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Homework Help: Help with Hess's law

  1. Nov 17, 2017 #1
    1. The problem statement, all variables and given/known data
    Using Hess's law, calculate the ΔH value for the following reaction:

    Use these three reactions:
    1. Fe2O3(s)+3CO(g)→2Fe3O4(s)+CO2(g) ΔH= -25.0 kJ

    2. 3Fe2O3(s)+CO(g)→2Fe3O4(s)+CO2(g) ΔH= -47.0 kJ

    3. Fe3O4(s)+CO(g)→3FeO(s)+CO2
    ΔH= +38.0 kJ

    2. Relevant equations

    3. The attempt at a solution

    This is what I have done so far:

    Reversed reaction 3
    3FeO(s)+CO2→Fe3O4(s)+CO(g) ΔH = -38.0 kJ

    I've tried some other things but each time I get the incorrect answer. The examples that I was shown were way easier than this.

    I'm not really sure what to do next. I reversed reaction 3 to get FeO on the left side. Any help on the steps I should take to complete this would be great... I find this question very confusing.
  2. jcsd
  3. Nov 17, 2017 #2


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    Staff: Mentor

    Your first step is definitely a correct one.

    Hard to comment on your attempts (and why they yielded incorrect answers) not seeing them.

    As there is an obvious mistake in the problem wording (first equation is not balanced, neither of the 1st and 2nd equation contains FeO(s), and there is no Fe(s) present anywhere) it is also impossible to point you in the right direction.

    Note: state symbols (s), (aq), (g) and (l) should be not written as subscripts. It is a common error, present even in textbooks, but IUPAC defined them to be written with normal characters.
  4. Nov 17, 2017 #3

    If I were you, I would try the following:

    Call ΔΗformation of each substance x (ΔΗf, Fe3O4), y (ΔΗf, CO2), z (ΔΗf,Fe2O3), v (ΔΗf, CO), k (ΔΗf, FeO). Then I would form 3 equations with 5 unknown variables.

    In order to find the unknown ΔΗ, I would search to find the value of (y-k-v). Perhaps, this is not impossible, since you relate the 3 unknown variables in a 3x5 system.Then, ΔΗunknown=y+ΔΗFe-k-v (ΔΗFe is considered given data-you can find it from the web).

    This way, I would transform the chemical problem into a mathematical problem, but still I don't know if it is possible/easy/difficult to find the value of (y-k-v).

    Edit Note : I agree with Post #2. There are problems with the equations that will make it impossible to solve the problem.
    Last edited by a moderator: Nov 17, 2017
  5. Nov 18, 2017 #4
    I did write it wrong, sorry about that. Is there any way I can have this thread deleted so that I can create a new one with the correct information? Also, thanks for letting me know about state symbols, in my lesson they are written as subscripts but I will write them as normal characters from now on.
  6. Nov 18, 2017 #5


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    Staff: Mentor

    Post the correct information and we will edit the original post, marking things that were modified.
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