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Homework Help: Thermodynamics Question

  1. Nov 21, 2016 #1
    1. The problem statement, all variables and given/known data
    You have 1.0kg of ice at 0.0 deg C in a pot on a stove that produces 900 W of power. Assuming that the pot and the stove are 'perfect' (i.e., no heat lost) and ignoring the heat capacity of the pot, compute the time it will take for this ice to boil away completely.

    2. Relevant equations
    Power * time = Mass * Specific Heat * Temperature (or maybe change in temp?!)

    3. The attempt at a solution
    I used the equation above ^ and used the specific heat for ice of 2106 J/(kg/K) and 4218 j/(kg/K) for liquid. but I don't think this is right because what would the change in temp be for the ice to liquid phase and vapor to completely gone phase? Really confused
  2. jcsd
  3. Nov 21, 2016 #2
    first off do you understand that 1 watt = 1J/s ? - meaning that a stove with 1 watt produces 1 joule of energy per second.
    so now forget about the stove
    how much energy (joules) do you need to boil the water?

    equation for change of state does not have change in temp, just becase there is no change in temperature does not mean you dont have to expend energy (joules) to make the state transition, makes sense no?

    furthermore for state changes the constant is not called "specifc heat" its caled enthalpy of fusion
  4. Nov 21, 2016 #3
    I do not know thats what the question says. I have no idea how many joules it takes to boil away completely. If i did I would have the question finished lol
  5. Nov 21, 2016 #4
    you must divde the question into parts:
    how much enrgy do you need to transform the water from ice to liquid;
    how much energy do you need to heat said (now liquid) water from 0 to 100 degrees celsius;
    and finally how much energy to transform the water from liquid to gas.

    just to complete my previous answer, you use the specifc heat of ice to know the energy you need to heat ice (say from -20º to -10), you use the enthalpy of fusion to figure out how much energy you need to turn that block of ice to liquid water (once ice is at 0º), understand?
    Last edited: Nov 21, 2016
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