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Hess's Law

HowAboutDAT

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  1. Pre-Dental
    Given the following information, what is the heat of formation of methane?


    C(s) + O2 (g) --> CO2 (g) Delta H = -94.1 kcal/mol
    H2 (g) + 1/2 O2 (g)--> H2O (l) Delta H = -68.3 kcal.mol
    CO2 (g) + 2H2O --> 2 O2 (g) + CH4 (g) Delta H = 212.8 kcal.mol

    I realize the next step is to get the "mother" equation, but when you get that, do you have to balance the equation leaving you with..

    C + 2H2 --> CH4?

    Thus the Delta Hf ~ -20?

    Correct answer: -17.9
     

    Kneecoal

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    1. Pre-Dental
      the "mother" equation should be the balanced equation.

      the second equation should be doubled to get your 1 mole of O2, so that it can add up with the 1 mole of O2 in the first equation to get the two moles to cancel out that are on the other side of the third equation. doubling that second equation also doubles its delta H. canceling everything out should then get you the balanced equation
       

      HowAboutDAT

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      1. Pre-Dental
        the "mother" equation should be the balanced equation.

        the second equation should be doubled to get your 1 mole of O2, so that it can add up with the 1 mole of O2 in the first equation to get the two moles to cancel out that are on the other side of the third equation. doubling that second equation also doubles its delta H. canceling everything out should then get you the balanced equation

        ahh i see, but does it matter if you balance the mother equation or the intermediate steps?
         

        Kneecoal

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        1. Pre-Dental
          i don't really get your allusion to the "mother" equation. the way you do these things is:

          C(s) + O2 (g) --> CO2 (g) Delta H = -94.1 kcal/mol

          2 (H2 (g) + 1/2 O2 (g)--> H2O (l)) Delta H = 2 (-68.3 kcal.mol)

          CO2 (g) + 2H2O --> 2 O2 (g) + CH4 (g) Delta H = 212.8 kcal.mol


          each mini equation within itself is balanced. your very first step should be to get the same numbers of products and reactants that are on both sides of what will be the overall equation, what i guess you're terming the "mother" equation. so i guess the answer to your question is yes, it matters. that's how you GET the balanced overall equation. does that clear things up?
           
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          HowAboutDAT

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          Jul 24, 2008
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          1. Pre-Dental
            i don't really get your allusion to the "mother" equation. the way you do these things is:

            C(s) + O2 (g) --> CO2 (g) Delta H = -94.1 kcal/mol

            2 (H2 (g) + 1/2 O2 (g)--> H2O (l)) Delta H = 2 (-68.3 kcal.mol)

            CO2 (g) + 2H2O --> 2 O2 (g) + CH4 (g) Delta H = 212.8 kcal.mol


            each mini equation within itself is balanced. your very first step should be to get the same numbers of products and reactants that are on both sides of what will be the overall equation, what i guess you're terming the "mother" equation. so i guess the answer to your question is yes, it matters. that's how you GET the balanced overall equation. does that clear things up?

            Yup, thanks man
             
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