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Optimizing Redox Reaction of FeO with H2

  1. Mar 6, 2018 #1
    Greetings,

    ME major here trying to brush the cobwebs off my chemistry. I can't seem to remember or recognize anything on my old chemistry textbooks regarding the optimization of a reaction. I question whether it was even taught.

    I'm trying to determine the most favorable conditions to ensure minimal limiting reagent and maximum Fe and H2O yield for the reduction of ferrous-oxide (wustite) with hydrogen gas.

    FeO + H2 --> Fe+2 + H2O

    It is my understanding that the catalyst for this reaction is heat. One source claims that it takes place at temperatures near 350oC [https://chemiday.com/en/reaction/3-1-0-5399]. However, this is just one source and I'm not sure how reliable the claim may be. I want to know how to determine the temperature required to completely react 1 mole of wustite with 1 mole of hydrogen gas at the highest reaction rate for a given pressure.

    I've been looking for research done on this matter, which studies the reaction in a range of temperatures, such as Further Insight into the Reaction FeO+ + H2 → Fe+ + H2O: Temperature Dependent Kinetics, Isotope Effects, and Statistical Modeling [https://pubs.acs.org/doi/10.1021/jp5055815] and Reactions of Fe with H2O and FeO with H2. A Combined Matrix Isolation FTIR and Theoretical Study [https://pubs.acs.org/doi/pdf/10.1021/jp010914n].

    I find them very heavy on jargon and I've never seen the addition of variables like D to a reaction, such as FeO+ + D2 → Fe+ + D2O

    If anyone could lead me in the right direction, that would be much appreciated.
     
  2. jcsd
  3. Mar 6, 2018 #2

    Borek

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    Staff: Mentor

    Several problems with your post and several comments that I have, hopefully they will be of some use to you (sorry if not).

    1. I don't like the idea of calling heat a catalyst. Yes, reactions are faster in higher temperatures, but it is not the same thing as using a catalyst to speed up the reaction.

    2. Not sure what you mean by "ensure minimal limiting reagent". The stoichiometry is given. There is no such thing as a minimum temperature required for the reaction to follow its stoichiometry. For equilibrium reactions you can choose a temperature that gives maximum possible yield, or is in some way optimal (particular combination of yield and speed that makes the process economically viable). Sometimes we use excess reagent to shift the equilibrium to the right. Neither of these cases fits what you wrote, so I wonder if you are not confusing things here.

    3. D means deuterium, they used heavy water in experiments (which is how the isotope effect came into play).
     
  4. Mar 6, 2018 #3
    My apologies, you are completely right. I'm sure you'll correct me if I'm wrong:

    There is no limiting reagent because the reaction is a 1:1 mole ratio. Furthermore, reactions take place spontaneously regardless of temperature, however, raising the temperature can increase the rate of the reaction and there exists a "Goldilocks" zone in which the maximum rate of reaction is reached at a given temperature.

    Performing basic stoichiometry (assuming I've done this properly), I determined that an arbitrary 19.7 kg of FeO requires 0.5527977284 kg of hydrogen gas to yield 17.50649532 kg of Fe and 2.746302405 L of water (I can go through the steps if you'd like to verify my reasoning).

    I'd like to know what a chemist takes into consideration when attempting to perform this chemical reaction.

    Thanks for replying!
     
  5. Mar 6, 2018 #4

    Borek

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    That's not how it works. If you have things reacting 1:1 but you mix one mole of one with half mole of second, the latter will be the limiting reagent.

    Your calculations start OK (yes, 19.7 kg FeO will react with 0.55 kg of hydrogen), but your masses of products are wrong. I wonder if it is not some mistake, 274 (same digits as in your water volume) is a number of moles involved.
     
  6. Mar 6, 2018 #5
    So, if I'm not mistaken, you're saying the limiting reagent is a case by case issue related to the supply of reactants and not an inherent limit of the chemical reaction itself.

    For the calculations I used the average mass number of elements as reported in the periodic table.
    Fe = 0.055845 kg/mol
    O = 0.015999 kg/mol
    H = 0.001008 kg/mol

    Because the mass of FeO is 19.7 kg, I divided it by the sum of the Fe and O mass numbers to get the moles of FeO.

    19.7 kg / [0.055845 kg/mol (Fe) + 0.015999 kg/mol (O)]

    FeO = 274 mol

    Stoichiometry shows that H2 is also 274 mol. So to get the mass of hydrogen gas required by this formula, I multiplied by it's mass number x 2.

