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Homework Help: Energy problem - ice melted by burning ethanol

  1. Apr 18, 2010 #1
    1. The problem statement, all variables and given/known data
    How many grams of ice at 0°C could be melted by the heat produced when 10.0g of ethanol is burned?


    2. Relevant equations
    So I wrote out the combustion reaction:
    C2H5OH +3O2 ---> 2CO2 + 3H2O


    3. The attempt at a solution
    First I though I had to find heat of formation for that reaction, which I found to be -1235.6kJ (So this would be the heat given off when ethanol is burned correct?)
    Hf=products-reactants
    Hf=[-393.6(2)+-242(3)]-[-277.6+0]
    Hf=-1235.6kJ

    I wasn't sure if I had to use that number or if I had to just pull the Hf number off my chart for ethanol (-277.6kJ)

    I have the answer given to me, but I'm stuck at converting my kJ to g of water. (Answer is 802g)

    Tried: 1235.6kJ[1mol/6.03kJ][18.02g/1mol]
     
  2. jcsd
  3. Apr 18, 2010 #2
    You need to constants. The latent heat of fusion of ice (let us denote it by [tex]L_{f}[/tex]) and the energy released by burning a unit mass of ethanol (let us denote it by [tex]q[/tex]). Then, you simply have:

    Heat needed to melt mass [tex]m(\texrm{ice})[/tex] = [tex] L_{f} \cdot m(\textrm{ice})[/tex]

    Heat released by burning mass [tex]m(\textrm{ethanol})[/tex] = [tex]q \cdot m(\textrm{ethanol})[/tex]

    These are equal, so, you have an equation for [tex]m(\textrm{ice})[/tex]:


    [tex]
    m(\textrm{ice}) = \frac{q}{L_{f}} m(\textrm{ethanol})
    [/tex]
     
  4. Apr 18, 2010 #3
    Thanks for the help. It worked out with that. Also the q value I was using was wrong, had forgot to take into account the 10g of ethanol being burned.
     
  5. Apr 18, 2010 #4
    cool. am i right to assume you're a chem student?
     
  6. Apr 19, 2010 #5
    Yeah I am. Right now just the high school course, next year I'm doing Physics with a minor in Chem
     
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