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Hess' Law Help

  1. Oct 19, 2012 #1
    1. The problem statement, all variables and given/known data

    The standard heat of formation of n butane cannot be measured directly but can be determined indirectly from other heats of reaction by applying hess's law. Given the folowing data for the combustion of carbon hydrogen and n butane in oxygen:

    C + O2---> CO2 H=-393.5
    H2 + 1/2O2 --> H2O
    C4H10 + 6.5O2 ---> 4CO2 + 5H2O

    2. Relevant equations

    3. The attempt at a solution

    Tried several times on scratch paper. The formation of butane is as so : 4C + 5H2-->C4H10

    Ive tried the actual formation using CO2 + H2O --> C4H10 But i cant cancel the carbons

    Sticking with my other normal equation 4C + 5H2 --> C4H10 (This is what I want, No?)

    Tell me if my equation is wrong, cause it proabably is.

    I tried multiplying the first step by 4, the second step by 5, and reversing the third step only to get everything to cancel except the oxygen. I get stuck with 6.5 oxygens on the left side and 9.5 Oxygens on the right. Dont know where to g ofrom there. Help please!
  2. jcsd
  3. Oct 19, 2012 #2


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    Staff: Mentor

    [tex]4 \times 1 + 5 \times \frac 1 2 = 6.5[/tex]
  4. Oct 19, 2012 #3
  5. Oct 19, 2012 #4
    Here is another question: Write the chemical equation for the standard molar enthalpy of formation of proponol. c3h8O
    Is this simply :: 3C + 8H + O2 ----> C3H8O ( Im pretty sure its this one.)


    Or is it 3CO2 + 4H2O ----> C3H8O + 9/2O2

  6. Oct 19, 2012 #5


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    Staff: Mentor

    Where did you get 9.5 molecules of oxygen from?
  7. Oct 19, 2012 #6


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    Staff: Mentor

    It is not balanced and it uses substances in non standard state. But it is much better than the other one.
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