    274 mol x 0.001008 kg/mol (H) x 2 = 0.55 kg

    Next, because 1 atom of Fe is bonded to 1 atom of O, I concluded that the product would result:

    274/2 mol of Fe or 137 mol of Fe. I divided the moles of FeO by 2 because half of those moles correspond to the Oxygen bonded to each Fe atom.

    274+137 mol of water or 411 mol of water. I added the moles of Hydrogen to the corresponding moles of O

    Then I calculated the mass by multiplying the mole values by the mass numbers as done previously. If my calculations are wrong, I'm thinking I was mistaken to divide the moles of FeO by 2?
     
  7. Mar 6, 2018 #6

    Borek

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    Yes.

    So far, so good.

    No. 1 mole of FeO contains 1 mole of Fe.

    It is 1 mole of Fe and 1 mole of O for 1 mole of FeO.

    Think about the mole as if it was just an overgrown dozen. If you have dozen molecules of FeO, how many atoms of Fe do you have? How many atoms of O? If every of these atoms of O reacts with two atoms of hydrogen, producing molecule of water, how many molecules of water will be produced?
     
  8. Mar 6, 2018 #7
    I see. So because 274 mol of FeO is still 274 of Fe and 274 mol of O, then 274 mol of O and 274 mol of H2 is still 274 mol of water. The formula was telling me that from the start because all the coefficients were equal, 1 mol of FeO + 1 mol of H2 is 1 mol of Fe + 1 mol of H2O

    I realize now the reason I got confused is because, in my private calculations, I actually wrote the equation in this form:

    FeO + 2H --> Fe+2 + H2O

    I moved the 2 in front because I thought it would be easier. As a result, I obtained:

    274 mol FeO + 584 mol H --> 137 mol Fe + 721 mol H2O

    But I see now it should've been:

    274 mol FeO + 584 mol H --> 274 mol Fe + 584 mol H2O

    This yields 15 kg Fe and 4.66 L or 4.94 L of water depending on the method.

    With that out of the way, assuming I finally got this right, how would you go about determining the parameters to implement this chemical reaction? I don't have access to a lab where I can mix FeO and H2 at different temperatures, I imagine there has to be a way to calculate this or perhaps existing research on the topic. Glancing at the research papers I linked above, do you believe I'm on the right track? Perhaps my answer is on those papers and I just can't recognize it.
     
  9. Mar 6, 2018 #8

    Borek

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    No, 2H still produce one H2O.

    Tricky. There is no problem with finding data that will tell you what the thermodynamical equilibrium is for this process, but you are interested not in thermodynamics but in kinetics - and that's where things get ugly. Experimental data and/or research papers are the way to go.

    Sadly, I can't check papers you linked to, either the site doesn't work or I am IP blocked for some reason. However, looks like these papers can be interesting:

    https://www.sciencedirect.com/science/article/abs/pii/S1004954112603577

    https://www.sciencedirect.com/science/article/pii/S000925091400428X

    Note that latter suggests using Fe/H2O for hydrogen production, which is exactly opposite to what you want to do. You should check if the thermodynamics is on your side.
     
  10. Mar 9, 2018 #9

    epenguin

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    Shouldn't that be Fe not Fe2+? Otherwise you have made charges appear from nowhere, and also the iron is not reduced at all, although the hydrogen is oxidised. Some of the quoted publications are probably irrelevant to someone who has cobwebs on his chemistry. I have never heard of FeO+ . As far as I can make out it looks to be something that appears in gas phase or molecular beams, probably not relevant to your concerns.
     
  11. Mar 20, 2018 #10
    In addition to what has already been written I'd say there are two problems to solve:
    1) Thermodynamics of reaction
    2) Kinetic of reaction

    I don't have data for the second but the approximate data I have for the first seem to indicate that it's endothermic and with increased entropy, so the reaction is strongly temperature-dependent.

    ΔGr = ΔHr - TΔSr.

    I have big doubts on the data I found, anyway are these:
    ΔHf H2O(g) =~ - 240*103 J/mol
    ΔHf FeO(s) =~ - 270*103 J/mol

    S{H2O(g)} = 190 J/(mol K)
    S{Fe} = 27 J/(mol K)
    S{FeO} = 60 J/(mol K)
    S{H2} = 130 J/(mol K)

    Then:
    ΔHr = (-240+270)*103 = 30*103 J/mol

    ΔSr = (27 + 190 - 60 - 130) J/(mol K) = 27 J/(mol K)

    ΔGr = 30*103 - T*27 J/mol
    ΔGr must be negative for the reaction to occur (independently from the speed of reaction):

    30*103 - T*27 < 0

    T > (30/27)*103 K =~ 830°C.

    --
    lightarrow
     
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