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Questions about Redox Reactions

  1. Sep 23, 2016 #1
    Hello,

    Firstly, I apolagize if this should go in "HW help". It isn't homework, and the question doesn't fit the format of "HW help", but it is so elementary, so I don't know.

    I'll write my questions in bold.

    Although I think I understand what redox reactions are--a reaction in which an element gains an electron from another element--I am confused about some of the language.

    The first sample exercise in the textbook regarding oxidation numbers is "Is the conversion from SO2 to SO42- oxidation or reduction". This question doesn't make sense to me: don't we say "does specific element get oxidized or reduces in such in such reaction?". When it solves the problem, it begins by saying "The process is oxidation if the oxidation number of sulfur increases, reduction if it decreases", so I interpreted this question as "Is sulfur reduced or oxidized when going from SO2 to SO42- ?" Is that correct?

    Even if the way they phrased it was shorthand for my question, I'm still confused. Both Sulfur and Oxygen have high electronegativtiy and are nonmetals; shouldn't the bonds be more covalent than ionic, and so oxidation doesn't really apply?

    Similarly, in a different question, the textbook asks whether H2O2 is oxidized or reduced in the reaction C6H8O6 + H2O2 -->
    C6H6O6 + H2O . How can H2O2 be oxidized or reduced!? Shouldn't they be asking whether the Hydrogen is oxidized or reduced, or whether the Oxygen is oxidized or reduced?
     
  2. jcsd
  3. Sep 24, 2016 #2

    Bystander

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    If you're going to do this, don't make me proof read, to wit:
    Answer: "No."
    Yes.
    No. That is not a correct interpretation.
    Hydrogen peroxide is an oxidizing agent; it is reduced to water by the organic compound, and the organic compound is oxidized into a different compound.
     
  4. Sep 24, 2016 #3

    Borek

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    The most easy to understand examples do deal with single atoms being reduced or oxidized (like Na/Na+), but there are at most just over 100 types of single atoms, while there is an infinite number of molecules/ions that can undergo oxidation and/or reduction. It is often pretty easy to say "oxidation number of an atom has changed, so it was oxidized/reduced", but is rarely the case - that is, it is rarely a single atom that is recipient/donor of electrons. In most cases it is a whole molecule/ion (or at least part of it) that is undergoing the reactions. SO2/SO42- is a good example of that.

    It may happen that the molecule gets destroyed/modified in the process, then it is hard to trace what was oxidized/reduced (as it disappears) and in such cases it is again easy to think in terms of "this atom/element is what reacted", but it is simply not true.
     
  5. Sep 24, 2016 #4
    The quetions about is this or that ocidized/reduced are usually simplified to badically just seeing which molecule has change its oxidation state(number) whether the compound is covalent or ionic is irrelevant.... when going from SO2 to SO4 -2 the molecule goes from covalent to a polyatomic ion but there are many examples like this
     
  6. Sep 25, 2016 #5
    Thank you for the responses. After reading them, it makes sense that a compound can be reduced or oxidized...compounds can lose or gain electrons too! But then it confuses me that SO2-->SO42- is a reduction instead of an oxidation. How do you see that SO2 has lost electrons? It makes sense that the Sulfur in the second compound is giving away more of it's electrons, but I gather from the second reply that it is incorrect to translate the question into "is sulfur oxidized or reduced during the reaction from SO2 to SO42-". Would someone walk me through seeing this correctly, without oxidation numbers (I'd like to be able to tell whether an element/compound is being reduced/oxidized without the use of oxidation numbers, as I don't get what is going on enough to use oxidation numbers with understanding). Or link me to something the addresses this?
     
  7. Sep 25, 2016 #6

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    It is an oxidation.
     
  8. Sep 25, 2016 #7
    :) Sorry I meant, how do you see that the compound is being oxidized not reduced. If anything, I naively see that the compound begins neutrally charged and ends negatively charged and see it as being reduced (which is incorrect).
     
  9. Sep 25, 2016 #8

    Borek

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    Oxidation state is not a property of a molecule, it is a (made up, not reflected in nature) property of an atom.
     
  10. Sep 25, 2016 #9

    Borek

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    Try to assign oxidation numbers to all atoms involved. How do they change?

    Technically oxidation numbers are just an accounting device helping to calculate electrons, but they are quite helpful when dealing with redox systems.

    Note: this can get a bit tricky as in every case we have two separate processes taking place and whether we call the reaction oxidation or reduction can depend on what we choose as reference point. In this case the obvious reference point is the central atom of both entities involved.
     
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