodynamic processes there:
    1) vaporization
    2) isothermal expansion form 1 bar to 0.1 bar.

    Because the temperature is the same, the variation in entropy can easily be evaluated by
    [tex]\Delta S=\frac{Q_1+Q_2}{T}[/tex]
    where Q1+Q2 represents the total transfered heat.

    Now you have
    [tex]Q_1=m\cdot \lambda_v[/tex]
    for the vaporization at 100 C
    [tex]Q_2=\nu R T \ln \frac{V_2}{V_1}[/tex]
    for the isothermal expansion

    and so on.......([tex]p_1 V_1=p_2 V_2[/tex] is the answer at your last question)
    Last edited: Oct 9, 2005
  4. Oct 9, 2005 #3

    Tom Mattson

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