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Homework Help: Enthalpy of transition of water to ice

  1. Nov 3, 2013 #1
    1.At constant temperature of 258K and constant pressure of 1.000 atm , super cooled liquid water (2.00mol) freezes irreversibly. Find the change in enthalpy(ΔH) by assuming that Cp,m of liquid water and ice are 75.48 KJ/mol and 37.15KJ/mol and are constant. (this problem was from my assignment sheet so there might be errors in the units of Cp,m. By the way , Cp,m is the specific heat capacity at constant pressure.)

    2.the only equation i could think of is ΔH=ΔU+ΔPV
  2. jcsd
  3. Nov 3, 2013 #2
    Hi Joel Martis. Welcome to Physics Forums.

    In terms of the enthalpy and temperature, what is the defining equation for the heat capacity at constant pressure?

  4. Nov 3, 2013 #3
    I think it is Cp,m= (∂H/∂T)p
  5. Nov 3, 2013 #4
    Good. So now you have to define a path form the starting point to the final point along which you know all the properties needed to calculate the ΔH. The starting point is liquid water at -15C, and the end point is ice at -15 C. But, you only know the heat of melting/fusion at 0C. So, how do you get form the starting point to the final point if you only know the heat of fusion/melting at 0 C, and the heat capacities for the liquid water and the water ice?

  6. Nov 4, 2013 #5
    Well, how about heating the supercooled water from 258K to 273K , freezing it, and then cooling the ice back to 258K?
  7. Nov 4, 2013 #6
    Sounds great.

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