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Phase change question!

  1. Nov 19, 2009 #1
    We have a tricky question for homework that is starting our new unit of phase changes, after we have been doing calorimetry

    120.0g of an unknown solid at 235*c is added to a 1.5kg piece of ice at -20*c inside a styrofoam container. After a period of time, both substances areat 120*c. What is the specific heat of the unknown material in kJ/g.*c and kcal/g.*c?

    Here's what I was thinking yesterday:

    Qsurround = Mh20Ch20ΔT ([120]-[20])
    ...

    but I'm confused and that wouldn't account for the phase change in the question.

    My teacher gave us this hint today:

    "Qsurroundings = Q1 + Q2 + Q3 + Q4 + Q5"


    Any help would be great thanks :)
     
  2. jcsd
  3. Nov 19, 2009 #2

    Borek

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    Staff: Mentor

    Not sure what surroundings have to do here. It is as usual - heat gained equals heat lost. If you started with ice and ended at 120 deg C, you had two phase changes on the way. Thus heating of the water can be expressed as sum of five parts - three being heating of water in different states of aggregation, two being latent heat of the phase change.

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  4. Nov 19, 2009 #3

    Yes, so if Ch20(s) = 2.05 J/g.*c and Ch20(g) = 2.08 J/g.*c and we need to find Qsurroundings of each of the 5 parts, what would I do?
     
  5. Nov 19, 2009 #4

    Borek

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    Staff: Mentor

    Sorry, I have no idea what you mean by surroundings. In the calorimeter we did everything possible to isolate everything involved from the surroundings, so we have to deal with heat gained by things that were cold, and heat lost by things that were initially hot.

    I can only guess that by surroudnings you mean things OTHER then the substance for which you make calculations, but it doesn't make much sense in this case.

    Let's start with water - there are three stages of heating, an you have already listed equation that have to be used (although you listed it in a very cryptic way - please use * for multiplication, guessing where are the borders between variable names in Mh20Ch20ΔT is a waste of time). Can you calculate amount of heat required for raising temperature of ice to the melting point?

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  6. Nov 19, 2009 #5
  7. Nov 19, 2009 #6

    Borek

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    Staff: Mentor

    How would you calculate the amount of heat required for raising temperature of ANY substance by ΔT?

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