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Homework Help: Changes of state and latent heat

  1. Apr 9, 2008 #1
    1. The problem statement, all variables and given/known data
    A 0.2 kg block of ice at -15*C is placed into a pan on a stove, heated to a liquid, and then to vapour with a final temperature of 115*C. Calculate the total amount of heat required for this process.


    2. Relevant equations
    Eh = mc(t2 - t1)
    Lf (or Lv) = Eh / m

    3. The attempt at a solution
    Etotal = Emelting + Evapourization

    Emelting = mct
    = (0.2kg)(4.2x10^3)(0+15*C)
    = 12 600 J

    Evapourization = mct
    = (0.2kg)(4.2x10^3)(115-0*C)
    = 96 600 J

    Etotal = 12600 + 96 600J
    = 109 200 J or 1.1 x 10^5

    What am i doing wrong? The answer is 6.2 x 10^5 J.
     
  2. jcsd
  3. Apr 9, 2008 #2

    Mapes

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    1) You need to use the correct specific heat capacity for ice, water, and steam.
    2) You need to incorporate the latent heat of the phase transformations that occur at 0°C and 100°C.
     
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