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Homework Help: Entropy change when converting water to steam

  1. Apr 7, 2013 #1
    1. The problem statement, all variables and given/known data

    At constant atmospheric pressure, 20g of water at 30C is converted into steam at 250C. Assume the heat capacity of liquid water is constant at 4.2 J/gK and the heat of vaporization at 100C is 2260 J/g. The molar heat capacity of water vapor at constant pressure is given by

    [itex]\frac{c_p}{R} = a + bT + CT^2[/itex]

    where [itex] a= 3.634,{ } b= 1.195*10^{-3} K^{-1},{ } c=1.350*10^{-7} K^{-2}[/itex]

    Find the entropy change of the water.

    2. Relevant equations

    [itex]\Delta S = \frac{dQ}{T}[/itex]
    [itex] dQ = \int c_p dT[/itex]

    3. The attempt at a solution
    My first thought was just to plug in the given equation for C_p into (1) and integrating, but I'm not sure how the heat of vaporization comes into play.
  2. jcsd
  3. Apr 7, 2013 #2


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    Homework Helper

    There are 3 different 'stages' to this problem. You need to think of what they are, then find the entropy change that happens in each stage.
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